[Reading Time: There is a lot of information here. This is a book length manuscript. It is not a short article or blog post. Grab some tea, a beer, wine, coffee or scotch and enjoy]

 

On the path to building our new sauna we learned that there is a lot of not so good (or downright bad and misleading) information on the web and from english speaking sauna vendors, and that there are a lot of details that are critical to get right and easy to get wrong.

Everything we found in English – books, forums, Reddit, Facebook and websites – consistently recommended ceilings no higher than 7’ to 7.5′, benches too low, ventilation that the laws of physics say won’t provide good ventilation and other things that just didn’t make sense from a physics standpoint and didn’t match with the saunas we’d experienced in Finland. But when we searched in Swedish, Finnish and German we saw much different information and information that does make good sense like for ceilings ‘higher is better’. Why such a difference? Why do folks in the U.S. build saunas so much different from those in Finland? And why are the saunas in Finland so much better?

The following is largely my notes from 2017-2019 as we were designing and building our new sauna. This is the result of trips to Finland, Sweden and nearby countries, numerous conversations with Finns, Swedes, Estonians, Germans and others about sauna and in particular what a sauna is, how they work and how to build one, a lot of translating of Finnish to English and dredging up physics concepts I’ve not thought about since Uni.

Finns get it right when it comes to building good saunas (and ’sauna’ is after all, a Finnish word). They know what they’re talking about. When they are so emphatic about ‘feet above the stones’, there are very good reasons. 

If you want a sauna in Finland then you hire a sauna builder and they’ll build a sauna as described here. Good sauna builders in Finland know all of this. That’s not currently the case in North America.

If you want this thing that Finns call a sauna then you, your builder or both should read and understand what’s here and ideally also Lassi’s “Secrets of Finnish Sauna Design’ book. (If you want to fully understand saunas, how they work, how they’ve evolved and how to build them then after reading this also read in order; ‘Sauna the Finnish Bath’ by H. J. Viherjuuri, ‘The Sauna Book’ by Johnson & Miller, ‘The Sauna Is’ by Bernard Hillala and  ‘Finnish Sauna: Design & Construction’ by Rakennustieto Publishing. Then re-read this and Lassi’s book. Then talk to Kimmo about a visit to Finland to tour some saunas and talk to some sauna experts and builders.)

Disclaimer: I am not an engineer nor doctor. Nor any kind of expert on sauna. The following is simply a bunch of notes on what we learned building our sauna and that we wish we’d known much sooner. Use at your own risk. I strongly encourage everyone reading this to do further research on all of these topics and in particular to read Lassi Liikkanen’s ‘Secrets of Finnish Sauna Design’ – the book that I really wish existed when we built our sauna. If you find any disagreement between what’s here and what Lassi says then Lassi’s book and website is the safer path. I also highly recommend Lassi as a consultant.

I hate seeing people taken advantage of. I have Asperger’s and that’s one of the interesting traits that comes with it. People in the U.S., due to our lack of knowledge about saunas, are frequently taken advantage of by sauna vendors. That’s not right.

This is a journey. I am still learning and totally enjoying it. I am, to the best of my ability, trying to communicate what this thing that Finns call sauna is. The goal of this is to be as accurate and informative as possible. If there is anything inaccurate below please let me know so that it can be corrected.

Trumpkin? One of my favorite characters in Chronicles of Narnia. If you want a sauna built properly then he is who you want building it – he’ll get the details right. Below he’s making a point about the importance of getting benches up to the proper height 🙂 .

 

What Is A Sauna?

A sauna is a wood lined room heated to a temperature of about 75-105°c by a large mass of stones. The stones may be heated by wood fire, electric, gas or nuclear fission. Water is thrown on the stones to produce steam that when combined with pure fresh air and even convective heat around bather’s bodies results in löyly. 

The essence of sauna is an experience where bathers are heated very evenly from head to toe, front to back and minute to minute by convective heat – hot air. Sauna is officially called a ‘hot air bath’ – see Official Definition of a Sauna in the end notes. When steam is created this steam similarly envelopes us evenly from head to toe and front to back, but not minute to minute as we want the steam to be temporal.  

Whether buying a kit, building from scratch or having someone build it for you, here are some of the more critical fundamental elements of a sauna that you should look for and that we’ll discuss in more detail further down.

• The Foot Bench is in The Löyly Cavity – typically above the top of the stones and/or lower third of the space (cold zone). 
• Bathers are heated evenly by Convective Heat.
• Direct Radiant Heat is minimal and other than on bathers backs is not noticeable.
• A Heat Cavity above the door – ideally the ceiling is at least 45cm (18”) above the top of the door opening.
• Proper downdraft ventilation (fresh supply above the heater, and for electric – mechanical exhaust below the foot bench).
• Stones! Ideally a MINIMUM of 6kg / m³ (1/3 lb / cf) of space and 17kg (37 lbs) per person.
• MINIMUM volume of 3 m³ (105 cubic feet) per person.
• Vestibule or sitting/changing/shower room to provide an air lock.

These notes are focused on building saunas and experiencing sauna in the same way as is done in Finland – By creating a löyly cavity in the sauna and placing bathers in it. 

It’s important here also to distinguish this thing that Finn’s call Sauna from two other forms of induced sweating that are popular in North America and that use (or misappropriate) the term sauna:

• IR or lnfraRed Cabins – Bathers are heated by Radiant / IR heat from electric IR panels on the walls.
• Sweat Cabin – Bathers are heated by Radiant / IR heat from a heavy steel or cast iron stove. Bathers may also experience some minimal Convective heat. These are kind of an amalgam of an IR Cabin, Native American Sweat Lodge and Sauna.

These are both very different experiences from sauna and neither is, I think, what folks in Finland consider a sauna. In a sauna any noticeable radiant or IR heat from a source “is undesirable” according to Risto Elomaa, president of the International Sauna Association. Eero Kilpi, president of the North American Sauna Society, says about radiant heat “if you can clearly point to a source, then it is undesirable” 

A sauna is an ‘air bath’ provided by convective heat which neither of these really are. While both of these might have wood walls and benches similar to a sauna, these elements are not as critical. Bench or ceiling height in an IR Cabin or Sweat Cabin doesn’t make much difference in the experience as it does in a sauna where we are heated by convective heat.

Neither of these two nor sauna are necessarily any better or worse than the others. Saunas, IR Cabins, Sweat Cabins, Banyas, Sweat Lodges, Turkish Baths and Onsens are simply different thermal experiences.

 

Löyly and Sisu

Löyly, pronounced kind of like Low-Lou, is critical to sauna. Löyly is a Finnish word that traditionally meant Spirit or Life but today is used as a descriptor for a good environment in the sauna. It has no english equivalent. A wood lined room with benches, heat and steam but without löyly is, according to Finns, not a sauna.

“Steam added to bad stale air is just that, steam added to bad stale air, it is not löyly” 

Löyly is not just the steam created from throwing or ladling water on the stones. That’s just steam. As we do, Finn’s have a word for that also, höyryä.

Löyly is, according to a 1988 Finnish research paper, “the purity, freshness, temperature and humidity of the air in the sauna”.

Löyly is about quality, not quantity. Löyly is what a bather experiences when they are heated evenly head to toe, front to back and minute to minute by a convective loop that creates a löyly cavity. The temperature at their head and shoulders is about 75 -115°c. The air is pure and fresh without high levels of CO2, colognes, mold, bacteria or other contaminants or scents. Undesirable direct radiant heat is unnoticeable. Ample soft wood walls and ceilings help to maintain a comfortable environment and absorb some noise so that the sauna is quiet and peaceful.

Water is thrown or carefully ladled on the mass of stones producing a burst of invisible steam that is carried with the convective loop up, across the ceiling and down to envelope bathers bodies for a brief period of one or two minutes and is then quickly exhausted returning the sauna to its original drier state – so that the burst can be comfortably repeated.

That, all of the above combined, is Löyly.

“Löyly is about the QUALITY of the heat more than the QUANTITY.”

Our goal isn’t getting to some temp, but to have good quality heat. When you experience quality heat and löyly and get use to it, you never want to go backwards.

—-

But there’s a bit more nuance to it. If a Finn visits my cabin and we go to my new sauna building down by the lake but the foot bench is below the top of the stones and so below the löyly cavity, they will have uneven temps around their body and cold feet – they’ll not experience löyly. And question if this is even a sauna. 

However, if we visit the sauna in my small 3rd floor city flat and it has similar too low of benches then my Finnish friend will proclaim it good löyly. Even though they might have had colder feet than the place at my cabin! What gives? 

The difference is that in my flat they knew that I had space constraints with no option to build it higher or larger so I did the very best I could. I showed perhaps, a tiny bit of Sisu. I was tenacious in achieving the best quality sauna experience I could given hard unavoidable constraints – I was determined and persevered. At my cabin I could have built a proper sauna with proper bench heights. There the sky is, quite literally, the limit. But I didn’t. I took a shortcut. I made a löylyless pseudo-sauna when I could have made a proper sauna. Just the opposite of Sisu. 

Sisu goes beyond individual saunas to saunas in general. Sauna builders in Finland constantly strive to learn and to understand how saunas work to make their saunas better so that the experiences of their customers will be the best possible. They want the saunas they’re building this year to be better than the saunas they built last year.

A great example of this is that after decades (or centuries) of fresh air supply entering near or in the floor of wood heated saunas, they are now doing a fresh supply vent above the stove so that these saunas can have as good of air and löyly as electric heated have.

There is a lot of room for aesthetic variation though, so long as the fundamentals are not compromised. A large window is not ideal but when there is a really great view it’s understandable. Benches below the löyly cavity or poor ventilation are not so understandable.

How do you know if you’ve got löyly? Most Finns are in enough saunas throughout their lives that they immediately recognize good vs acceptable vs bad. How can we know?

• If you blow on your feet and can feel any heat then you might have cold feet and could benefit from higher benches. 
• If you ever feel light-headed, dizzy, tired or fatigued, confused, have a tingling or pins or needles feeling, or feel like you need to leave the sauna for some fresh air then you likely need better ventilation. In a sauna with proper ventilation and good löyly your breathing should be very relaxed and normal, you should always be able to comfortably, easily and steadily breath through your nose and never feel like you’re suffocating in any way.
• If you close your eyes you should not be able to tell where the stove is, you should not feel any heat directly from it, and you should feel even heat around your entire body head to toe, front to back and minute to minute. Your head should not be noticeably warmer than your feet and your back not cooler than your front. 
• With your eyes still closed, have someone throw a few ladles of water on the stones. You should feel a wave of heat descend on you from the ceiling (it should not come at you as a frontal wave) and then feel it work it’s way down your entire body from your head to your feet, caressing you evenly on all sides in its warmth. It should perhaps be just short of unbearably hot but quite wonderful for this brief period and then just when you think you’ve had enough of it, it disappears and is exhausted.  

One huge advantage that people in Finland have is a country full of good saunas and a bunch of builders who know sauna. We don’t have either. While measurements aren’t very romantic, they are a good way to take some of the guess work out of knowing how good a sauna is and allow us to know objectively how a sauna performs and if it has the capability for quality heat and löyly. Some targets to aim for might be:

A good sauna builder will aim for and meet at least the criteria for an Acceptable sauna and often for a Good sauna. 

There are some elements that can’t be so easily measured though such as heat being even front to back, how soft the steam is, etc.  For these we have to rely on our senses.

Jesse Hämäläinen of Narvi Sauna Heaters says “There is no shortcut to löyly, it is always about stones and proper ventilation”.

 

Lost In Translation: Why Are American Saunas So Bad

“90% of saunas in North America are bad. The other 10% are worse.”
– Board Members, Finnish Sauna Society
– Mikkel Aaland
– Finns of all sorts

Why do they say this? And why is what is written here in this document so different from most of the rest of what’s written about saunas in english?

If you talk to sauna builders and enthusiasts in Finland you’ll consistently hear three things; ‘feet above the stones’, ‘ventilation’ and ‘bigger is better’. Most of what is written in english has however said the opposite; Saunas should be no more than 7’, should be small, and well sealed or have only a gap under the door for ventilation. And just about every sauna in North America has been built like that so it’d certainly be easy to assume that 7’ is correct.

Why the difference? Just a few decades ago:

Finns said “Feet above the stones, which can usually be accomplished with 1900-2300mm (6’2” – 7’6”) high ceilings.

Americans said “Cool, 7′ it is.”  (We do like to simplify things!)

That was with much smaller heaters though. Just as we went through a period of poor quality building (homes, cars, etc.) in the 60’s – 80’s, Finland went through a period of poor quality saunas. This I think primarily with stoves that tried to economize with lessor stone mass. As Finns re-realized how critical stone mass really was the heaters got bigger and they raised their benches accordingly. But we were stuck on 7’. And we totally ignored “jalat kivien yläpuolella” – “Feet above the stones”.

Similar with ventilation. Finns changed how they do ventilation in the 1990’s but that was all written in Finnish. It never made the leap to english so we’ve been doing either 1970’s ventilation or in many cases the even more misguided idea of no ventilation.

The crux of the problem then is, I think, one of simple translation. We’ve not been getting the same info about saunas as Finns.

A second problem on top of this is UL standards that were written by someone who didn’t understand sauna nor any of the physics involved.

 

Sauna Temperature

Finns don’t ask what the temp was, they ask how the löyly was.

Temperature is important but temperature alone is quite unimportant. As or even more important is the quality of the heat and environment – löyly. It’s important that the temps on bathers bodies are fairly even all over, head to toe and front to back and minute to minute, that the air is fresh without high levels of exhaled CO2 or built up humidity, bathers experience the convective loop of the löyly cavity and steam descending down on them from the ceiling, and no undesirable radiant heat. These are the ingredients of löyly. It doesn’t matter if the temperature in a sauna ‘got to 200°f’ if your feet are only 140°f, you’re being baked on one side by radiant heat and you’re feeling light headed because of bad air.

So, that said… The official recommended temperature is 80-105°c (± 10°c) (167-221°f (± 20°f))  – measured at a point 1m (39”) above the middle of the longest sitting bench opposite the heater. A good sauna should be able to maintain any temperature in that range.

 

A Brief History Of Our Understanding Of Sauna Physics

How did we get where we are today? Over time we (humans, mostly Finns) have come to understand the physics of how saunas work better each year which has resulted in increasingly better saunas.

1961: Feet Above The Stones – Sakari Pälsi mentioned this in his 1961 book ’Sauna’. Known today as ‘Pälsi’s Law’ or ‘The First Law of Löyly’ it is one of the most fundamental elements of good sauna design. It was known prior to this but Pälsi is the first that I’m aware of to state it succinctly.

1960’s: Overall volume of at least 3m³ (105 cubic feet) per bather. While most saunas usually met this requirement it was documented officially to combat the health problems being seen from an increase in much smaller saunas beginning to be built. We see this also in English in Johnson & Miller’s 1977 ‘The Sauna Book’ and Alan Konya’s 1987 book ‘Finnish Sauna’.

1980’s: Stone Mass For Electric Heaters – Until this point most electric sauna heaters had very little stone mass and for some the stones were little more than aesthetic. This is one reason why electric heaters got such a bad name. During the 1980’s there was a growing realization of the importance of stone mass in electric heaters for better quality convective heat and steam. This resulted in increasingly larger heaters and so also commensurately higher ceilings and benches to keep ‘feet above the stones’. Calls for even greater stone mass continue.

1992: Mechanical Downdraft Ventilation For Electric Saunas – Electric heated saunas were largely using the same ventilation strategy as wood heated – typically a gap under the door and an exhaust vent high on an opposite wall. This didn’t work and was another reason electric heaters got a bad rap. During the 1970’s and 80’s builders across Finland began experimenting with other options with Mechanical Downdraft seeming the best. VTT did some studies on this in 1992 and found that it was indeed the best and really the only good option. Soon after the official RT system was updated to reflect this. 

Late 1990’s: Open Sided Mesh Heaters – Along with the realization of the importance of stone mass came the first open sided mesh heaters from Iki. These were initially marketed as eliminating the need for higher ceilings and ‘feet above the stones’. Some still believe (and market) this even today but better builders quickly learned that this wasn’t fully the case and that benches and ceilings still needed to be high up. These heaters still provide benefits in stone mass and aesthetic appeal. 

2020’s: Combustion Downdraft Ventilation For Wood Heated Saunas – Noting that the air in newer electrically heated saunas with mechanical downdraft ventilation was fresher and less stale than that in wood heated saunas, sauna builders began incorporating a fresh air supply above the heater in wood heated saunas. 

 

How A Sauna Works and the 3 Heat Zones

How a sauna works, from physics and physiology standpoints is, …very very cool 🙂  

The stones and heater are the heart of a sauna and the convective loop that creates the löyly cavity is its soul. Let’s look quickly at how these work, what makes a sauna so special and different from other forms of sweat inducement, and how these affect our design.

There are two powerful forces at work in a sauna; Convective loops that work for us and thermal stratification (due to buoyancy of hot air and stack effect) that works against us. And it all begins in the heart – heat from the stove.

Convective Loop – The heat produced by the stove heats the stones. The stones, thanks to a lot of mass, surface area and air flowing up through them, convert this heat to hot air (convective heat) that rises to the ceiling, travels across the ceiling (pushed away from the rising plume from the heater) towards the far bench wall and then as this air cools it flows down the bench wall where it begins to heat us. Note that this air began its journey at perhaps +160°c and cooled as it progressed along so it’s cooling to perhaps 110°c which is what we want.

The hot air rising above the stones creates a bit of a vacuum in that space and this vacuum pulls the downward flowing air from the bench wall back towards the top of the heater where it repeats this cycle by being reheated, rising, etc. This is a convective loop and the convective heat that it carries is what warms our bodies evenly on all sides. 

When we throw water on the stones, thanks to all of the hot surface area of the stones we get steam, which flows with the convective loop up, across the ceiling, descends down on us and (thanks to efficient heat conduction from the hot water in the steam to our skin) temporarily causes a significant and quite wonderful increase in the amount of heat that we feel. This image below shows the journey of the steam from when it first begins to a bit later when we are experiencing it.

The area where this convective loop is, the green area above, is called The Löyly Cavity – it’s where löyly is.  This is why Finns so often repeat ‘Feet Above The Stones’ and why that is called the First Law of Löyly, because being up in this convective loop is key to our experiencing löyly.

If we have proper ventilation then this steam is removed fairly quickly, in about 2 minutes, and we are back to the dryer convective heat.

“The heater and stones are the heart of a sauna and the convective loop and the löyly cavity are its soul”

This is the core of what a sauna is and is what makes sauna so special. This convective loop is critical to good sauna and to löyly so the area of this loop is called the Löyly Cavity or Löyly Pocket. In designing a sauna we want to be careful to facilitate this loop, take advantage of it, and not interrupt it.

One interesting bit is that this air or steam at the ceiling will flow in the direction of lowest resistance – which is towards the furthest corner from the stove. It’s not unusual in a sauna in Finland to see people doing a wave. The steam gets to the furthest person first and so they lean forward, then the person next to them and so on.

But wait, there’s more behind the scenes of this convective loop…

 

Thermal Stratification – In any space, hotter buoyant air rises and cooler denser air sinks. This is thermal stratification. In a typical sauna the air at the floor will be about 40°c (104°f) ±. So if we’ve heated our sauna to 94°c (200°f) at the ceiling then we’ll have a 54°c (96°f) difference between the floor and ceiling with graduated temperatures between. In a 250cm (98”) high sauna for example we gain about 0.2°c / cm (1°f / inch) above the floor – on average.

Our body is happiest when it’s about the same temp head to toe. Sitting on a bench in a sauna however there’s about 150cm (58”) difference in height from our toes to our head and so, thanks to thermal stratification, potential for about 32°c (58°f) or more of temperature difference. Our body doesn’t like this, it results in cold feet and isn’t wonderously comfortable.

In this chart of a typical 7’ high American sauna the teal line is the temperature 4″ above the floor, orange is 20″ high (so 2″ above an American foot bench), grey is 40″ (so about the height of a Finnish foot bench), yellow is 60″ and blue is 80”, just below the 84” ceiling. The foot bench here is perhaps 100°f, about 85°f cooler than a bathers head. The less head to toe temp difference the more comfortable and enjoyable the sauna. Finns shoot for no more than about 28°f difference and ideally less. Also notice that the temp at the floor (teal line) hardly changes. And at 20″ above the floor there is change but very little.

There are three key things that help us overcome stratification and achieve the more even temps we desire; higher ceiling & benches, a good convective loop, and downdraft ventilation via the convective loop. 

The greater the distance between the floor and ceiling, the higher the ceiling, the less temperature difference there is per inch so the less difference from head to toe. This is good. Note also that a lower ceiling DOES NOT result in a warmer floor (teal line above). The floor will be about the same temp no matter how hot the ceiling is and no matter how powerful the heater.

A higher ceiling also allows our benches to be up higher above the colder air nearer the floor. That’s good also.

Where there’s a downward flow of air, such as with the far side of the convective loop on the bench wall, there’s less stratification because warmer air is being pulled downward. So not only is the convective loop warming us, it’s also reducing thermal stratification in that part of the sauna to make it more enjoyable. 

Where there’s a plus, there’s often a minus. The top and bottom extremes, 100°f at the floor and 200°f at the ceiling, haven’t changed so if there is less stratification up where the convective loop is then there must be greater stratification down below where the convective loop is not. And not only is there greater stratification but there is also the colder air that sinks to this area. This is called the Cold Zone and is about the lower third of the volume of the sauna. It’s the area where the air temperature is more than about 30°c (54°f) colder than the temp at the ceiling. We want to avoid any part of our body being in this lower third Cold Zone. 

And finally, ventilation that flows upwards from low to high will carry colder air from the cold zone up to us in the löyly cavity. That’s not good. Downdraft ventilation with air entering the convective loop above the heater, flowing with the convective loop to provide fresh air for us and then exhausting stale CO2 laden air from below the foot bench or the bottom of a wood fired stove enhances the convective loop rather than work against it. This is good.

This convective loop and the löyly cavity it creates are critical and one of the key things that distinguishes sauna from other forms of induced sweating.

So, we have three critical zones:

1. The Cold Zone is about the lower third of the volume (and thus height in a cabin sauna) of the space – dense heavy colder air will sink (stratification) and it needs a space to sink to …and this is it. This area also has greater temperature stratification. We want to avoid any of our body being in this area. Imagine the room is a third full of cold water and you want to stay dry.
2. Sauna builders in Finland call the area where the primary convective loop is Löyly Onkalo, the Löyly Cavity or Löylytasku, the Löyly Pocket. This is above the Cold Zone and often above the top of the stones. This is where we want to be – every bit of us. It is this convective loop that warms us evenly and thanks to the convective loop there is less stratification in the Löyly Cavity than below it.
3. The area above the door opening is the Heat Cavity. This is an area that contains and preserves heat when the door is opened. Generally the larger the better. More on this later.

A Larger Heater Does NOT Help – A larger or more powerful heater cannot overcome stratification and often makes it worse. When you add heat to a room it goes to the ceiling. No matter how much you add though the floor will stay about the same temp. We saw this in our line chart above – as the sauna heats up, the higher you are the more the temps increase and the lower you are the less they increase. Higher ceiling temps plus the same floor temp equals greater stratification. 

A larger heater can heat the room faster or maybe achieve higher temps but it cannot eliminate stratification nor cold feet from too low of benches. Proper heat in a sauna is more about finesse than braun.

Something American’s often mistake for reducing stratification is direct radiant heat on their feet and legs from the heater. This however is very different from the proper convective heat that sauna is about.

This knowledge of the heat zones and what happens in a sauna will help us to determine the best dimensions, bench and ceiling heights. In general we’ll want a somewhat larger room and higher ceiling for several reasons;

1. Getting the foot bench and so all of our body up in the Löyly Cavity.
2. Getting us above the Cold Zone (sometimes more important than ‘above the stones’).
3. Having sufficient overall volume of space per bather.
4. Getting the sitting and foot benches high enough for heat to kill mold and bacteria.
5. Creating a large Heat Cavity for greater comfort and less wasted energy.
6. Reducing direct radiant heat from the heater and stones.
7. Reduce energy loss/costs.

 

Radiant Heat (DRAFT)

For those interested…

Everything around us emits radiant heat and anything that’s colder than any received radiant heat will be heated by it. Perhaps the most common example is feeling the sun on our face or warmth from a campfire. Radiant heat can feel quite wonderful.

When you are sitting in a sauna, everything in there, including the walls and the person sitting next to you, is emitting radiant heat. Any radiant heat that reaches us and is warmer than our skin will be absorbed and potentially felt.

Radiant heat is directional. You feel it only on the side from which it is radiating towards you and it can easily be blocked. This is why an IR booth (radiant heat) has panels on all walls but a sauna needs only one heater (convective heat). People in IR Booths and Sweat Cabins often move around a lot to try to even out the radiant as best they can; their arm blocks it from their side so they move their arm, there’s too much on their feet so they turn away from the source a bit.

Radiant heat declines as it spreads out so the further you are from something the less likely you are to be affected by it. 

Key radiant sources to consider in a sauna:

Heater – This can, depending on the design, emit a lot of radiant heat, is point-source and so very directional on bathers bodies and can vary considerably from one place on our body to another. This is uncomfortable and undesirable so we want to minimize radiant heat from the heater as much as we can with heater design and placement far enough away from bathers.

Walls, Ceiling & Benches – This radiant heat is usually quite minimal, fairly even on our bodies and is for our purposes somewhat inconsequential. We cannot really control it very well anyway other than using materials such as soft wood and avoiding materials that might create an uncomfortable amount of radiant.

Since this radiant is so minimal and mass source rather than point source it is much more even on our bodies and a minimal amount of this from the wall on our backs can be desirable, especially on a cold day with lower sauna temperatures.  One considered benefit to log or timber construction is that the greater thermal mass of the walls produces more of this lighter and more even radiant heat on bathers backs. Sometimes too much though as occasionally benches need to be moved forward from the walls.

Some say that you must have a lot of radiant heat from the stove or heater in order to heat the walls. This is not true. The walls in a sauna are heated almost exclusively by convective heat. We can see this with IR images of saunas that indicate the wall temperatures track the stratification of the ambient (convective heat) temperatures.  

If radiant was much of a factor in heating the walls then they would be quite even from floor to ceiling (radiant and conduction heat travel in all directions, convective heat stratifies (e.g., hot air rises) in fluids such as air or water). Or actually hotter at heater level that is much closer to the radiant heat from the heater and cooler near the ceiling that is further away and receives much less radiant heat.

How much radiant do we experience? This will vary from sauna to sauna. If our sauna is 94°c (ambient temp @ 1m above the sitting bench) then the walls at that height are about 91°c. At the foot bench the ambient air temp is about 66°c, the walls about 61°c and the bench about 64°c. After 8 minutes in the sauna my skin, according to our FLIR, is about 57°c (134.6°f) on average – slightly warmer on my face and slightly cooler at my feet. 

So the highest heat I experience on my 57°c skin at chest level is ambient air of 79°c. Next then is radiant heat of perhaps 71°c on my back and about 0°c elsewhere (74°c @ the wall declines to 71°c at my back, 74°c at the side walls declines to below my skin temperature by the time it reaches my sides or front so I will experience no effect from it).

 

Cold Feet

Cold is a relative term (and personally I think a quite poor description for this but nobody asked me). In a sauna the ‘cold zone’ is actually quite warm, perhaps 50°c (122°f) on average. But compared to higher up where we’re sitting in 90-110°c (194-230°f) temps it can feel, relatively, ‘cold’. It’s similar with ‘cold feet’ or ‘chilly backs’. Temps at the foot bench might be 50°c (122°f) which in a normal room would be hot but if our head is in 94°c (200°f) air then our feet can feel ‘cold’ at 50°c. We won’t much notice a head to toe difference of less than about 15-20°c (27-36°f) but most people will begin to notice differences greater than that.

Many people in North America say that they don’t have ‘cold feet’ in their sauna. This primarily because they’ve never experienced a proper Finnish sauna. After raising their benches and fixing their ventilation, they have a revelation of what they’d been missing and how much better a sauna can be when the head to toe differential is decreased.

Cold feet can also make our head, and so us, seem much hotter than it really is. In a sauna with a lot of stratification resulting in cold feet we might think that 140°f at our head is really hot. But then in the same sauna but with higher benches and ceiling and so no cold feet we find that 200°f is much more comfortable and enjoyable than our old low bench sauna at 140°f. A core element of this is that our bodies want to always be in homeostasis – a stable equilibrium. When our head is much hotter than our toes our body prioritizes this and reacts to this heat differential rather than just heat. That is not what we want in a sauna. One doctor also mentioned to me that such high differentials between head and toes may not be healthy for us.

Cold Feet / Hot Head – In a sauna with too much stratification, usually from too low of benches and ceiling, our body will perceive temps as hotter than they are. Our body wants to be in homeostasis – equilibrium – and if our head is more than about 15-20°c warmer than our toes it will react to this temperature difference rather than to heat in general. That’s not what we want in a sauna. 

 

The Notes – Some Important Details We’ve Learned

So here, some quick notes on what we have learned about sauna design and building (some that we wish we’d learned much sooner). Special thanks to Kimmo Raitio, Jarmo Lehtola, Risto Elomaa, Eero Kilpi, Lassi Liikkanen, Allison Bailes, the folks at VTT in Espoo FI, the numerous sauna builders across Finland, Sweden and Germany who responded to my endless queries and many others for their ideas and patience in answering my questions and everyone’s perseverance in making sure that I not only built a proper sauna but understood sauna.

 

Stove Room Space – Finnish official recommendation (RT Card) is overall minimum volume of 3 m³ (105 cubic feet) per person or larger. Builders have told me that if necessary a minimum of 2 m³ (70 cf) per person plus one m³ (35 cf) for the elf (technically for the heater so yes, it is important) is also possible.  Larger (4-5 m³ per person) is better, smaller not. The more volume per person the fresher the air will be and the less colder air (ratio to hot air) needed to maintain healthy air quality and löyly. Larger spaces tend to have more even convective heat on bathers bodies than smaller spaces. Smaller spaces can also have people walking too close to the heater increasing the possibility of accidentally getting burned.

A good starting point recommendation from Lassi Liikkanen for a basic four person sauna is an 8’x8’ interior floor space and 8’4” interior height resulting in 130cf/person (3.5 m³ / person). This is comfortable for 4 people, benches aren’t too close to the stove, gets the foot bench above the stones for most stoves, provides a good heat cavity and it will be easier to maintain good air quality.

 

Stove Room Shape – Generally a square or tall square is best with a rectangle or tall rectangle a close second. If you understand sauna physics then many shapes can be made to work well.

Be cautious about trapezoid and similar shapes. The narrower bottom of this sauna (below) will result in a much higher cold zone (approx lower 1/3 of total volume of interior space). The cold zone here will be about the lower 1/2 of the height instead of just the lower 1/3 as in a sauna w/ straight sides. The low ceiling means that bathers will be sitting too low as well so they will have cold feet and legs. The lack of a heat cavity will result in a lot of heat loss every time the door is opened and this shape has much too little overall volume as well. A trapezoid can work but needs to be large enough that the foot bench is above the cold zone, has a sufficient heat cavity and has proper overall volume. The owners of this one below will, hopefully, still enjoy it as they are probably not familiar with sauna and so won’t know what they’re missing.

Ceiling Shape – Flat, coved, partial barrel vault/curved, vaulted/cathedral, and sloped/shed can all work well. The highest point of any of these, relative to the top of the walls, should not be greater than about 1/4 the heater wall to bench wall distance and ideally less. So for an 8’x8’x8.5′ sauna the interior peak of a vaulted ceiling should not be greater than 10.5’. Higher than this (steeper) can stall the convective loop, allow hotter air to stay trapped in the peak and increase stratification. 

For vault or partial barrel vault the peak should run parallel to the longest bench wall so between the stove wall and bench wall. Try to avoid it being perpendicular. If perpendicular can’t be avoided then height is your friend – the higher the benches and ceilings the less it matters. For a partial barrel vault / curved ceiling, setting the radius point at floor level midway between the stove and bench walls usually works well. 

For a sloped or shed ceiling the high side should always be along the primary bench wall and the low side the stove wall. There should be a bit of flat or reverse slope on the high side to avoid any corners less than 90°. As above, somewhat minimal height difference from one side to the other is best. 

Domes and other 3D shapes can work but are tricky to get right and getting them wrong can result in a quite bad sauna.

Remember to include any extra volume above the top of the walls in determining where the Cold Zone is.

Avoid steeply vaulted, coved or similar ceilings – In theory and in experience a near flat ceiling is best as it results in the most even temps. A slight rounded vault (radius greater than overall height) in the ceiling or angled coves in the corners can help airflow.  Heavily vaulted, coved or sloped ceilings result in heat being up too high, reduce desirable air movement of the convective loop and result in bathers being much lower in the overall volume of space and often almost entirely in the cold zone.

Avoid Ceiling Beams or other elements that will interrupt the air flow of the convective loop.

Note that the best interior ceiling shape will often run contrary to exterior aesthetics. Given the importance this is one case where I would not sacrifice function for form.

 

Heater and Bench Relationship: Creating The Löyly Cavity – In smaller saunas (less than about 9’x9’), the heater should ideally be on the wall opposite the benches, or with L shaped benches in the opposite corner. So we’ll have a Bench Wall or Bench Corner and opposite that a Heater Wall or Heater Corner. We also want the heater and the stones to be fairly close to the heater wall. This is to create a good convective loop and löyly cavity. The heat and steam rise from the heater, travel across the ceiling towards the bench wall until it hits the wall and then descends down that wall on to bathers. We want to be on the wall that it’s going down as this is comforting hot air or steam coming down and enveloping us on all sides.

This works also for a heater in the middle of a U or parallel bench platform. The heat and steam rise to the ceiling, then travel in all directions to the walls and then down the walls. These seating arrangements should ideally be in a 3m x 3m (10’x10’) or larger space though somewhat smaller does seem to work with some heaters.

Benches on the same wall as the heater or next to the heater don’t get the full benefits of this convective loop and so bathers experience greater heat stratification and are less comfortable. In particular they can miss out on true löyly when the steam follows the convective loop and comes down enveloping us.

 

Heater On The Door Wall – Often it is good to have the door and heater on the same wall and with the benches on the opposite wall. There are several reasons for this. 1) Both the heater and door are ideally far from bathers to reduce radiant heat, create a good löyly cavity and so that bathers are not much interrupted by cold incoming air when the door opens. 2) The updraft from the heater is much stronger than the downdraft side of the convective loop so this causes the least interruption of the convective loop. 3) If the door is away from the heater then door openings result in a loss of heat from the heat cavity.

 

Heater to Bather Distance: Reduce Harsh Radiant Heat & Face Slap – Space between bathers and the heater is a good thing. The foot bench (or bathers toes) should be about 4x – 8x the clearance to combustibles for the heater. Or typically about 1 – 1.5m (40-60”).

While the radiant heat in a 120°f IR cabin or from a ceiling heater on a 45°f patio will feel quite nice, radiant can feel harsh in the higher temps of a sauna. Radiant is also uneven and heats us only on the side its coming from. A sauna is about being heated evenly all around by soft convective heat. An IR Cabin is about being heated by radiant heat. This is also one reason why having the foot bench above the stones is important as that also helps to reduce direct radiant heat.

Having heat and steam rise in to bathers faces isn’t enjoyable. Greater distance between the heater and bathers helps to reduce this.

 

Bench and Ceiling Heights: Staying Out Of The Cold And In The Löyly Pocket – Bench and ceiling heights are critical for a good sauna and the higher the better as higher results in less stratification, a larger heat cavity, greater likelihood of keeping bacteria and mold at bay and other benefits. A higher ceiling and benches can also sometimes reduce the importance of other elements such as ceiling shape. And this is regardless of heater design or power, no matter the heater, higher benches and ceiling will result in a better sauna experience.

“Think of Harley Davidson cruising with a 5 horsepower engine.”
– Sauna Sherpa Kimmo Raitio on saunas with too low of benches and poor ventilation

The Sitting Bench (the highest bench where people sit) should usually be 100-120cm (40-48”) below the ceiling. Ideally 120cm (48”) as this is in the best heat, comfortable, reduces the likelihood of being hit in the face by the convective loop, and allows using a vihta. If you are forced to have a shorter ceiling (below about 250cm or 8’4″) then reducing this to as little as 112cm (44”) or even 100cm (40”) can be a good option. You generally don’t want the sitting bench to be more than 120cm (48”) below the ceiling though there are three exceptions; 1) an exceptionally tall group of people will be using it or 2) in a larger sauna where the convective loop will have traveled a greater distance and spread out more where some extra height keeps bathers from being slapped in the face by the heat and 3) if the ceiling is about 300cm (10’) or higher.

The Foot Bench or Foot Platform is then 40-45cm (16-18”) below the sitting bench. Below that then might be a stepping bench, steps, a walking or access platform or some combination of these.

Following are five methods for determining the appropriate heights (if this seems confusing, here’s a hint – they all come up with about the same result in most cases):

 

Bench and Ceiling Heights Method 1: Traditional Finnish Method – Traditionally, sauna builders in Finland have subscribed to ‘Feet Above The Stones’, known as the First Law of Löyly or Pälsi’s Law. They always try to get the foot bench ABOVE the top of the stones. And for very good reasons as this is where the better heat and less stratification is.

This was originally noted by Sakari Pälsi in his 1961 book ’Sauna’, has been the standard for a few decades, and has been included in Finnish RT Specifications (guidance for building) for some decades. 

Start by setting your foot bench above the stones, about 10cm (4”) above is good, more is better (and at least 85cm (34”) above the floor to avoid the cold zone so whichever is higher). Your sitting bench is then 40-45cm (16-18”) above that and your ceiling about 112 – 120cm (44-48”) above the sitting bench and you’re good to go. This applies equally to wood, electric and gas heated saunas.

 

Bench and Ceiling Heights Method 2: Top Down Shortcut – Start with the ceiling at 250-275cm (8.4 – 9′). This will get bathers above the cold zone. Set the sitting bench 112 – 120cm (44-48”) below that. Set the foot bench 40-45cm (16-18”) below that. Going down from there you can do another bench level or two, a platform, or a combination. So long as your foot bench is above the stones by a bit you’re pretty good. This is a basic starting point that many Finnish builders use. They might then raise the ceiling and benches if a larger heat cavity is desired, owner desires an taller open mesh pillar style heater or just to improve the sauna experience. 

 

Bench and Ceiling Heights Method 3: 3 Benches – Some Finns will say to simply make sure to have 3 levels of benches; 45cm (18”) Step + 90cm (36”) Foot + 132-135cm (52-54”) Sitting. Then the ceiling 112 – 120cm (44-48”) above the sitting bench.

Note that in many saunas in Finland and elsewhere the ‘step’ level is actually an air permeable platform covering the entire room. These saunas will appear to have about a 7’ ceiling (and it is 7’ from the platform) but in reality there’s another 18-40” or more of space below the platform and this space is critical as it is a key bit of the cold zone. 

 

Bench and Ceiling Heights Method 4: Mesh Shortcut – If you have a taller open sided mesh style heater such as an Iki or Helo Himalaya then a good target for the foot bench or foot platform is about 90cm (35”) plus half the distance to the top of the stones. So if you have a 48” high heater then the foot bench should be 35” + 7” = 42”. Higher is better, lower not so much.

There is however a bit more margin with these heaters. You can have lower benches, but at a cost. Bathers will experience more head to toe stratification but the difference for every inch you go down isn’t as great as with closed sided heaters so long as the benches have sufficient air gaps. Also keep in mind that you still want a sufficiently large heat cavity above the door so lowering your benches and ceiling also reduces that. 

 

Bench and Ceiling Heights Method 5: A Bit Of Physics – The advent of open sided mesh heaters generally and open sided mesh pillars in particular has changed things a slight bit. ‘Feet Above The Stones’ still applies and is still the best/simplest rule to follow but with mesh heaters we have more options if we need them.

First, the higher the space, the higher you are in the space, the greater the cold zone below you and the greater the heat cavity above the door – the less of a head to toe difference you’ll experience, the less heat you’ll loose when the door opens, the more enjoyable your sauna will be. Higher is better. This is the case regardless of heat source or style of heater.

(There are points of diminishing returns though. Getting the foot bench above the stones and above the cold zone makes the greatest difference and should always be done unless really impossible. From here (usually about a 255cm or 100” ceiling) to about a 400cm (13’) ceiling provides benefit with each cm (inch). Above 400cm the benefits become more marginal and largely unnoticeable.)

Let’s look at why.

No matter how high the ceiling is, the temps near the top and bottom will be about the same given other conditions (fresh supply air, heater, changing room, outside temp, etc.). If the temps are 100°c top and 30°c bottom in a 7’ high sauna then they will be the same in a similar 9’ or 10’ high sauna. Raising a 7′ ceiling by 16” to 8’4” allows bathers to also be 16” higher and have less of a head to toe temperature differential – bathers will be more comfortable and are less likely to have cold feet.

Your feet are 19°f warmer and so you’ve 19°f less head to toe difference in a sauna with an 8’ ceiling than one with a 7’ ceiling. That simple 1’ of extra height makes a huge difference in comfort and enjoyment. The more even head to toe temps are, the less difference there is, the more comfortable and enjoyable a sauna is. 

(Heater cooling with fresh air supply below the heater that pulls colder air upwards will make stratification and cold feet worse. High to low ventilation with fresh air supply above the heater and that pulls warmer air down around our feet will lessen stratification a bit resulting in a more pleasant and comfortable experience. Do not count on ventilation or air movement to overcome too low of benches though – Proper ventilation can help but not overcome stratification. On the other hand, low to high ventilation can significantly increase stratification and wipe out the benefits of higher benches.)

Here’s an IR image of our sauna showing how quickly the air cools as you go lower. How cold do you want your feet?  (Also note that where the studs are is much cooler than between the studs. This is due to the studs acting as thermal bridges – sucking heat out. This sauna has 2” of continuous insulation on the exterior and would be much worse without that).

So, our first priority is for the foot bench and so our body and feet to be above the cold zone, so above the approximate bottom third (or more) of the space. A good starting point for foot bench to ceiling is 165cm (66”) which is comfortable for someone sitting and allows room for using a vihta. So taking half of this (1/3 of the overall height) is 83cm (34”). Setting our foot bench at least 83cm (34”) high then is our desired minimum. 10-40cm (4-16”) higher is better yet if space allows or even higher if we desire a larger heat cavity. 

The sitting bench is then 16-18” above the foot bench. A ceiling at 44-48” above the sitting bench provides more comfortable room for using a vihta and as Risto Elomaa says “this is important”.

If you want a greater sitting bench to ceiling or foot bench/platform to ceiling then a taller room is your friend. The higher the ceiling the less important these distances are because there is less stratification and you are higher above the cold zone. So if you want a 50-52” sitting bench to ceiling for taller people or you want people to be able to walk upright on the foot bench/platform then just make sure that you keep the foot bench/platform above the cold zone and ideally well above it.

If inside and this is higher than your maximum ceiling height then you can play with things to get the best outcome. Select a heater that can be lower perhaps, tighten up the bench to bench and bench to ceiling heights a little. However, try not to have bench heights below about 15” (38cm) or bench to ceiling less than 42-44” (100-110cm).

Our second priority, in addition to the above, is to also have feet above the stones. If you have an enclosed sided heater this is quite critical for a good sauna. These heaters result in a quite well defined löyly cavity due to the convective loop. Placing the foot bench at least 10cm / 4” above the top of the stones is a good minimum to aim for and we want to avoid going much lower as temperature stratification can get quite significant.

Open sided mesh style heaters result in a somewhat less defined löyly cavity and so ‘Feet Above The Stones’ is somewhat less important. But there’s a good and bad to this. Good because it’s not quite as critical to be above the top of the stones. Bad because there is a bit more stratification in the löyly cavity than with enclosed sided heaters. So the first thing to understand with mesh heaters is that to get a sauna experience equivalent to an enclosed sided heater you actually need a higher ceiling and benches (the lessor stratification of the higher ceiling helps to make up for the greater stratification of the less well defined löyly cavity). 

Feet above the stones also increases heat felt evenly around your body from löyly and decreases direct radiant uneven heat from the heater. The former is more comfortable and less harsh than the latter. This is particularly important for bathers closer to the stove.

You still don’t want to be too far below the top of the stones or anymore below than your ceiling height will allow. In other words, higher benches and ceiling are still better. 

 

A low 7’ Ceiling Does Not Save Energy – The primary sources of energy loss in a sauna are opening the door, overheating to avoid cold feet, poor insulation and poor air sealing. A higher ceiling is NOT a significant consumer of energy and often results in an decrease in wasted energy.

In a typical room where we sit near the floor and want to remain comfortable at body level then a higher ceiling requires considerable extra heat above our body to remain a comfortable temp at our body that is in the lower and cooler part of the room. A 7’ high ceiling needs to be about 70°f at the ceiling for our body to be 68°f but a 9’ ceiling will need to be much warmer, about 74°f, at the ceiling for that same 68°f temp where we are. That extra 4°f of heat will use more energy and cost more.

A sauna is quite different. Since we are keeping our body in the upper part of the space then any additional volume is added/subtracted lower down. Regardless of how high the ceiling is we still want the ceiling to be the same 210°f and we still have the same löyly cavity.  Example: An 8’6” sauna costs $1.08 to heat initially and then $0.42/hr to maintain that temp (8’x8’, R-20 insulation, 30°f outside temp) so $1.92 for warm-up and 2 hrs of use.  Lowering the ceiling to 7’ costs $1.02 to heat plus $0.40/hr for $1.82 or 10 cents less per session. And though much higher temps than our home, we are only heating a sauna for a portion of each day/month/year rather than constantly so energy use/cost will often not be an issue.

But there’s a more important bit. A higher ceiling gives us a larger heat cavity and less heat lost each time the door is opened. Depending on how often the door is opened and for how long this can and in most cases will make the higher ceiling with the larger heat cavity less expensive to heat than a lower ceiling. This is a key reason that commercial saunas often have the foot bench or walking platform above the top of the door – because it costs them less to heat (and keeps bathers more comfortable).

And one more. Some people who’ve installed proper ventilation in electric saunas with a supply above the heater and a mechanical powered exhaust below the foot bench discover that turning the exhaust blower up pulls more warm air down resulting in warmer feet and a more comfortable experience overall. This is good, but they are often ventilating at a much higher rate than necessary for ventilation itself and so are wasting heat that would not be wasted if their ceiling and foot bench were at proper heights allowing them to have the same comfort with less wasted heat.

Reducing Energy Use – For the most energy efficient sauna:

• Insulate Well
• Reduce Thermal Bridging
• Have a Large Heat Cavity:
• Low Door Height + Higher Ceiling
• Use a Sauna Curtain
• Have a vestibule/changing area as an air lock.

To preserve heat as well as possible, older saunas in Finland often had doors that were about 1m (40”) high by 1m (40”) wide. They wanted the top of the door to be below the stones, and bathers and so the foot bench to be above the stones and above the top of the door. This is still done today – saunas with a lot of people coming/going, particularly public saunas, will have the foot bench above the top of the door.

A sauna curtain hung in the doorway can be a good tool for preserving energy and steam. Be careful that it’s not too close to the heater though!  Lassi Liikkanen has noted that they also work well for families with children who might want to open and close the door frequently.

 

Going Down To Go Up – If building codes limit your height, such as the 2.5m height limit in parts of the UK, then digging down to get the floor of your sauna lower may be a good solution. Ideally you want the entry door to also be lower to provide good heat cavity so one good option is to have a sunken patio or grotto outside of the sauna building. A sunken patio can also aid privacy (and personally I think these look very cool). If you do this, make sure to have good drainage.

Some builders in Finland will dig a pit in basements in order to fit a sauna in with the proper height. 

 

If A Low Ceiling Can’t Be Avoided… Maybe get a Saunum. The Saunum is a newer product designed specifically to help alleviate problems of stratification and cold feet in saunas that cannot have a proper 8.5’ or higher ceiling. It is designed based on the principles of a Convection Sauna built for people with disabilities. 

If you are handy and enjoy engineering you can possibly build something similar yourself. If you do, it may be worth experimenting with airflow in the opposite direction; intake at the bottom and outlet at the top. Also, intake on the bench wall with outlet on the stove wall above the stove.  And possibly include fresh ventilation air along with this. Unlike the Saunum that works against the convective loop that creates the löyly cavity, this would work with that loop.

Saunum Note: Saunum collected good data on both heat stratification and CO2 levels in saunas. On the latter noting that they often measured CO2 levels of greater than 2000 ppm which is in agreement with my earlier studies. They err in their solution to the CO2 problem though in stating that the Saunum increases O2 (Oxygen) levels for bathers. O2 and CO2 are not really connected. There is a massive amount of O2 in the air around us and in saunas, there is no need to increase O2 nor to re-distribute O2 in a sauna. The need is to remove CO2 – which the Saunum product does not do.

A higher ceiling and benches are still much better but if a low ceiling can’t be avoided then a Saunum (and proper downdraft ventilation) may help considerably.

 

“I Just Put My Feet Up” – Is a phrase heard often with saunas in North America. To avoid the cold feet problem common in U.S. saunas people often put their feet up on the sitting bench. And this is for the most part totally fine (and I personally like to sometimes sit like this even in a proper sauna). Some drawbacks are that this increases dirt and bacteria on the sitting bench, it’s not so good for blood circulation, it’s not comfortable for very long and it doesn’t work unless there are fewer people and more bench space. If the sauna is designed properly however, with proper high benches and ceiling, it’s not necessary and everyone can sit up straight like modern humans.

In barrels this is only marginally effective since the stratification from bathers heads to the sitting bench where they are putting their feet is often still greater than the 30°c maximum that’s minimally acceptable (though some builders say that 25°c or even 20°c is the maximum acceptable) for head to toes.

Bench to Bench Height – This is traditionally 45cm (18”) and that does work well. However, reducing this to perhaps 40-42cm (16-17”) has some advantages. It gets the foot bench up a bit higher which is always good but especially beneficial in a space that’s forced to have a ceiling shorter than 8.5’. Shorter is more comfortable for some people and is especially more comfortable when you want to lean forward with your elbows on your knees. It makes for easier step ups for people. Ours are 18” but we have some 2” and 3” risers that people can pull out which is also an option though these could also be a trip hazard.

Bench Widths – In Europe the recommendation is 60cm (24”) of sitting bench AND foot bench per person. 26-30” may be more comfortable for some Americans. If a L shaped bench then a bit more so that the people in the inside corner don’t have their legs on top of each other. A good rule of thumb is to measure seating capacity along the front edge of the foot bench.

A Bench to Lay On – If possible at least one sitting bench should be 76-80” or more so that someone can comfortably lay down.

Bench Depth – 24-28” is the recommended minimum as this is both comfortable for sitting and wide enough (24” is kind of tight) to lay down on. Lassi recommends as deep at 40” for the top sitting bench which is ideal if  space allows. This places you out away from the wall so that your back is heated by löyly and not cooled by the wall.

Bench Slat Dimensions – My preference is roughly 1×4 nominal (7/8 x 3 1/4) or actual. These are thick enough to be structurally sound and the 3.25” width provides a comfortable seating surface with good air gaps.  However, numerous other dimensions can work. Be careful of too thick though as that can result in taller gaps and too little airflow. Too wide can result in too few gaps and too little airflow. Whatever you choose, make sure that the top edges are rounded over.

Bench and Platform Air Gaps – Good air circulation throughout the stove room space is critical. A minimum of about 1/6 of bench and platform surfaces should be evenly air permeable so a maximum of about a 5:1 wood to air gap. More air gap like 4:1 or 3:1 is better. Gaps should be wide enough for good air passage/circulation (minimum 3/8”) but not so wide (7/8” maximum?) as to be uncomfortable to sit or walk on. 

Too little air permeability interrupts the convective loop that forms the löyly cavity and causes it to shelve or bottom out at the sitting bench (image below) rather than reaching down to bathers legs and feet. This then results in less even heat on bathers bodies, cold feet and potential for increased mold and bacteria growth. 

Sufficient gaps are needed also for the wood slats to dry. It’s easier for sweat to penetrate in to tiny gaps than for air to flow through enough to dry the wood.

A larger gap of about 1 – 2” at the wall (or a gap between the bench itself and the wall) that allows better airflow behind bathers can help reduce stratification below the bench and so reduce cold feet and will also keep bathers backs warmer (though I think perhaps shouldn’t be so large that all of the air flows there and none to bathers front).

Avoid Tall Gaps – Ideally you want the gaps between slats to be wider than they are tall. If the gap is significantly taller than it is wide then it can result in static pressure reducing airflow. This isn’t as important as overall bench heights and other things so if your heart is set on benches with milled 2×4’s set on end or some other design resulting in tall gaps then it’s not a huge loss but otherwise something to keep in mind as it will affect the experience.

Removable Benches – Building benches so that the bench or a portion is easily removable may be appreciated when cleaning time comes. It may be easier to take them outside for a good scrubbing than to do it with them in place. This especially if you don’t have a drain and want to do a really good annual scrubbing. This is doubly important if you want to give them a light sanding as this will save you cleaning up sanding dust (which I can tell you that you will rarely get fully cleaned up in a sauna). Lighter weight removable panels that sit in a stronger structural frame are a good option. FWIW, mine are not removable and I’ve not found it a problem.

Bench Skirts – It is generally best to not have skirts. Many people like to tuck their feet under the bench, especially if someone is sitting there, and they impede airflow. If you include skirts then recessing them back several inches behind the front of the bench can be more comfortable for bathers. They should also be at least 30% permeable (3:1 wood to gap) for airflow.

Access To The Benches – Can be via steps or a platform can be built 17-18” below the foot bench with steps up to it from the changing room floor. If room allows, three benches can be beneficial and especially if the middle of these, that is both a sitting bench itself and a foot bench for the highest bench, can be a bit extra deep like 26-30” to provide more comfort for feet and rumps. It’s critical that ALL benches and platforms be openly air permeable with sufficient gaps to the space below as otherwise you’ll have a higher cold zone.

Foot Bench vs Platform – Many saunas in Scandinavia do not really have a ‘foot bench’. Rather you go up 2 or 3 steps to a platform that is 90cm (35”) or more above the floor and then there are sitting benches around the platform. These benches need not be fixed in place and can even be individual chairs or loungers. In others you might go up a step or two to a 45cm (18”) high platform and then there will be foot benches around the platform with sitting benches above them.

Below you step up to a central platform and then on either side (one side shown) is a second platform a bit above the stones around which are the sitting benches. There is no real foot bench. (Note the fresh air supply in the ceiling).

 

Note the fresh air supply in the ceiling above the heater, exhaust in the ceiling back left corner, and thermostat sensor on the back wall near the ceiling.

Chairs – It is also possible to do chairs or loungers rather than benches. Build a platform at 90cm (36”) or higher and then rather than have built-in sitting benches you can have wood chairs such as those from Saunasella. FWIW, I don’t necessarily recommend this as it requires considerable space per person but could be a good option for some.

 

Mezzanine / Loft Sauna – It is not uncommon in Finland and Sweden for the hot room to have two levels. A stove level and then steps or a ladder up to the sauna mezzanine level. The stove level may be just large enough to enter and access the steps but is often large enough for one or two people to bath standing up. Or, as in the second photo, comprises all ancillary space for changing, bathing and relaxing. The sauna level is an air permeable platform about 1-2m (40-80”) above the stove level with sitting benches on or around the platform.

This gets sauna bathers above the stones, above the cold zone and results in very little head to toe temperature difference thanks to the higher ceiling and benches so a ver enjoyable sauna experience. If the mezzanine is above the entry door such as these are then there is no critical need for a vestibule to act as an air lock and the overall height somewhat lessens how important some elements are such as ceiling shape and sitting bench to ceiling height.

Here’s an example of a typical family sauna in Finland.

And another in Soini, Finland. The mezzanine door is not used during sauna except for someone with disabilities. The heater is to the viewers right. And yes, the main door on the right is quite short which is also not unusual as it helps creates a larger heat cavity.

 

 

Large Heat Cavity Above The Door – Every time the door is opened some heat escapes and it’s the heat from the top of the door opening downward where this happens. Heat and steam that are above the door opening largely remain in the sauna (think of a cup of air inverted in a tub of water). In many saunas this is the greatest source of heat/energy loss.

The larger and higher the Heat Cavity the better.  Being larger means that a larger volume of heat is retained within the sauna and a lessor percent is lost with each door opening. Being higher results in the hottest air being retained and cooler lost. Also, the higher the ceiling is above the door opening the less air pressure there is at the top of the door opening and so the less heat pushed out.

A 7’ sauna with a 6’6” door will lose a good bit of 190°f air while protecting only about 6% of the heat, while a 9’ sauna with the same 6’6” door will lose 181°f air, and less of it, while protecting 26% of the hottest air. 

In practical terms bathers will feel less colder air directly, the overall sauna temp will decline less and the sauna will recover to proper temps sooner. Overall much less discomfort for bathers. This can help any sauna but the more people coming and going the more critical it is. So if you will have many guests then a 9’ or 10’ ceiling might be considered to allow for a larger heat cavity. Keep in mind that if you raise your ceiling for a larger heat cavity then you need to also raise your benches. The sitting bench should always be within 100-120cm (40-48”) of the ceiling and the foot bench no more than 18” below. Besides the larger heat cavity this will also reduce head to toe stratification so is overall not a bad thing.

A sauna curtain in the doorway is also an option for creating a larger heat cavity.

In saunas that will have many people coming and going, such as public saunas, a builder will try to have the foot bench and often the platform in the heat cavity above the top of the door (so a platform at about 6’6” or higher and ceiling at 12+’ vs changing room floor). This can also allow for a door directly to outside since bathers are fully in the heat cavity and so protected from cold air entering when the door is opened. 

Sauna Door – The wider and in particular the higher the door opening the more heat that escapes each time the door is opened. Height is most important as this determines the size of the heat cavity. A 6’-4” door opening can often work quite well. Shorter would be better but to preserve people’s heads it’s best here to go short enough so that it is noticeably different so perhaps 5’-6” or less. Doors of 4-5’ aren’t unusual in Saunas in Finland as they result in such a larger heat cavity. While a 22” width works well for a 6’-4” door, a shorter door needs a bit more width.

The door should seal relatively well (wood on wood is fine) so barn doors and other non swinging doors are often a poor option.

Door Opens Out and Is Unable To Be Latched – This is primarily a safety thing. Anyone in the sauna hot room needs to be able to exit quickly and easily at any time with a simple push on the door. There should never be anything that can accidentally block the door (such as the door to outside from the changing room) nor should there be any type of latch that can accidentally be latched or as a prank. Similarly a sliding door can be too difficult to open in an emergency.

Cognitive and motor abilities can decline quickly and suddenly with heat stress and this especially if CO2 levels are high from poor ventilation. This can make any effort beyond simple pushing difficult or impossible. 40-50% of sauna deaths (which are quite rare but still 40-60 per year in Finland) are alcohol related with falling asleep in sauna the number one cause. One person told me that being unable to exit is the cause of some deaths every year which baffles her because she said it’s difficult to find such saunas in Finland as they are all made with easy exit for safety. Someone could also die from CO2 poisoning in an unheated sauna with poor ventilation if they are accidentally locked in.

Door Window – It’s good to be able to see if anyone is on the other side before opening.

Wood – The use of wood is functional more than aesthetic. Wood is hygroscopic which helps to even out temp and humidity extremes to provide a more comfortable experience. Some woods such as Abachi, may remain cooler to the touch, and importantly, softer woods absorb some noise to make for a quieter and more peaceful environment. Ideally you want at least 80% of the wall surface area and at least perhaps 50% of each individual wall to be soft wood. As well, all of the ceiling and benches should be soft wood. However, if you’ve a really really great view then it may be worth it to sacrifice some wood surface for more window.

The functional aspects of wood are of increasing importance the higher in the space they are and particularly from just below the foot bench to the ceiling. So, if you’re building a sauna in an old building with great looking old brick that you want to leave exposed (I would) then doing this in the lower portion like the lower quarter of the sauna and keeping the upper 3/4’s all wood is the better option. And this is also generally better aesthetically as heavier materials (brick, stone, steel) look more natural below lighter (wood) materials than they do above them.

Almost any wood can be used though some are somewhat better than others. Softer woods are better than harder woods so you want the majority to be softer. Mixing for aesthetics works well though keep in mind that different woods expand/contract at different rates so always allow for that. Avoid using woods that get hot in places with a lot of skin contact.

Spruce, Fir, Alder and Aspen are the most popular in Finland, Sweden and elsewhere. True Nordic White Spruce such as that from Koskisen is the primary wood choice in Finland. North American Spruce is likely a close second. Lassi used Monterey Pine (Pinus Radiata) for the ceiling of his new sauna which is certainly a good recommendation for that.

Be careful of woods with too much sap or knots, especially for benches, platforms and backrests, as a knot with sap or pitch can get hotter to touch than surrounding wood, knots can pop out with hot/cold cycles and sap dripping from the ceiling isn’t too desirable. Clear select grade pine can work well so don’t totally rule pine out and knots without sap are often not a problem. Be cautious of differences in woods based on geography or common names. 

Abachi, Alder and Linden/Basswood are good options for benches as they remain a bit cooler than some other woods. 

A good discussion on wood: Why Nordic White Spruce. 

Cedar – Popular in the U.S. but you’ll not see it used for saunas in Finland, Sweden or other European countries.  Cedar can be too fragrant for many people and for a few it’s a direct respiratory or skin irritant. Personally I like the smell of cedar but find that in a sauna it detracts from the sauna experience. Cedar can also conflict with other oils/fragrances for those who want them. With cedar that’s the smell you’ll always have. With low fragrant woods you can freely use whatever you want (Eucalyptus is overwhelmingly the most popular).

Contrary to popular belief, mold and bacteria do grow in cedar. The only way that I know of to control/kill mold and bacteria is heat – 65°c for 20 minutes.

Toping the list of negatives is potential toxicity. The primary concern is likely the chemical Thujone which is a neurotoxin and possibly a carcinogen. A secondary concern is the substance Cadinene. When you walk in to a cedar closet or cedar sauna and ‘smell the cedar’ you are breathing in Thujone and Cadinene. The big question is one of dose. Are you breathing in enough over time to cause health problems? In my very limited research the answer for us and our sauna is more than likely. Spruce was a much better and safer choice for us.

Cedar wood and cedar oil are used for storage closets and chests because they are an insecticide and repel, likely due to the aerosol emission of Thujone, many animals including insects. While cedar dust is one of the more toxic in woodworking (and one that comes with numerous warnings to always wear a good respirator), I do not know how this plays out as an aerosol in a hot sauna. There is speculation/evidence that cedar, yew or hemlock in a sauna may contribute to eczema flareups and other skin irritations.  

Neurotoxins and carcinogens like these are often very slow acting and build up in our bodies slowly over time – dose. It can take years or decades for the harmful effects to become apparent as cancer, respiratory disease or other ailments. So be cautious of anyone who says that they’ve used a cedar sauna for years with no ill effects. They do not and cannot know.

Perhaps the primary reason that you don’t see cedar used in Finland is that it doesn’t grow there. However, some Finns and Swedes have commented ‘why would you want to use something with such strong smell in a sauna?’ 

Note that some ‘cedar’ products such as fence posts and pickets are actually made from Juniper which may be more toxic to humans than cedar.

Finally, sauna is a social activity. While you may prefer cedar for yourself, your guests may not like the smell or may be allergic to it.

Personally I would avoid cedar. For me the negatives more than outweigh any benefits. If you want to use it I would suggest doing some more research first. It may be a totally OK option for you, or not.

Board Size – 1” x 4” nominal (0.75” x 3.5” actual) is generally considered a good size for interior cladding. Wider, such as 1” x 6”, or thinner can usually work OK in warmer climates or indoors but can cup in colder climates. This can be quite climate dependent so talking to a local woodworker who knows how various woods respond in your climate might be worthwhile.

Groove Down – With T&G walls the tongue should face up and the groove down to avoid moisture, water or sweat collecting in the grooves. Similarly shiplap should always be oriented so that the top board overlaps the board below it on the interior to help prevent moisture from creeping back behind. T&G is a better option as the interlocking helps to keep everything straight. 

Horizontal vs Vertical Interior Cladding – Vertical can result in moisture creeping around the boards and getting trapped behind. With a proper air gap this is not likely to be a huge problem (vertical is somewhat popular in Finland and this doesn’t seem to be a problem) but it can be. Horizontal T&G or Shiplap done properly should avoid this though.

It Gets Hot – Be careful of any metals, woods or other materials that get hot easily as getting burned on these can be less than pleasant.

Avoid plastic, vinyl, PVC, treated lumber, spray foam insulation or similar materials – When heated to sauna temps they can give off noxious gasses and worse these are often odorless so you do not realize you are breathing anything noxious. They may be OK in limited quantity outside of the vapor barrier (and insulation). The goal of sauna is to be enjoyable, not to get cancer.

Be careful of Glues and Binders. These can produce unhealthy and unappealing fumes when heated. Glues and products such as OSB or Plywood should be avoided or minimized (though OSB sheathing that is on the outside of the vapor barrier and insulation is fine). 

Floor – The ‘floor’ of the sauna hot room can be bare cement. There is no need for anything decorative as it will either have platforms built above it or wood slat duckboards on it.

Floor Drain – A floor drain is highly recommended and some say is critical. My personal opinion is that it is definitely good to have but not extremely critical. The presence or absence will not affect the normal sauna experience in any way. Having a drain makes cleaning easier since you can just hose everything down. If you want to be able to wash in the sauna, throw a LOT of water on the walls and benches or have a cute Finnish singer pour a bucket of cold/ice water over your head (if you’ve never tried this, you should) then a drain is more critical.

It may be helpful to see if your drain can be run to a nearby drain pre-trap. A trap filled with water is necessary to prevent sewer gases, primarily methane, from entering. If a drain is not used enough then the water in the trap can evaporate and allow sewer gases to pass. Running to a nearby drain that is used more often helps to eliminate this. Being able to do this depends on local building codes and some practical bits of whether it will work properly or not. Otherwise, make sure that your sauna drain gets enough water in it to keep the trap full. This may require occasionally pouring a bucket of water in it.

Trough Drain – A trough drain can work better and make construction easier than a round drain. The floor remains flat but slightly slopes towards the drain. Here’s one partially constructed (more details here).

Warm Dry Floor – Stepping on to a cold or cold and wet floor while still in a hot room doesn’t make for a pleasant experience. A wood slat floor (duckboards) makes for a more enjoyable end to each sauna round. They will also help to keep benches cleaner as debris on the bottom of feet are more likely to remain on the boards or fall through them vs a hard floor where debris are more likely to stay on feet to then be deposited on to the foot bench. Duckboards actually seemed quite rare in Europe however as the entry to almost every sauna hot room was a step, followed by another one or three steps to get up to a platform and the benches.

Hygiene requires higher temps – It’s critical that after a sauna has completed its duties for the day that heat is used to kill bacteria and mold. The sauna should be able to maintain a minimum of +65°c ( +149°f) at the foot and sitting benches for a period of 15-20 minutes after use and after excess moisture has been exhausted.

Bacteria thrive in temps between about 4°c and 70°c (40-160°f). Mold between -5°c and 40-55°c.  A sauna is an ideal breeding ground for both. Once bacteria or mold gets a start in a sauna the only options to get rid of it is heat or removal of the material. Chemicals, which you never want to use in a sauna anyway, have too large of particles to penetrate to the roots of mold or home base of bacteria. 

It’s also important to note that not all molds and bacteria are unhealthy and there’s actually a problem in the U.S. of people not being exposed to enough organisms resulting in poor immune systems, but that’s a much deeper topic for another day.

For more: About the Hygiene of Sauna

Thermostat – Normally it should be placed at a height equal to 1m above the upper sitting bench and at least 20cm away from the heater. This will provide an accurate temp for sauna bathers. In North America however, due to the 90°c (194°f) limit imposed by UL, many people place the thermostat much lower so that they can get higher temps as otherwise they have an American warm room instead of a sauna. This could violate UL and manufacturer guidelines though which may be problematic.

Another solution is to take a page from the heater manufacturers and use fresh cool supply air to cool the thermostat sensor just as they use it to cool the high limit sensor. 

UL Labs: Promoting Unhealthy Sauna Since 1977 – In the U.S. (and I believe parts of Canada) UL Labs states that the thermostat should be placed directly over the heater, 4-6” below the ceiling and that temps at the thermostat be limited to 90°c for electric heated saunas. This recommendation results in three problems;

1) Actual temps for bathers, at their heads and shoulders, are then only about 60-80°c which is well below the 85-105°c temps recommended by the Finnish and Int’l Sauna Societies.

2) Most critically this results in too low of temps at the foot bench for good hygiene. Higher temps are needed to kill bacteria and prevent mold growth and UL labs restrictions prevent this.

3) To get around this many people in areas under UL requirements place thermostat sensors lower in the sauna (taking advantage of heat stratification) so that the sauna can reach proper temps. Because the thermostat sensor is lower it does not see the wider temp swings higher up at bathers heads and it cools off more and faster when the door is opened. This results in the heater over-correcting so bathers experience much greater temperature swings than they should. So if users desired 96°c it may spike up to 109°c for a bit and then slowly come back down.

As well, UL may be requiring sauna heater manufacturers to include a high limit temperature switch on their heaters with a very low high temp limit (mine appears to be 350°f). This results in a couple of problems;

1) Heater manufacturers appear to be requiring a low fresh air supply vent in order to cool the HL probe in order to prevent trips. This low vent results in poor ventilation for bathers and so high levels of CO2 in U.S. saunas.

2) It is somewhat common for people in the U.S. to separate the HL sensor from the heater to prevent constant trips. If the HL probe and switch is needed from an engineering standpoint rather than just to meet a UL requirement then this could be problematic.

3) On hotter days it may be impossible to cool the HL sensor enough to avoid trips since the air used to cool it may be over 100°f.

UL’s requirements do not appear to align with those of other countries and like new wine in old wineskins may be doing more harm than good.

Huum heaters appear to not have these problems as they are apparently imported and sold under a different classification than sauna heaters in order to get around these issues.

 

  

Stones 

A good sauna is heated by the stones and the stones by the heater. The stones help to soften and even out temperatures over time so that there are not uncomfortable high/low swings, produce convective heat rather than radiant, and effect the quality of the steam that can be produced.   Nothing makes up for proper stones.

More Is Better. If possible a MINIMUM of 6kg / m³ of space is a good starting target though some sauna builders in Finland and Sweden I’ve talked with say 8kg / m³ is their minimum. More is better so 10-12kg / m³ is better up to about 16kg / m³ (or about 1/3 lb / cubic foot minimum to 1 lb / cf). Beyond that may be getting in to some diminishing returns so while perhaps still better, only marginally. More stones result in more even temps, more even softer steam and thus a more comfortable experience. More stones results in more negative ions which may make us feel better.

For perspective, a smoke sauna in Finland will have about 90kg of stones per m³.

These targets can be difficult to reach with some stoves from Harvia and others in which case we do the best we can. Maybe 4-5kg / m³ is the best we can do.

Bigger Is Not Better – Stones should be about 5-15cm (2-6”) in size. Surface area is important so too big of stones will result in too little surface area and too small of stones will disintegrate and not last long.

Rough and Irregular Is Good – Stones should ideally have a rough and irregular surface – a high surface area to mass ratio. This increases their ability to produce convective heat and helps hold tiny pools of water to make better steam. Smoother or rounder stones do not do this so well. If you want the look of rounder stones then maybe use them only for the top layer. 

Commercial Virgin Quarried Stones – Landscape stones or river rock may have organic matter (think cow dung) that is unhealthy and can produce unpleasant and unhealthy odors when heated and this can take years to burn out. There is also potential for stones to contain arsenic, asbestos, sulphur or other undesirable elements. Commercial sauna stones are relatively assured to be able to tolerate heat and to be free of undesirable compounds.

Stones from along lakeshores can be quite good if they are not under water constantly (if they have green stuff growing on them you don’t want them). Whatever you choose, wash them well with only water.

Olivine diabase are a volcanic stone that some Finns believe are the ideal option. Gabro and Peridotite are also good.

Warm New Stones Slowly – Stones may sometimes have pockets of water inside them. Three hours @ 50°c, 3 hrs @ 75°c and 3 hrs @ 100°c with a day or half of cooling down between without anyone in the sauna will allow the water to either dissipate or if the stone is going to explode do so without hurting anyone.

Deeply Stoned – You want at least 3-4 courses of stones on top of each other and on top of the heat source so about 40cm or 16” of depth minimum. 

Flat Up – Placing stones so that the larger flatter surface is up and very slightly tilted towards the center of the heater will result in the most steam.

Harvia has a good discussion on stones here.

 

 

Heaters

A good sauna is heated by the stones and the heater that heats the stones is critical. A sauna heater should:

1. Have a properly large and deep stone capacity.
2. Heat the stones.
3. Contribute to the Convective Loop.
4. Produce very little radiant heat to the front, back, sides or above.
5. Keep the heat source close to the floor and the stones low.

In a sauna you want to experience heat from convection not so much from radiation. Heat from radiation is uneven and directional, it only heats your body on the side it’s radiating from and is very uneven inch to inch and second to second. Heat from convection is more all encompassing, in a well designed sauna it will heat all sides of our bodies evenly which is more comfortable, can carry the steam to us via the convective loop, and is what a sauna is all about.

Note: Everything produces radiant heat. There is a lot of radiant heat in a sauna just as there is everywhere around us everywhere we go. We can’t escape the radiant heat that’s produced by the wood walls and even the person sitting next to us. But radiant is directional and uneven so we want to minimize it and keep it as even as possible.

You want as much of the heat produced as possible to rise up as convection airflow and contribute to the convective loop that forms the löyly cavity and as much of this as possible to do so via heating the stones. You want as little as possible to go to the heater carcass which will result in undesirable radiant heat. In other words, you want all of the sides of the heater to be as cool as possible.

This also helps with placement in the room as clearances to combustibles then is less and allows you to get the heater and the stones closer to the heater wall which results in a better convective loop and so a better löyly cavity, and is greater distance from bathers which further reduces radiant heat on them. This applies equally to wood or electric heated and hard sided vs open mesh sided.

The design of electric heaters is fairly straightforward as the calrod heating elements are in the middle of the stones. The outer stones stay cooler and help to reduce radiant heat and so most of the heat rises up heating the stones above and contributing to the convective loop.

Wood stoves are trickier because they have a big steel fire chamber in the middle. The traditional design for these is to have three layers on all sides; fire chamber walls + air channel + thin outer carcass. The air channels carry heat from the sides of the fire chamber upwards through the stones. This airflow is critical for the stones, helps the convective loops in the sauna, gets more of the heat produced by the fire to the stones, and reduces the amount of heat radiated to the sides.

While a wood stove to heat a room with radiant heat will be made from thick (1/8” – 1/4”) heavy steel, a low radiant sauna stove is intentionally made from much lighter and thinner materials like 1/8” for the firebox, 1/16” for the inner carcass and 1/32” for the outer carcass. The outer carcass being thin does not conduct heat as well as thicker heavier steel and so also helps to reduce radiant heat on bathers.

Note how the upper part of the firebox and the flue are used in all of these to transfer as much heat to the stones as possible to produce the greatest amount of convective heat.

Here is how this works in a heater from Helo. 

And from Harvia:

A Narvi NC-20 (by C Macqueron):

 

An alternative introduced by Iki in the late 1990’s is to replace the side air chambers with stones and have an open mesh outer carcass. This further reduces the amount of radiant heat,  increases the stone mass and increases the amount of heat contributing to the convective loop. Other manufacturers soon followed with similar designs.

The Löyly Cavity – A closed sided heater will generally have a more defined löyly cavity than an open mesh sided heater. For any given ceiling height and stone height then the close sided will likely provide a better löyly cavity with less stratification than the mesh will. The mesh is more forgiving if the foot bench must be below the top of the stones.

Safety Distance & Radiation – One way to get a very rough idea of how much radiant heat a stove will produce is looking at the safety distances to combustible material (not the safety distance to non-combustible). For example, an original IKI is 100mm (4”) to combustible (largely on all sides) so we know it will produce very little radiant heat. A Harvia Legend is 200mm-500mm (8-20”) so considerably more than the Iki. A Helo Karhu is 300mm (12”) on sides and 500mm (20” to the front) and the Harvia Pro 36 is a wapping 500mm (20”) on all sides (though it is intended for quite large saunas). Perhaps a good rule of thumb is that you want bathers (typically knees) at least 4x this distance from the stove and ideally 8x. 

Some wood burning stoves don’t insulate the front thinking that’s unnecessary but it is very necessary so something to consider.

To reduce radiant heat on bathers then you ideally want the front of the foot bench to be perhaps 5-8x the heater’s rated distance to combustibles. So if the side of the heater facing bathers is rated for 8” to combustibles then you want the foot bench to be 40-64” from that side of the heater.

Wood, Gas or Electric? First, there is no difference in the sauna experience between wood and electric (assuming the sauna has been built properly). A good sauna is heated by the stones and the stones …don’t really care how they are heated. Someone who is blindfolded will not be able to tell the difference in the two. The heat, steam and löyly are totally identical.

Wood is more traditional and more romantic. Wood fires are enjoyable to see, hear and smell (for many people). Even the routine of preparing the fire has benefits and for many of us is quite enjoyable, relaxing and a great way to prepare for a good day of sauna – it’s foreplay for sauna. A wood sauna causes you to slow down a bit and be intentional about your sauna. Drawbacks are that it is not as convenient, uses natural resources (trees) and is a direct source of air pollution. There is also the issue of impact on neighbors – the smoke from a wood stove is fine in a rural area but could be quite inconsiderate in a more urban or suburban area.

Electric is more convenient, especially with a phone app that allows you to begin preheating before you arrive home. Electric can maintain more even temps than wood. How the environmental impacts of wood vs electric compare is a much longer discussion and may depend on how the electricity is generated. From a cost standpoint, a well designed 8x8x8.5’ sauna will use about 8kWh to heat and then about 4kWh to maintain temp during use. So at $0.15/kWh will cost $1.20 to heat up and then about 0.60 per hour so for 2 hrs of use about $2.40.  

I think electric got a bad reputation early on because of too little stone mass and problems with ventilation. These were corrected by the late 1990’s with more stone mass and downdraft mechanical ventilation. Even so some people still hang on to the idea that electric is not as good.

One significant drawback to electric in the U.S. are the UL requirements and the problems they pose outlined earlier.

Gas is not as prevalent but can be a good option. It’s critical that a gas stove be well designed and well vented as you don’t want the byproducts of gas combustion in the sauna with you. Some local codes also will not allow remote app control so you don’t have the convenience of electric nor the romanticness of a wood fire.

With wood you have a choice of loading wood from inside the stove room or outside. With the key benefit of wood fires being aesthetic – seeing and hearing the fire, there is often not a good reason to have an exterior loading stove as it looses that benefit. There are two instances where exterior loading would seem to make sense though. One is an off-grid sauna but where the owner doesn’t want to carry wood in to the stove room. The second is if the loading is done from the vestibule/sitting/changing/shower area where the fire can be enjoyed by people sitting in there.

Hybrid? I built a hybrid gas/wood fireplace for our house. Natural gas is used primarily to get the wood going but is sometimes kept on if wood is greener than it should be. Similarly, it should be possible to create a gas/wood hybrid sauna stove that can act as a traditional wood stove (with a convenient gas starter) or have the convenience of a gas stove when desired. 

One final point. There has been speculation that a wood stove produces beneficial negative ions while an electric either does not or produces positive ions. There has been no study that I’m aware of to support this nor is there a good foundation for it to be true. The steam that’s created when we throw water on the stones will produce a significant amount of negative ions and this regardless of how the stones are heated. 

Bigger Is Not Necessarily Better – Too large of a heater can have a number of drawbacks.

1. It Will Short Cycle. Electric heaters work by constantly turning on and off. If you’ve set it for 100°c then it will turn on until the sauna reaches perhaps 103°c and then turn off until it has cooled to maybe 97°c when it will turn on again. With a properly sized heater (and lots of stone mass) these cycles are largely unnoticeable but if the heater has too much power for the space then the heating up can happen quite quickly and be noticeable followed by a long period of the elements being off.
1. A secondary problem of short cycling is that the elements are not on long enough to keep the stones sufficiently hot to produce good steam.
2. Larger Temp Swings. Rather than 97°c to 103°c, a too large electric heater can result in perhaps 95 to 110 or 115°c. 
3. More Direct Radiant Heat. In a sauna we want convective heat, not direct radiant heat. The larger the heater (electric or wood) the more direct radiant heat it produces. This isn’t so much of a problem if the heater is sized properly so that larger heaters have larger spaces. 
4. The steam can produce a harsh feeling löyly
5. Evenly maintained lower temps allow for a greater range of löyly from dryer to wetter steam. Too large of stoves cannot maintain lower temps.
6. If interior height is tight then a stove on the smaller side may get the stones down lower in relation to the foot bench providing a more comfortable experience while too large of a stove will have the stones up higher so bathers will have colder feet and actually be less comfortable with a larger stove than smaller.
7. Fire Hazard. Too large of a heater can be a potential fire hazard. 

Finnish RT Guidelines state 1.0kW per m³ (per 35 cubic ft) for saunas below 10 m³ (350 cubic ft) and slightly less for larger spaces. This assumes that walls and ceilings are insulated, that at least 90% of the wall surface and 100% of the ceiling is soft wood and a proper mass of stones in the heater. For windows or other uninsulated wall areas add 1-1.5kW per m² (per 11 sq ft) of window surface. For stone or similar wall surfaces add 0.7kW per m² (8 sq ft). 

Now, to maybe contradict myself. Stone mass is critical so sometimes it’s necessary to upsize a heater purely for stone mass.

And in general stay within the recommendations from manufacturers. 

Convenience of Electric and Romance of Wood? – It is possible to have both an electric and wood heater in the same sauna. It requires a bit more space but is doable. The wood stove here is a bit oversized for this sauna as a demo but fun to look at.

Helo-WT and Similar ‘Technology’ – Many heaters like the Helo Himalaya include a water tank in the middle of the stones. The idea is that you fill it with water and it then produces constant mild steam and humidity throughout your sauna session. Some people do like this and for those its fine. It’s perhaps not really sauna though. On most you can remove the tank, put more stones in the heater (this is a very good thing) and the heater and sauna will function like a traditional Finnish sauna. With the tank you get constant mild humidity and because of the lessor stone mass you get a lessor steam when you throw water on the stones. Without the tank you get a more traditional (or proper according to many Finns) sauna that is naturally rather dry and then a stronger burst of steam.

Manufacturers – Some very brief thoughts.

• Narvi – Generally considered the best among Finns for design, function and build quality. These have a lot of stone mass and very little radiant heat which both contribute to good löyly.
• Iki – Introduced the open sided mesh concept in the 1990’s and are highly regarded in Finland. 
• Helo – Good build and functional quality. A step below Narvi perhaps but above Harvia. This is unfortunately also a confusing array of branding including Sauna360, Tylö, Kastor, Finnleo and Amerec.
• Harvia – Harvia is a good value brand. Lower quality for both build and function but also lower cost. Some of them have a bad reputation for too little stone mass and producing too much radiant heat so that’s something to watch carefully for with these.
• Aino – Some of the more interesting heaters from an aesthetic standpoint. These can work well but hot rooms must be planned carefully to achieve good results.

 

• Lamppa Kuuma – These appear to be excellent build quality from a company with a reputation for quality products but are not really sauna heaters. These appear to produce more radiant heat than is desired in a sauna and not as much convective heat. These would likely make a great heater for a sweat lodge/cabin. If they introduce the Kuuma Air I’ll likely be a hearty supporter.
• Nippa, Royale, Grill’D, Ilo, Other – I don’t know about the build quality of these. Similar to Kuuma these do not appear designed to function well as sauna stoves. Grill’D for instance is 500mm (20”) to combustibles on all sides which is a lot. 4-6” is good with as much as 12” perhaps acceptable for home saunas.
• Huum – Nobody should support this company for any of their products. They have had some significant quality and ethics problems including;
• Heating elements that last less than a year when other mfr’s last 20 to 50.
• Not covering the elements under warranty.
• Putting their customers at significant financial risk by skirting UL regulations which, besides being unethical, could result in consumers not being covered by insurance in case of a fire as using one of these in a sauna may go against UL and National Electrical Code regulations.
• Heating element failure potentially increases fire risk. There has also been one report of a Huum heater catching on fire, including video of the fire.
• There have been increasing reports of the stones they supply (and I believe require to have any warranty) breaking down very quickly (and they are round which may not be a good idea).

 

Big Iron – There are reasons that you don’t see heavy iron or steel sauna stoves for sale in Finland, Sweden and elsewhere. A lot of the heat produced by the fire goes in to the steel instead of the stones and the steel then radiates that heat to bathers. That works well when we’re trying to heat a normal room where radiant is desirable but is the opposite of what we want in a sauna.

In a sauna you want the heated mass to be the stones, not the stove itself. The stones have a very high surface to mass ratio that works well for creating convective heat and good steam, a steel stove carcass has a very low surface to mass ratio so does a poor job of both.

A well designed heavy steel stove with air channels in the sides that reduce radiated heat and carry that heat up to the stones can work OK in larger saunas of perhaps 300x300x300cm (12’x12’x12’) but not so well in smaller saunas where the stove is closer to bathers. So while it’s tempting to weld a stone basket to an old steel wood stove to use in a sauna, there’s a bit more to it. 

Converting a Big Iron to a Sauna Stove – Perhaps A good way to understand the differences in a big iron stove and a sauna stove might be to look at how we’d change the design. Let’s start with a Kuuma. 

Of 24 kW of heat the fire produces, the stove produces in to the sauna perhaps 2 kW convection / 17 kW radiant / 1 kW conductive*. Ideally we want 24/0/0 but that’s impossible so maybe 18/3/1 (with 2kW still going up the flue) would be a good realistic target.

1. It has some stones but little airflow so we’ll add airflow vents, about 1” high, along the entire bottom of the sides of the stone basket. Now we’re at perhaps 4 kW / 15 kW / 1kW
2. It has quite minimal stone depth so let’s add about 12” of height to the stone basket. So now we’re at maybe 8/11/1.
3. Let’s route the flue through the stones to extract a bit more of that heat to become convective. Now we’re at 10/11/1.
4. If we add the std Kuuma heat shields we get to maybe 12/9/1. We know the Kuuma is still producing a lot of radiant because its clearance to combustibles with a heat shield is 15” on the sides and 48″ in the unshielded front, a bit more than the 4-11” of Narvi, Helo, and Harvia’s various wood stoves. Our stove is now beginning to generate enough convective heat to start forming a little bit of the löyly cavity that is critical to a sauna.
5. Heat shields are actually not what we want though. We want to convert heat energy to convective heat, not just try to shield it. So we need to think differently. Let’s make the heat shield thinner, remove the insulation on the inner side and make sure we have good air flow. This will allow the air moving up in the channel between the firebox and heat shield to take on more of the heat and carry it up as convective heat. Yes, it will also now be taking on more radiant from the firebox but this s/b less than is being converted to convection so we’re good. So now we’ve got perhaps 13/8/1.
6. Instead of the std heat shields let’s do a permanent shield fully surrounding it and make it with a lighter material that won’t transmit as much heat from the firebox out to the surface of the outer carcass. This can be done in a way that preserves the big iron Kuuma aesthetic. So now we’re perhaps 17/4/1 and have a pretty good convective loop and löyly cavity.
7. Making the stone basket out of lighter material will reduce radiant here a bit. So maybe 17.5/3.5/1
8. The top of the stones are really high (44″) which will force the löyly cavity it’s now producing up quite high and thus require the benches to be much higher than normal (≈52”) so let’s take a bunch of steel off the bottom to lower everything down and get a side benefit of a bit less steel mass to give us our 18/3/1.

* This is NOT calculated scientific but should give a general idea of the differences in a heavy steel stove vs a sauna stove and why sauna stoves are made the way that they are.

Important Note: I believe that Garrett and the other folks that own and run Kuuma are a good bunch and do good work. I think they, like largely everyone in North America, have simply been misled about what sauna is and how a sauna works. And this is not anyone’s fault but simply a consequence of language and distance.

Perhaps they can call the original Kuuma the Kuuma SC (Sweat Cabin) and this new one then can be the Kuuma Air 🙂

 

Consideration For Others – If you’re debating between wood and electric heat, give some consideration to your neighbors. While wood is enjoyable and romantic, it does not otherwise make for a better sauna. An electric sauna has every bit as good of heat, löyly and overall environment along with some great convenience benefits. Many Finns actually prefer electric as the heat is more even. If you live close to others please give some consideration to how smoke, even if legal for you to do, might negatively effect them. This is in response to someone’s concern that neighbors might complain:

Just because you have a right to do something doesn’t mean that you should and personally I think that attitudes like this are at the core of many of the conflicts we’re seeing in the communities around us and this certainly doesn’t help the reputation of sauna people.

 

 

Ventilation

“Löyly is the Purity, Temperature and Moisture Content of the air contained inside the sauna as well as its thermal radiation.”
– 1988 Finnish paper on sauna health benefits

“Steam added to bad stale air is just that, steam added to bad stale air, not löyly..”
– 2020 Trumpkin’s Sauna Notes

Ventilation in a sauna is critical and discussed in: Proper Ventilation For Saunas.

DO NOT skip proper ventilation.

 

Other Thoughts

Location – Sauna, More Than Just The Hot Room – Going to sauna is not just sitting in the hot room and sweating. That’s actually a rather minor part of the overall experience. Sauna is hot/cold/hot/cold/hot/cold. Cooling off and relaxing is just as important an element of each round as sweating. Ideally the sauna space, be it a dedicated building or space within a house or other building, should be a space that you enjoy and want to spend time. 

A sauna should be located where bathers can quickly and easily cool off each round. Ideally with a shower in an adjacent changing room and a convenient door to outside. Being able to go outside each round if possible is quite important.

Social aspects and being able to invite friends to join in is also important so a sauna located somewhere other than a master bathroom is good.

A separate sauna building with proper sitting, changing and shower facilities is best. Just as going to a cabin allows us to escape being at home and the distractions that come with that, a separate sauna building does the same. This even if the sauna building …is at our cabin. A separate building allows us to have a sauna-frame-of-mind and so relax and enjoy it more. We’re away from any distractions and stress in our home or cabin.

A sauna in a bathroom is totally fine and can work well for many people but can sometimes be a long trek to outside and perhaps not the best social space. A sauna with an adjacent changing room and shower in a lower level walkout might be better. Below is one way that a Finnish builder will include a sauna (red square) such that it works well for the family and for guests.

 

A Shower (or two) Is Important – A shower directly adjacent and without having to go outside sweaty in cold weather is almost critical. It’s important to shower with soap (and dry off) before first entering the hot room and a cool shower is a good way to cool down after each round. Rinsing sweat off before going outside in cold weather is not really a luxury, it’s basic (unless you plan to jump through a hole in the ice Avanto! or roll in the snow!). The more convenient the shower the more likely it is to be used and the more pleasant an experience.

A thermostatic temp controlled shower is nice to have. It’s much more pleasant to set the temp to 68°f and know that’s what it will consistently be rather than go through the starting kind of warm then getting really cold then slowly warming back up then too warm so you now have to turn it down routine (that we have to go through since we decided to save some money in the wrong place).

We have two for our sauna; one inside (below) and one outside. In hindsight I wish we had two inside. In non-winter environs something as simple as a hose (or pre-made hose shower) on an outside wall can work.

Lounge/Changing Room/Shower/Vestibule – Besides changing and showering this space provides a critical air-lock function to lessen cold chilling air from blowing in to the sauna. It also provides a safe way to lock the building without the risk of locking someone in the hot room.

Finns and Swedes say that this is a critical element of the overall sauna routine so sauna designers in Scandinavia shoot for this to be at least 1.5x the size of the hot room and often larger. So an 8’x8’ hot room for 4 people will have at least an 8’x12’ changing/shower/vestibule.

A Large Changing Room – The changing room can be a great place to relax, enjoy a Finnish Long Drink, read a book or take a nap. Making this a larger area is never a bad idea. Or maybe even have a central gathering room with the changing/shower off one way and the sauna off another.

A Covered Porch – When it’s raining or snowing it’s nice to be able to go in and out without the weather blowing down in to the changing room and a sheltered place to sit outside to cool down is quite wonderful. We don’t (yet!) have a covered porch and wish we did.

A Window on the world – Being able to see outside while in the sauna is quite enjoyable. One or two small windows works well. If you have a great view then a larger picture type window might be warranted though keep in mind; 1) The more window, the greater the thermal stratification, 2) The more glass and less wood the harsher the experience, and 3) people in the sauna can feel like they’re on display with too large a window. For the latter some well placed vegetation outside can provide privacy and still allow for a view. So, glass, but don’t go overboard with glass.

Too Much of a Good Thing – A window to see outside and let in some light is a very good thing. Too much window and especially a full wall window not so much. There are the obvious things like energy efficiency and that windows feel cold and that it’s critical to have a lot of soft wood wall and ceiling surface in a good sauna and the more window you have the less wood you have. There’s more though. A sauna should be a quiet, peaceful, serene and relaxing place of solitude away from the rigors of the world – a bit of an escape. A large expanse of glass can reduce or totally eliminate this. A large window or wall can also make people feel like they are on display which is uncomfortable.

If you have a really spectacular view it may be worth doing a half or even full window wall but otherwise be judicious.

Skylights – Skylights are tricky in a sauna stove room for a number of reasons. It is possible to do them successfully but I generally do not recommend it. We want the ceiling of our sauna to facilitate the flow of the löyly cavity convective loop for which we want a fairly smooth ceiling and a skylight will, in most cases, interfere with this. The extreme temperature and humidity swings of a sauna result in a lot of structural movement that can make it difficult for a skylight to remain sealed well year after year. A skylight is exposed to the highest heat. It’s replacing desirable wood surface with glass. It is possible to install one insulated glass at roof/curb level and then install another at ceiling level but it’s critical that the cavity between them be very well sealed or you’ll get condensation on the glass.

During a typical sauna session we actually don’t spend that much time in the sauna stove room. 10-15 minutes per round so 30-45 minutes of a 2-4 hour session. We spend a lot more time outside so not having that view of the stars for that brief period is not that big a deal (and makes them that much more special and enjoyable when we are outside looking at them!). A skylight, roof lantern or conservatory ceiling in the lounge area, especially if it can open to vent out hot air, would be a much better investment.

Heated Floors – If you have a concrete floor then adding in-floor radiant heat can make for a more comfortable experience, especially in the changing room and shower but also in the hot room. Extending this to the porch and nearby walks or patios (snow melt system) isn’t a bad idea either.

Spigot – A spigot for filling the water bucket and washing the shower can come in handy (shower works for bucket filling too). Be careful that it’s not from a high chlorine source though as you don’t want that on your stones or in your sauna. Make sure that it’s threaded for a hose – a short hose with a sprayer on the end can make cleaning day much easier.

Privacy – Sauna is best enjoyed nude. Providing for some privacy for both inside the sauna building and for an outdoor patio can make for a much more enjoyable experience for all. Note that some materials such as brick can suck moisture out of bare feet that over time result in painful skin cracks so may not be good materials to use.

Four is better than One or Two?  Consider at least a four person (8’ benches) sauna. One of the joys of sauna is socializing and enjoying it with others.

Garage Saunas – Be very careful with placing a sauna in a garage. If you ever park petrol cars in there or do woodworking or painting in the garage I would not do it. Saunaing results in stress on your body and breathing in any latent fumes from a car, dust from woodworking or other chemicals during this stress could be extremely harmful to your health. Fresh pure air both inside the sauna and when exiting is critical.

 

Some Perspective On Temps – The Finnish Sauna Society and International Sauna Association recommend temps of 80-105°c (176-221°f) at bathers heads and shoulders (thermometer or probe placed 1m (39”) above the middle of the longest bench). Many Finns say hot enough that 10-15 minutes is as long as you can stay in. The majority of Finns and Swedes I’ve talked with prefer 90-105°c with occasional warmer or cooler sessions. Lassi Liikkanen is a proponent of temps a bit below 80°c being sauna. Dr. Jari Laukkanen’s study on the benefits of sauna bathing for cardiovascular health found average temps of about 78°c for these older participants (Finns, avg age 63). Russians seem to prefer either slightly cooler with much higher moisture (nothing like a steam room or hammam though) or much higher temps of 130-140°c (266-284°f) with lower moisture. My personal preference is 94-96°c most days with occasional 110-120°c days or sometimes longer rounds of 75-80°c. SO, there’s a lot of variety in ‘correct temps’.

A good electrically heated sauna then should be able to maintain any temp from 70-105°c or better to 125°c or higher.

I think that many people in the U.S. confuse bad air from poor ventilation with heat exhaustion. FWIW, I’m ready to leave a 90°c sauna with bad air after about 12 minutes and the last few minutes doesn’t feel too great. In a sauna with good ventilation I feel quite good for 12-20 minutes at +100°c and every second until I step out feels great. Good ventilation is critical to being able to get the most out of heat and löyly. Poor ventilation resulting in high CO2, common in U.S. saunas, can also result in Hypercapnia.

Fresh Water – You need water to throw on the stones for steam and to dump over your mates heads. Since the Flint Michigan water incident many communities in the U.S. have been significantly increasing the amount of Chlorine and other chemicals in their water systems. These are quite bad in a sauna, especially when thrown on the stones. Make sure that your water supply filters these out or that you use something like a Berkey or RO system to provide clean unadulterated water.

Paint, Stain and Wax – I am a strong proponent of leaving everything in the sauna natural. If however you do want to wax your benches or stain your walls then look for products from Tikkurila. They are a Finnish company and have a variety of stains and waxes. They have a UK Site in English and a some of their products are available online in the U.S. Something that Lassi L points out is that some of these that seal the wood will also reduce its hygrothermic properties which is not desirable for most people.

 

Notes on Construction

Disclaimer: I am not an engineer nor a builder so I am far from any kind of expert on wall assemblies (but I sometimes know a little bit about physics). 

For some good information on building science I’d recommend two books; ‘Pretty Good House’ by Michael Maines, Daniel Kolbert, Emily Mottram and Christopher Briley. And ‘A House Needs to Breathe…Or Does It?’ by Allison Bailes (I believe this will be available in Nov 2022). Also the website and forums at Green Building Advisor.

Foundation / Floor Structure – Concrete and Wood can both work well. A drawback to concrete is that it can get cold (it pulls heat out of the air, our feet, etc.) which is uncomfortable on feet and can increase stratification. In-floor heat can make concrete quite wonderful though and reduce stratification. A bit of an exception is in summer when you don’t necessarily want to warm the concrete floor.

The frame for a wood floor should usually be off the ground and can be built on top of poured concrete piers, pre-cast concrete piers, driven piles, helical piles, ground screws, etc. What works best will depend on your local climate and soil conditions. A good example of how a Finnish sauna wood floor with drain is often built is here.

 

A Basic Wall Assembly:

This is the recommended wall assembly in Finland (and Sweden, Germany, etc.). Wood framing (2×4 in North America) + foil faced rigid foam (PIR) + air gap + interior cladding.

Vapor Barrier – A good sealed vapor barrier is critical if the walls are not solid timbers. Many people say that a sauna is a ‘dry sauna’ but this is not really the case. 20% relative humidity in a 100°c sauna (actual moisture content ≈ 134 g water / kg air) is about 99% RH at 61°c and at temps below that is… water. For perspective, 90% RH at 26°c, a hot muggy day, is only about 19g of water / kg air. There is a lot of moisture in most saunas.

Saunas are also kind of high pressure; 1) The heat increases pressure (hot flows towards colder), 2) hot air rises and creates even higher pressure in the upper part of the sauna and 3) steam creates even higher pressure again. This higher pressure inside the sauna is pushing out against the walls and ceiling and wanting to take all of that moisture with it. If not stopped by a solid vapor barrier this moisture will condense in the insulation leaving the insulation wet which reduces its effectiveness and given the heat makes an ideal environment for mold and rot. This will not usually dry to the inside or outside. It was driven in to the insulation by pressure and without similar pressure or airflow to drive it out, most of it will stay there. 

In Europe the recommended and typical method uses foil faced PIR (polyiso rigid foam). It’s easier to work with than roll foil, seals better, and importantly reduces thermal bridging. Some popular products in Finland and elsewhere include:

• FinnFoam FF-PIR which is made specifically for saunas and has T&G edges for easier installation and better performance.
• Kingspan Sauna-Satu is another good option made specifically for saunas.
• Recticel FF-PIR.
• ISOVER is now also making a foil-faced mineral wool panel.

Unfortunately I’ve not found anyone in the U.S. that sells any of these. Kingspan does have distributors in the U.S. so they might be the best option. Some options in North America are:

• Johns Manville AP Foil Faced PIR should likely work well and is usually readily available in North America.
• Roxul foil-faced mineral wool panels may be the best option in North America as it has less embodied carbon and less risk of off-gassing.

Note that this needs to be a vapor or air barrier, not just a vapor retarder. It should have a perm rating of 0.1 or less. It should also be rated for the higher temps of a sauna so at least 120°c or higher.

For those in North America who want to use roll foil, a kraft backed foil rated for sauna temps is likely the best alternative.

Bubble wrap type foil should probably be avoided. It’s more difficult to install, nearly impossible to seal well and over time the bubbles burst resulting in holes and the tape coming loose. Without a good seal the moisture in the sauna can get driven in to the wall cavities eventually resulting in mold and rot.

You DO NOT Want Two Vapor Barriers – The ONLY vapor barrier should be the foil. The structure needs to be able to dry to the outside of the foil. For an outside structure you do ideally want a rain screen / gap behind your exterior cladding.

When insulating avoid paper faced insulation that acts as a vapor barrier.

Insulation – Good and proper insulation in a sauna is important, not so much for energy efficiency as in a house, but for bather comfort. A wall that sucks a lot of heat out can increase thermal stratification and make bathers backs feel cold relative to the front of their bodies and chilly backs can be quite uncomfortable (especially for women). The walls of a sauna will always pull heat from inside to outside – heat always flows towards cold. There is no amount of insulation that can prevent this but we do want to minimize it as much as practically possible. Well insulated walls provide for a better, more even and more comfortable heat on all sides of your body.

Reducing thermal bridging through the studs is quite important to accomplish this. Traditionally (in Finland, Sweden, etc.) foil faced polyiso (such as this FF-PIR) on the inside provides a good vapor and thermal barrier. Architect Christopher Wegscheid points out that environmentally this is not a good choice and a layer of mineral wool on the outside would be better. A proper rain screen is always important but critical with exterior mineral wool. Thick enough exterior insulation can eliminate the need for insulation between the studs however I’m not sure I’d do that for a sauna, especially in a colder environment. Attention must also be paid to the dew point and so condensation in wall cavities which varies by environment.

PIR looses its insulation value as it gets cold so either exterior insulation or insulation in the wall cavities or both is still important.

Well Sealed – For a sauna to function properly we want fresh air to enter only where we want (primarily above the stove and maybe some down lower for combustion or heater cooling if needed), not where we don’t. For ventilation in a wood or electric sauna to work well we want to seal up air leaks as best we can.

This includes the floor. Replicating an old sauna with an open floor that allows air through is romantic (and kind of cool when you think about the physics of it) but is similarly romantic to using newspaper for insulation in your new house. Just as the newspaper will result in an uncomfortable house, an open breathable floor (or walls) will result in a not-so-great sauna.

A Sauna DOES NOT Need To Breath – You do not want air being sucked in through various leaks which compromises air quality and reduces proper ventilation, and you certainly don’t want the moist air in the sauna to be driven in to the wall cavities where it will reduce insulation value, create mold and possibly rot the structure. You do want air to enter where you want it to (in to the convective loop above the stove for instance) and you want it to exit where you want it to. Nowhere else. You do want an air gap and some minimal air flow between the interior cladding and the foil vapor barrier and you do want the structure outside of the foil vapor barrier to ‘dry to the outside’. Two good articles on this here and here.

Radiant Heat – For the most comfortable sauna we want balanced or neutral radiation on all sides of our bodies – we want the surfaces around us to radiate the same amount of heat as our bodies. If a surface, such as a not well insulated wall behind us, radiates a lot less heat then the portions of our body facing that surface will feel cooler. And likewise, any surface radiating more heat will make us feel hotter on the side facing it such as if we’re too close to the heater. If the surfaces all around us radiate the same as our body then we’ll be heated evenly by convection and löyly which is what we want. Here’s a fun article that touches on this: Why Doesn’t Frost Form In The Carport.

Joe’s Suggestion – Joseph Lstiburek is one of the more highly regarded people in the world of building science. Here is his take on the best wall assembly for a sauna interior. Note that he shows the furring as horizontal for vertical interior cladding – it should be vertical if you want horizontal T&G.

I might make two changes to what Joe suggests; 1) Leave a bit of an air gap between the interior wall cladding and interior ceiling cladding and 2) include a plinth rather than just flashing at the floor (and make sure to stop the wall cladding at least 1” from the finished floor to prevent water on the floor from wetting the wood). For the gap at the top one option would be to stop the ceiling 1” short of the foil on the walls, then stop the wall cladding 1” short of the ceiling cladding, then add a decorative piece to the ceiling a bit out from the wall cladding. 

Mind The Gap – As a general rule you never put wood against a non-permeable surface. If you use radiant foil or foil-faced PIR as a vapor barrier (and you should use one of these) then you should include an air gap (ideally 0.5 – 0.75” but even 0.25 or 0.125 helps) between the foil and interior wall boards using furring strips (run perpendicular to interior cladding). There are three important reasons for this; 1) No gap could result in increased thermal bridging and a colder more uncomfortable wall because the foil can act as a cold sink helping to suck heat out, 2) so that the back side of the wall boards can dry out and not get mold* and 3) so that the radiant foil can provide some radiant benefit. If there is no air gap then the foil does not act as a radiation barrier and does not reflect any heat back towards the sauna. The air gap is critical for this. Done properly the walls of your sauna will be a bit warmer than without the gap and so provide a more comfortable experience (more details on this in Lassi’s book below).

Wall Assemblies Options

• Best – Exterior cladding + rain screen + exterior mineral wool (1-2” minimum) + studs (and insulation?) + foil faced PIR (1 or 2 layers) + air gap + interior T&G.
• Better A – Exterior cladding + rain screen + studs (and insulation?) + foil faced PIR (1 or 2 layers) + air gap + interior T&G.
• Better B – Exterior cladding + rain screen + exterior mineral wool (1-2” minimum) + studs (and insulation?) + foil vapor barrier + air gap + interior T&G.
• Good – Cladding + rain screen + studs and insulation + foil vapor barrier + air gap + interior T&G.
• Not So Good or Bad – Foil with no air gap

Note that some exterior mineral wool acts as structural sheathing and some not so take that in to consideration. Any external mineral wool must be vapor permeable – you do not want a second vapor barrier.

Thin Timber Construction – Thin timber construction (timbers less than about 5” thick so perhaps 2-4” thick) can save a good chunk of money. While a properly insulated and vapor-barriared sauna is definitely best, particularly in colder climates, I do think that given the cost and time savings that thin timber is not necessarily a bad option in some cases. Given monetary constraints it’s much more important to get benches at proper heights and have proper ventilation. Even with thin timber walls the ceiling/roof should be built with proper insulation and vapor barrier.

Thin timber can, I think, be done a couple of ways; 1) The timbers are the interior, exterior and structure or 2) The timbers are the exterior & structure while foil faced PIR + air gap + interior cladding is added to the inside. The latter perhaps better for colder environments while still saving some money over stick frame. I think it’d also be possible to build the former and then some time later add the interior FF-PIR & cladding.

Sound Isolation – The softer the wood used for interior cladding and benches the better along with reducing hard surfaces such as glass, stone or steel. Not having parallel wall/ceiling/floor/bench surfaces or 90° corners will reduce echo in the sauna as well as reducing sound entering or leaving.

Foil faced PIR or similar poly-iso panels rather than just foil directly on the studs along with having the air gap between the foil and interior cladding will reduce structural sound xmission. 

On the opposite side of the sauna there are a few options to help reduce sound xmission further; Iso channel between the sheetrock and studs, double layers of sheetrock with green glue, or pre-made sound iso sheetrock.

 

Lighting

Many saunas have no artificial lighting, nothing beyond the light through a window, and function quite well so the first thing to consider is that lighting is not necessarily necessary. Traditionally the next step up was a lantern hung in a window to augment whatever light there was from outside. And today there are saunas with 22 channels of RGBA lighting.

There are three functions to think about with lighting:

• Safety
• Ambiance
• Cleaning

For Safety we want to make sure that people can see steps and benches well enough. And maybe also the top of the door.

For Cleaning we want to be able to see everything including under the benches. The fewer shadows the better. It’s convenient to have this lighting built in and easily available with the press of a button but it’s also possible to use a portable light for this as well.

Lighting for Ambiance could be an entire book (and there are many books on architectural lighting). So here some very quick thoughts… Some options include:

• Downlights in Ceiling
• Twinkle or starlights in ceiling (typically fiber optic)
• Sconces on the walls
• Rope light under benches, behind a ceiling cove or elsewhere
• Accent light behind heater

Think about the source of the light, the area it will light and the surface patterns it will create as all of these have aesthetic appeal. Putting a downlight in each corner about 8-15cm (3-6”) out from the walls will provide some general ambience lighting and will create some nice patterns on each of the walls. Similarly, doing them along the walls every 40cm or so will create an appealing scalloped wall pattern. This is where looking at lots of photos online is really beneficial.

Heat – Yeah, a lot of standard lights won’t work in the heat of a sauna. Some will, some not. Some LED rope will work lower down where temps are lower but fail when higher up. FWIW, we’ve got LIFX GU-10 downlights in our changing room that is often over 40°c (105°f).

Color Quality – Many LEDs have very poor color quality. Slightly better than florescent but far below incandescent or natural light. Ideally you want high TM-30 scores. For countries that don’t require that the next best thing is a high CRI (at least 90 but over 95 or 98 is better) and a high R9 (over 80 or 90). Another aspect to color is being Warm-Dim capable. Some LED’s will have good color quality at full brightness but quite poor when dimmed. Better LED’s will, similar to incandescent or the sun, produce a warmer color temp as they dim. So full bright might be 3700° but as it dims it gets warmer (lower color temp) so at 20% might be 1900°.

Some LEDs have flicker that can be uncomfortable for some and cause headaches and nausea so something to pay attention to.

Control – Control can be as simple as one switch or as complicated as an RGBA control system with programmed scenes. It wouldn’t be too unusual to have 20 RGBA lights with each individually controllable and programmable to create different scenes. In our case we already have a Control4 home automation system for our home so we use that to control our sauna lighting as well. 

 

Barrels:

Moved to: Trumpkin’s Notes On Barrels.

 

Form, Function and Löyly

The above is largely about function and Löyly. It’s about the things that are needed for a good proper Finnish sauna experience.

Form is important also and form can enhance function. Sauna can be more enjoyable in a room that is aesthetically pleasing than one that’s not. Should sauna function ever be compromised for form? Perhaps. But not for our sauna, the one we use several times per week. For our routine saunas we want as great a sauna experience as we can get.

Some people may be more interested in form and want only what function fits the form they desire. And that’s OK, it just may not be sauna. Or the löyly is slightly compromised for a really amazing view. And that’s OK (and I’d love to come enjoy the sauna and the view with you). Saunas Of The World is one of my favorite Instagrams because I love architecture and it has some fabulous looking saunas and places, but most of what they feature are either not sauna’s or not good sauna’s. I’d still love to visit many of them though.

Someone may prefer to have all glass walls for instance, and that’s certainly OK. Or they may want lower benches. What’s important is that they understand what affect these will have so that they can make an informed choice – what bit of better sauna experience are they giving up for a better view or not having to climb up higher (and there are some great options for those with disabilities to be able to enjoy a proper sauna).

What’s important is not that every sauna be ideal, that it hits the golden bullseye above, but that people know and understand what compromises to the sauna experience are being made. 

 

Breaking The Rules

Sometimes constraints prevent us from achieving the sauna that we want and in these cases we simply do the best we can. Other times we might choose to intentionally break a rule or two to achieve an aesthetic element. And sometimes we can get away with breaking rules…

Case Study 1 – I’ve recently been involved in a conversation for a ‘campfire’ sauna. The owner wants a U shaped seating arrangement around a central pillar style heater that might have a bit of a campfire feel. I think that this is a great idea, even though it breaks some rules.

Here’s a photo of the sauna at Villa Furuvik just outside of Helsinki that is the inspiration for this project.

Here’s why this works. First is that this is what some Finns refer to as a party sauna so while it should still provide a good sauna experience, that experience may be somewhat secondary to other aspects including social elements and being safe for people who might have had a bit more alcohol than normal. It’s important to note also that this is retrofitted in to an older shed and so had some unavoidable constraints including overall size and height.

So, let’s break some rules.

Feet aren’t above the stones. They are though just about above the cold zone which is more important here. ‘Feet above the stones’ serves four purposes in one; 1) There is less stratification above the stones and the less head to toe temp difference the better, 2) getting the foot bench above the stones also usually gets feet above the cold zone, 3) feet above the stones forces the ceiling higher to create a good heat cavity and 4) feet above the stones reduces direct radiant heat.

The foot bench / platform here is about 27” above the floor and so is almost above the cold zone. 

Open sided mesh heaters are more forgiving. The convective loop can extend down a bit below the top of the stones.

Even though the room is not perhaps as large as they would have ideally liked, it is still somewhat large, about 11’x10’. And, the heater is in the middle so there is even space all around to the walls. These two elements help with stratification. While in a typical smaller residential sauna the stratification changes quite dramatically at about the top of the stones, here it will continue down a bit before we start getting the more quickly changing air temps*. So while in a smaller sauna the floor of the Löyly Cavity is above the top of the stones, here it is a bit below.

And there is a fairly ample heat cavity above the door. 

Being above the stones might still be best but this will work relatively well so long as there is good ventilation to also help pull warmer air down.

Too close to the heater. Will people sitting here feel a lot of uncomfortable radiant heat from the heater? If they have the 12” diameter Tylö-Helo / Finnleo Himalaya that we have in ours, yes. Like with many sauna heaters you need to be about 4-7’ away from it for the most comfortable experience. The Iki in the photo and planned for the new sauna is 21” in diameter though and as Lassi Liikkanen points out, should have less radiant heat from the sides due to the much greater amount of stones (and that 21” width of stones).  While temps on the sides of the Himalaya can range from 100-150°c, the Iki shouldn’t much exceed 100°c to the sides. Even so, they’ve apparently recently added solid wood around three sides of the heater to reduce direct radiant on bathers.

This sauna may not be one you’d want as your everyday sauna (though I could be wrong on that) but it should serve quite well as a party sauna and will likely be a better sauna experience than 99% of saunas in North America.

 

Case Study 2 – On first glance this sauna at Löyly in Helsinki breaks a number of rules including feet aren’t above the stones nor above the cold zone. Oh, and a lot of extra space above the sitting bench, much more than the 1200mm (48”) recommended.

From outside though we can see just how big the stove is and how large of a cold zone was included. The cold zone below the foot bench / platform is well over one-third of the total volume of space. The overall volume of space along with having bathers up this high also allows for a bit more room above the benches. The Iki stove has over 1300 lbs of stones.

 

Ideal Sauna Shape?

This is a theoretical / exploratory discussion for those interested.

Earlier we discussed wanting to facilitate the convective loop that forms the löyly cavity but also compress it, and we want to experience as little stratification as possible. In theory then an ideal shape might be;

1) Have the heater wall slope in a bit, perhaps 5-20°. This will narrow the space near the ceiling resulting in less stratification and widen the space at the bottom providing more space for the cold zone and effectively lower it. This will also help to facilitate the convective loop as it will begin directing the rising plume towards the bench wall. 

1b) Have the side walls slope in a bit as well for the same potential benefit to stratification.

2) Do a slightly vaulted ceiling (1:3 pitch) with the peak cheated towards the bench wall.

3) Slightly trapezoidal room that’s wider on the bench wall. So if you have an 8’ wide bench wall then perhaps a 7’ or 6’ wide heater wall.

 

Cost

Costs vary considerably so for the most part every project is different.

First, doing a lower ceiling, lower benches and lessor ventilation – taking shortcuts for an American psuedo-sauna – will save only about 3-5% but will result in a much worse sauna experience. It’s not worth it. The lower ceiling will in most cases also cost more in energy use per session due to the smaller heat cavity. 

It’s possible to build a typical Finnish cabin sauna with an 8’x8’x8.5’ stove room, 8’x12’ vestibule/changing/shower and porch for about $12k (DIY) or about $20-30k if you hire a good contractor. In general materials are about 50% and labor about 50% so full DIY can cut the costs in half.

A DIY build of a smaller 6x7x8.5 garden sauna can be done below $5k (2022 $’s) and then a vestibule/sitting/shower added later.

Nicer aesthetic elements are often the biggest driver of high cost so on the upper end you can easily spend $120k or much more to have someone build it for you with nicer materials, finishes and details. Here labor can easily be 80% or more of the cost. The tile I wanted to use in our shower would have cost about $1300 more than the tile I ended up choosing. Doing decorative wood designs can easily add $35k or much more in labor.

While insulated frame construction is best, there are some significant cost savings to be had in doing thin timber (2.5” – 4”) walls and I’d take a well designed thin timber sauna over no sauna any day. A hybrid of all walls being thin timber except the primary bench wall could provide some cost savings and somewhat eliminate the biggest downside to thin timber which is bathers having chilly backs. A thin timber cabin with proper bench heights and ventilation could be a great starter sauna.

 

 

Thermometers and Other Measurement Devices

Moved to: Notes on Thermometers and Other Measurements

 

 

Cleaning & Maintenance

The best resource I’ve found on cleaning: How To Clean The Sauna and 9 Tips For Cleaning A Sauna Ecologically

 

Resources:

Web Links:

Sauna Ventilation – Finding Good Pure Air 

SaunaLogia

Building A Sauna In 13 Minutes Video – By Andrejs Vasevics in Latvia.

Books:

EVERYONE building a sauna should read Lassi Liikkanen’s ‘Secrets of Finnish Sauna Design’. I’ve read over 30 books on sauna and sauna design. This is the best and most accurate I’ve found. I wish it had existed when we built our sauna as it would have saved me a lot of research time, a lot of headaches and two remodels.

Lassi also provides consulting services that can be well worth it for those interested.

 

Other Worthwhile Books:

• Villas and Saunas Of Finland (Hautajärvi)
• The Sauna Book (Johnson & Miller) – Overall good with some good info on construction details. Some info such as on ventilation is quite dated.
• Finnish Sauna: Design & Construction (Rakennustieto Publishing) – Overall good though somewhat dated information. This book is very interesting from a historical perspective but Liikkanen’s book above is largely considered to have superseded it.
• The Modern Sauna (Konya)
• The Opposite Of Cold (Nordskog & Hautala) – One of the best coffee table books for those in North America.
• Cathedrals Of The Flesh (Brue)
• The Sauna Is (Hillala)
• The Best Part of a Sauna (Peterson, Dupre)
• Sweat (Aaland)
• International Handbook of Finnish Sauna (Konya)
• Sauna Magic (Conover)
• Finnish Sauna (Konya)
• Sauna: The Finnish Bath (H. J. Viherjuuri)
• Pretty Good House (Maines, Kolbert, Mottram and Briley)
• A House Needs to Breathe…Or Does It? (Bailes)
• Thermal Delight in Architecture (Heschong)
• The Architecture of Bathing: Body, Landscape, Art (Pearson)

Anyone who builds saunas professionally should read or at least browse in depth: This document, ’Secrets of Finnish Sauna Design’, ‘Sauna: the Finnish Bath’, ‘The Sauna Book’, ‘The Sauna Is’,  ‘Finnish Sauna: Design & Construction’, ‘Pretty Good House’ and ‘A House Needs to Breathe…’. The latter two are not specific to sauna but to construction practices. Together these should provide a good foundation on some history, how sauna has developed over the past 50 years, and what current best practices are.

I’ve about twenty other books on sauna not listed because they are simply not worthwhile. 

 

Official Definition Of A Sauna

International Sauna Association (ISA) – Adopted at the ISA Congress in Aachen, Germany on 5 Aug 1999

Sauna bath – Saunaing is a healthy and relaxing hot air bath, alternating between warming up and cooling off. When taking a sauna, the whole body is heated several times in a wooden-surface room with a typical temperature of about 80-105 º C, measured from a height of about 100 cm above the level of the upper sitting bench. Warming is followed by cooling in the open air or with cold water.

Sauna room – The sauna is a wood-paneled room with stepped benches, a stove with stones, with a temperature of about 80-105º C measured at a height of about 100 cm above the level of the upper sitting bench, and low humidity, which is briefly added by throwing steam.

 

Still To Learn:

We see extremely high particulate matter (1.0, 2.5 and 10.0) when ladling water on to the stones. Part of this and possibly all of it is bits of stone breaking off and disbursing with the steam. I’d guess some of it is also from the calrods which isn’t good.

We’re still trying to figure out a good reliable way to measure CO2 near bathers faces during a sauna session. It’s easy to do at lower temps like 60°c but not so much at 90°c which is above the temp range for most meters. Airflow will be different then too so why I’d like to find a good way to measure it.

 

Misc Resources:

Paint / Stain:

Tikkurila

Teknos

Lighting & Other Stuff:

Cariitti

SAAS

LEDify

FTLight

HideALite

Ensto

Airam and Airam Blog Article

Spectra (Sauna360)

 

 

 

_________

* If the vapor barrier is well sealed then there is not likely to be much or any moisture on the back side of the interior boards.