Trumpkin’s Notes On Building A Sauna

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.

Tower of Babel

Everything we found in English – books, forums, Reddit, Facebook and websites – consistently recommended ceilings no higher than 7’, benches too low, ventilation that the laws of physics say won’t provide any 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. Why such a difference?

My Swedish wife and I along with our son and his wife traveled to Finland and Sweden to experience sauna and learn from experts. We noticed a huge difference between saunas in the U.S. and saunas in Europe. Sauna’s in Europe are consistently much better. We wanted to know why. Why are saunas that people in the U.S. sell and build so much worse than even bad saunas in Finland?

If you hire a good sauna builder in Finland or Sweden, they will build a sauna based on the same principles in this document and you’ll be assured of having a good sauna.

Unfortunately that’s not the case in the U.S. and other english speaking countries. The vast majority of what’s available in english is based on outdated and misunderstood information from the 1950’s and 60’s that’s been regurgitated and passed down over and over from one decade to the next. We’ve been stuck in the 1960’s while Finns and others have been improving their craft. To put it bluntly, we’ve been misled. 

If you want a proper Finnish sauna then you, your builder or both should read and understand what’s here and ideally also Lassi’s 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.)

The following is largely my notes from 2017-2019 as we were designing and building our new sauna. Most of this (and the other Trumpkin’s Notes linked in the menu above) are not actually anything new but things that have been well known and practiced in Finland and elsewhere for decades. These are only new to the english speaking world. Finns get it right. They know what they’re talking about with sauna. When they are so emphatic about ‘feet above the stones’, there’s a reason. Actually several reasons.

I dislike people being misled or taken advantage of (I have Asperger’s and this is one interesting key bit of it). I hope that this information will help others to avoid the mistakes that we made or almost made.

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 I’d lean towards what Lassi says.

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 🙂 .

TrumpkinLow

For a brief intro to Finnish Sauna and how to properly take a sauna: Intro To Sauna

* We had not yet found books such as Rakennustieto’s ‘Finnish Sauna: Design and Construction’ or any of Allan Konya’s books that while somewhat dated do accurately convey some of the important principles to the extent they were known at the time.

 

What Is A Sauna?

A sauna is a wood lined room heated to a temperature of about 80-105°c by a large mass of stones upon which water can be thrown to produce steam that when combined with pure fresh air and even heat around bather’s bodies results in löyly. The stones may be heated by wood fire, electric or gas.

That’s about it. One important thing I’ve learned is that you won’t find much of a detailed definition of what is, or is not, a sauna – a specification of any sort. How can we build a sauna without a spec? Without knowing what we’re trying to accomplish or aiming for? I even tried to get a spec written and received considerable, though very polite, pushback. Finns don’t want to box sauna in too tight. And rightly so I’ve learned.

What you will find is that saunas considered to be good in Finland, saunas that have löyly, all follow a number of key principles like having a lot of stone mass (a sauna is heated by convection via the stones and the stones by a heater) that you throw or ladle water on to control humidity, foot benches above the cold zone and often above the stones, fresh outside air ventilation for bathers, the ability to get to or above 100°c (212°f) for those who desire it. These are ideals that good sauna builders there consistently aim for. And for good reason – because each has a functional purpose – a reason for being done. Each one of these along with the other elements detailed below contributes to a more enjoyable and healthy sauna experience.

“There’s a point where magic happens. When the löyly is so good, so close to perfect, that you know you’re experiencing something special. The air is fresh and pure without suffocating levels of CO2, even temps surround your entire body head to toe and front to back, no chilliness of any sort, little or no radiant heat from the heater or elsewhere and soft warm hygroscopic wood all around. The temp is somewhere above about 94°c, perhaps above 100°c, and it is wonderfully refreshing. Fresh water is thrown on the mass of stones, the steam rises and your entire body is enveloped in löyly. This is sauna.”

Sauna builders in Finland have a clear target that they aim for. They will do everything they can to get benches at the best heights, ceiling shape as perfect as possible, excellent ventilation, and on and on. And they very often hit their target. 

SaunaPolar05

Occasionally though, constraints force compromises – and good builders then do the best they can within the constraints placed on them. And that’s OK. Not meeting these 100%, being further off the mark, doesn’t necessarily mean that something isn’t a sauna, just that it’s not necessarily an ideal sauna so may be a lessor or different experience. 

Too often North American consumers are not provided the information to make good choices. We buy a kit or hire someone to build a sauna for us and it’s built to often misguided American beliefs rather than Finnish ideals. We’re told it’s a ‘Finnish Sauna’, but it rarely is. And this is all without any discussion. That’s not right.  

If someone in Finland wants a high peaked ceiling in their sauna, a Finnish builder will push back quite hard and explain to them the negative effect that will have on the sauna experience. Many Finnish (and Swede, German, Dutch, etc.) builders will not build such a structure. Builder’s in the U.S. will often simply say ‘sure’ and do it – because either we don’t understand the physics of sauna and how poor an experience that peak will cause or our macho don’t-sweat-the-details attitude kicks in. Either way the result is a bad sauna.

Sauna builders and vendors in the U.S. don’t know what target to aim for. We’re intentionally aiming for ceilings no higher than 7’ and small spaces when sauna builders elsewhere are trying hard to avoid those – because they know they result in a bad sauna experience. We should aim for the same ideals as Finnish and other builders.

This is all just the technical and functional side though. A sauna should also be a room that we enjoy being in and likewise the adjoining spaces, the vestibule, changing room and shower, should be a place we desire to spend time in not just be functional.

 

Löyly and Sisu

Löyly is critical to sauna. It’s a Finnish word that traditionally meant Spirit or Life but today is used as a descriptor for a good environment in the sauna and 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 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 the temperature at their head and shoulders is about 75 -115°c and more importantly is fairly even around their entire body – head to toe, front to back and minute to minute. The air is pure and fresh without high levels of CO2, colognes, mold, bacteria or other contaminants. Direct radiant heat is minimal so bathers are heated evenly by only convective air all around their bodies. 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 envelopes 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 wonderful 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.”

When you experience quality heat and löyly and get use to it, you never want to go backwards.

—-

But there’s a bit more 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 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 have constantly striven 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.

How do you know if you’ve got good löyly? Most Finns are in enough saunas throughout their lives that they immediately recognize good vs not so good 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 and you should feel even heat around your entire body. 
  • 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 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.

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

 

We Don’t Know What We Don’t Know

I thought I knew what good sauna was but it was not until I experienced a lot of proper saunas in Finland that I realized just how bad saunas in the U.S. are.  Glenn Auerbach thought he knew good saunas until he was surprised that he never got dizzy in saunas in Finland. I’ve known people who were positive that the sauna they’d built 10 years ago and used multiple times every week since was a great sauna . …until they fixed the ventilation and experienced fresh sauna air for the first time.

U.S. saunas are like freeze dried instant coffee. Great …until you try the real thing. 🙂 

Many people will build an American pseudo sauna where they experience stale air and cold feet, and be quite happy with it. We’ll think that we’re experiencing good sauna because we don’t know any better – we’ve never experienced proper sauna. And if we’re happy with it that’s actually kind of OK – ignorance is indeed bliss. And this isn’t a slam against anyone – how could we know when our experience is limited to saunas built to American rather than Finnish principles.

The problem is when we invest a lot of time and money in building a sauna and then learn how bad it is, that it’s not the Finnish sauna that we thought, that there are much better options. We were misled by vendors advertising ‘Finnish’ sauna when what they were selling wasn’t. This happened to us.

 

Sweating The Details

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

There are reasons for that statement and very good ones. U.S. saunas suffer from bad information, a love of mediocrity and don’t-sweat-the-details, and a lack of sisu. U.S. traffic engineers not sweating the details on road design is why we have the most dangerous road system of all developed countries, why a child in the U.S. is 11x as likely to be killed walking or riding a bicycle as a child in Europe and why children in the U.S. don’t walk/bicycle to school as children in healthier and more academically successful countries do. One of many examples for why we have, not just mediocre, but the lowest life expectancy of all developed (and many third-world) countries. Yep, details can be quite important. And we need to stop saying ‘don’t overthink it’ before we’ve begun to understand something.

We think that cold feet and feelings of mild suffocation are normal for sauna, but they’re not. These are only normal in poorly done saunas, most of which are in the U.S. (though several Finns have pointed out to me that there are some quite poorly done saunas in Finland as well).

Some things we noticed on our travels in Finland: Saunas there are BIG by American standards. EVERY sauna required climbing up several steps to the benches from the changing room – feet are always above the stones if possible. EVERY sauna except one had good ventilation. They all had an adjacent shower and with a tiny few exceptions, a window or two. All critical details that make for a much better experience.

The biggest thing though is how much more enjoyable sauna is in Europe. More even enveloping heat front to back, head to toe and minute to minute. No cold feet or chilly backs. And that there’s a huge difference in leaving a sauna having benefited from heat and löyly versus leaving because you feel like you need air to breath. 

These details are more critical for women. Women are often more sensitive to temperature and more uncomfortable with cold feet or chilly backs. More importantly they are, by design, more impacted by high levels of CO2 and so poor ventilation makes for a much more uncomfortable experience for women than for men.

If ten men and ten women experience typical U.S. sauna’s; 5 men will like it and 1 woman will, the rest not so much.  This is why so many saunas in the U.S. fall in to disuse – they are not that great of an experience.

Those same 20 people experiencing proper sauna; 9 men and 9 women enjoy it and can’t wait until the next time. These are the saunas that get used frequently forever.

A bit of effort up front sweating the details, aiming for better than mediocre, provides thousands of hours of more enjoyable and beneficial sauna for years to come. It’s worth the effort.

 

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.

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’.

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. During the 1970’s and 80’s, realizing that this didn’t work well for electric, some builders began experimenting with other options with Mechanical Downdraft proving to be the best option. VTT did some studies on this in 1992 and found that it was indeed the better option. Soon after the official RT system was updated to reflect this.

1990’s: Stone Mass For Electric Heaters – Until now most electric sauna heaters had very little stone mass and for many the stones were little more than aesthetic. During the 1990’s there was a growing realization of the importance of stone mass in electric heaters for better quality heat and steam. This resulted in increasingly larger heaters and so also commensurately higher ceilings and benches to keep ‘feet above the stones’.

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 and others. 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 the case and that benches and ceilings still needed to be high up. These heaters still provide some 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 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. 

 

Building Or Buying A Sauna

SaunaLaws

Whether buying a kit, building from scratch or having someone build it for you, here are some critical things to look for. These are the things that are most often done incorrectly in North America and that are the cause of the “90% of saunas are bad” statements.

– Foot bench should be above the top of the stones and/or lower third of the space. 

– Proper ventilation (for electric: fresh supply above the heater, stale exhaust (mechanical) below the foot bench).

– Proper amount of stones. MINIMUM 6kg / m³ (1/3 lb / cf) of space and 17kg (37 lbs) per person.

– Minimum volume of 2 m³ (70 cubic feet) per person plus 1 m³ (35 cf) for the heater.

– Vestibule, often a changing room and shower, to provide an air lock.

– Proper vapor barrier and insulation.

If buying a kit, also be cautious of people capacity. Some manufacturers will advertise a much greater capacity than the sauna can actually accommodate.

I strongly recommend against any barrel less then 8-10’ in diameter. More here: Notes On Barrels.

IR is not sauna. Nor can IR be combined with sauna as the IR panels reduce the amount of wood surface needed for a sauna and when used as a sauna the IR panels radiate heat which is uncomfortable and something you do not want in a sauna. Finnleo offers an InfraSauna that attempts to combine the two but the result is a quite poor sauna experience. IR is not necessarily bad though. More: Notes on IR Booths and Cabins

Some reviews of sauna kits: Kit Sauna Reviews

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.

 

Sauna Heat Zones

This and ventilation are the two most important concepts to understand about sauna.

There are two powerful forces at work in a sauna; thermal stratification (due to buoyancy of hot air and stack effect) and convective loops. These result in three critical zones in a sauna.

  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. 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 above the top of the stones Löyly Onkalo, the Löyly Cavity or Löyly Pocket. This is above the Cold Zone, often above the top of the stones, is where the primary convective loop of warm air is …and this is where we want to be – every bit of us. 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.

SaunaHeatZones01b

Here’s that convective loop that makes the Löyly Cavity so critical. When we’re designing our sauna and the ceiling in particular we’ll want to facilitate the flow of this loop but we’ll also want to compress it down a slight bit. This convective loop reduces stratification and carries the steam when water is thrown on the stones.

SaunaConvectiveLoop1

Note the fresh air supply vent in the diagram above. To provide ventilation to bathers fresh air needs to be injected and mixed in to this convective loop between the heater and bather. More on this later.

A larger or more powerful heater cannot overcome stratification. A larger heater can heat the room faster or 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. When you know proper convective heat, there’s no going back to bad radiant heat.

Here’s a simulation (below) using NIST/NFPA software of steam in a sauna. The left image shows the initial bit a few milliseconds after water is thrown on the stones as the plume rises and begins to travel across the ceiling. On the right the steam has traveled across the ceiling and down the bench wall. Note that the steam and thus löyly is largely above the top of the stones – the löyly cavity – it’s following the convective loop. Someone sitting on the bench in this sauna will have their upper body in warm löyly while below the knees will be much less to none. We want all of our body up in the löyly cavity.

SteamSimulation

One more thing to note, the convective loop that forms the löyly cavity and that the steam follows would normally go below the sitting bench a few inches to maybe halfway to the foot bench. My guess is that this simulation used solid benches rather than permeable. This shows the importance of proper air gaps in the benches and the larger gap at the wall to allow this upper convective loop to flow down to bathers feet. Without these gaps the convective loop will shelve at the bench.

There is more on stratification and some interesting charts from VTT in the Ventilation article (in the menu above).

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 seven reasons;

  1. Getting the foot bench and so all of our body in the löyly cavity.
  2. Getting us above the colder lower third of the sauna (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.

 

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 rather ‘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 seem much hotter than it really is. In a sauna with a lot of stratification resulting in cold feet we might think that 175°f 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 more comfortable and more enjoyable than our old low bench sauna at 175°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.

 

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 Shape – Generally a square or tall square is best with a rectangle or tall rectangle a close second. 

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.

BadSaunaDesign72

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 width. So for an 8’x8’x8.5′ sauna the interior peak of a vaulted ceiling should not be greater than about 10.5’. Higher than this (steeper) can stall the convective loop and allow hotter air to stay trapped in the peak. 

For vault or partial barrel vault the peak should run parallel to the longest 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 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.

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.

Above we mentioned that we want to facilitate the convective loop that forms the löyly cavity but also compress it. Quite possibly the ideal ceiling shape actually begins at the wall behind the heater. The upper 1/2 of this wall should be angled in slightly, perhaps 5-20°. Then a slightly vaulted ceiling (1:3 pitch) with the peak cheated towards the bench wall. The room wouldn’t be rectangular BTW, the bench wall would be wider than the stove+door wall so if the bench wall is 8’ then the stove+door wall would be perhaps 6’. 

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.

 

Stove Room Space – Finnish official recommendation (RT Card) is overall volume of 3 m³ (105 cubic feet) per person or larger and most saunas meet this requirement. 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 (3-4 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.

 

Heater and Bench Relationship: Creating The Löyly Cavity – 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 take advantage of the upper convective loop that creates the 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.

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.

 

Reduce Harsh Radiant Heat – 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 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.

 

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.

Note The 7’ myth prevalent in North America has numerous sources – including from Finland. Even older Finnish RT specifications mention ceiling heights of 7’ but these same sources also mention ‘Feet Above The Stones’ as the critical driving element. Part of the misunderstanding is that heaters were once smaller and lower so it was normal to have a 7’ ceiling and still have the foot bench above the stones. As heaters became larger and higher, Finnish designers and builders raised their benches and ceilings accordingly. Through this they also discovered (or rediscovered) the importance of having a heat cavity, of being above the lower third cold zone and having the foot bench above 65°c (150°f) to kill mold and bacteria. Americans though, even today, latch on to a bit of Finnish text that said 7’ and ignore the rest of it and everything that has transpired in Finland since the 1950’s

The Sitting Bench (the highest bench where people sit) should be 100-120cm (40-48”) below the ceiling. Ideally 120cm (48”) as this is in the best heat, comfortable 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. Note that you generally never want the sitting bench to be more than 120cm (48”) below the ceiling. The exception is a sauna for an exceptionally tall group of people where you may go 125cm (50”) or even 130cm (52”) below.

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.

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

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, Finnish, Swedish and other sauna builders 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 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 eventually 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. 

SaunaHeat

(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?

SaunaIR01

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 that they create. 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 increase in wasted energy.

The 7’ myth prevalent in North America likely originated with the energy crisis of the 1980’s, a misunderstanding of sauna, a misunderstanding of physics, and nobody ever questioning it.

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 – mostly in the cold zone below us. 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 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

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 Huum or Saunum. (NOTE: Reports of failed heating elements on Huum heaters have reached the point that I would recommend staying away from them until their quality improves.)  An open mesh style heater results in a less defined löyly cavity due to a less defined convective loop. With a proper higher ceiling and benches this isn’t necessarily so good because that well defined löyly cavity has less head to toe heat stratification and is more comfortable. However, with a lower ceiling that forces the foot bench below the top of the stones the less defined löyly cavity helps us by bringing a little bit of heat down lower around our legs and feet. So rather than cold feet we’ll perhaps just have cool feet. 🙂  

A Huum Drop, Hive or Hive Mini is an open sided mesh and also has the stones down lower than many other mesh style heaters so in a room with a lower ceiling should result in a more comfortable sauna than an enclosed heater with feet below the stones or a taller mesh pillar such as a Harvia Cilandro.

My guess is that the Drop has the least defined löyly cavity and so would be best for the lowest ceilings, the Hive-Mini the next least defined and then the Hive. 

Huum Warning: I believe Huum heaters are not UL listed for use in a sauna so insurance may refuse coverage in case of a fire – your house could burn down and insurance will not cover one bit of it. That’s a big risk. Also, there have been a number of reports of heating elements on Huum’s going bad rather quickly so there may be a quality problem with them. There has been mention of this being due to poor quality stones supplied with Huum’s that break down faster than proper stones allowing the elements to touch and burn out so possibly a Huum with better stones is a good option. I like Huum heaters but buyers should be aware of these issues.

I think that with a lower ceiling that any of the Huums should likely result in a better sauna experience than the alternatives that I’m aware of currently available.

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 Huum or 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 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.

 

A Note On Builders – If everything you’ve ever read says that a sauna ceiling should be no higher than 7’ and every sauna builder you know builds 7’ ceilings and every sauna you’ve ever been in had 7’ ceilings then you’ll likely and quite rationally conclude that 7’ ceilings are the thing to do. So you’ll build nothing but saunas with 7’ ceilings. And then after building 80 saunas with 7’ ceilings over the past two decades some yahoo comes along saying that ceilings should be 8.5’. Or HIGHER! I’d probably think anything but 7’, along with anyone saying so, is nutty too.

History is replete with whole populations thinking ideas are nutty though. Fortunately it’s much easier to learn about physics, how Finns build saunas, why they do things the way that they do, and other bits in this document, than prove that the earth isn’t flat.

 

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 Board Gaps – Good air circulation throughout the 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 on. 

Too little air permeability interrupts the convective loop that forms the löyly cavity (see the NIST/NFPA image above in the Heat Zones discussion) and causes it to shelve or bottom out at the sitting bench rather than reaching down to bathers legs and feet. This then results in less even heat on bathers bodies, colder feet and potential for increased mold and bacteria growth. That latter both on the foot bench due to lack of heat and on all benches/platforms if the air gaps are too small to dry out.

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 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 the 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).

Sauna paarakennuksessa

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 Sauna – It is quite common 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.

Sauna Huitukka

And another in Soini, Finland. The mezzanine door is not used during sauna (except perhaps 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.Tonttu01

 

 

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 likely the best option and is the primary wood choice in Finland. North American Spruce is likely a close second. Be careful of woods like pine 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 though 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 – Is 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.

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.

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 recommended and some say is critical. My personal opinion is that it is definitely good to have but far from critical. The presence or absence will not affect the normal sauna experience in any way. Having a drain may make 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 during sauna or 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.

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 / 40°f and 60°c / 140°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.

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.

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. MINIMUM 6kg / m³ of space is a good starting target though several 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.

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 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.

 

 

Stoves and 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 no or very little radiant heat to the bottom, front, back or sides.

In a sauna you want to experience heat from convection but not too much heat 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, can 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. If you want radiant heat then save a wad of money and stand by a fire or buy an IR booth.

That said… 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 heater and the wood walls and even the person sitting next to us. But radiant is directional and uneven which we want to minimize. 

You want as much of the heat produced as possible to rise up as 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 + air channel + thin outer carcass. The air channels carry heat from the sides of the fire chamber upwards. 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. 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.

Here is how this works in a heater from Helo. For this particular stove, thanks to the Coandă effect, the hotter the fire the more air that flows up through the stones. 

WoodStove01

An alternative introduced by Iki in the late 1990’s is to replace the side air chambers (and front insulation) 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.

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.

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.

Big Iron – There are reasons that you don’t see many heavy iron or steel sauna stoves for sale in Finland, Sweden 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.

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. 

Wood, Gas or Electric?  Wood is more traditional and more romantic. 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. 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 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 certainly more convenient, especially with a phone app that allows you to begin preheating before you arrive home and electric maintains more even temps. Electric may be less healthy, this perhaps due to the calrods used for heating (though this, if it is a problem, could be eliminated with better heater design). Electric may have similar environmental impacts to wood though as resources are used to produce electricity and production of electricity often produces pollution (and making solar panels does as well). How the environmental impacts of wood vs electric compare is a much longer discussion.

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. Some local codes will not allow remote app control.

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/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. 

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 perahaps 103°c and then turn off until it has cooled to maybe 95°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.
  2. Larger Temp Swings. Rather than 95°c to 105°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. 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.
  5. 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. 

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.

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:

LetThemComplain

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.

FJxfSpo

 

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. 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.

Sauna05 100

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; the more glass and less wood the harsher the experience so don’t go overboard with glass, and 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 allow for a view.

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 it should be avoided.

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.

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.

 

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 both work well. One drawback to concrete is that in some environments the floor of the vestibule/changing/shower can get cold, however, in-floor heat can make concrete quite wonderful. What works for a well done garden room or shed in your environment should likely work for a sauna.

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.

Sauna

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 inside 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. 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. This is what I recommend. Johns Manville AP Foil Faced PIR is a good option. 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 about 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. 

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.

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 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.

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.

BSI10 Figure 03a sauna

BSI10 Figure 03b

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) 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.

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 is 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). Saunas Of The World is one of my favorite Instagrams because I love architecture and it has some fabulous looking saunas and places, but many or even 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.

VF 01

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 above the cold zone which is more important here. ‘Feet above the stones’ kind of serves four purposes in one; 1) There is usually 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.

VF 02b 100

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

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.

Iki loyly helsinki

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 almost half 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.

Iki loyly helsinki from outside

 

 

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 you can easily spend $120k to have someone build it for you with nicer materials, finishes and details. A DIY build of a smaller 6x7x8.5 garden sauna can be done below $5k (2022 $’s) and then a vestibule/changing/shower added later.

Nicer aesthetic elements are often the biggest driver of high 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 25 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.

 

Folks in the U.S. may also want to read Glenn Auerbach’s ‘Sauna Build from Start to Finnish’. Some practical information for sauna construction. Link for it is on the SaunaTimes website (for some reason the link doesn’t work properly here and results in a really giant image). Note that many of the sauna design principles in this e-book are for an Americanized sauna, not a Finnish Sauna.

 

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 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 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)
  • Sweat (Aaland)
  • Sauna Magic (Conover)
  • 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.

 

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

Lighting & Other Stuff:

Cariitti

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.