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.

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. But when we searched in Swedish, Finnish and German we got 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?

The following is largely my notes from 2017-2019 as we were designing and building our 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 in Finland and elsewhere for decades. These are only new to the english speaking world.

If you hire a good sauna builder in Finland or Sweden, they will build a sauna based on the same principles as in this document (and some apparently use 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. If you want a proper Finnish sauna then you, your builder or both will need to read and understand what’s here.

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 I’d had when we built our sauna.

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.

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

 

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 include certain details like a lot of stones (a sauna is heated by stones and the stones by a heater) that you throw or ladle water on to control humidity, foot benches above the stones and cold zone, 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.

 

Löyly and Sisu

Löyly is a Finnish word for the environment in a sauna. It has no english equivalent and is critical to sauna. A wood lined room with benches but without löyly is not a sauna.

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 “the purity, freshness, temperature and humidity of the air in the sauna”.

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

Löyly is about quality, not so much 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 burst can be comfortably repeated.

That, all of the above combined, is Löyly.

“Löyly is about the QUALITY of the heat as much, or more perhaps, than the QUANTITY.”

Löyly is just as much about the quality of the heat, not just quantity. Quality is all those things above. And when you experience quality and get use to quality 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 together but the foot bench is below the top of the stones and so below the löyly cavity and so they have uneven temps around their body and cold feet, then 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 very tiny bit of Sisu. I was tenacious in achieving the best sauna 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, the sky is, somewhat literally, the limit. But I didn’t, I took a shortcut. I made a 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.

As Jesse Hämäläinen of Narvi Sauna Heaters said “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.

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

Many people will build a ‘North American’ 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. And if we’re happy with it that’s actually OK.

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. This happened to us.

 

Details Are Important

“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. Details can be quite important.

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 it’s been pointed out to me that there are also some poorly designed and built saunas in Finland as well).

Some things we noticed on our travels in Finland: 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 and head to toe. 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.

 

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

 

Sauna Heat Zones

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

  1. The Cold Zone is about the lower third of the space – dense heavy colder air will sink 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. This is above the top of the stones and above the cold zone and is where the primary convective loop of warm air is and this is where we want every part of of our body to be.
  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. There is another lessor convective loop of air flowing down a bit lower to the bottom of the heater but it’s only about 5% as much airflow as the upper loop.

SaunaConvectiveLoop1

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. 

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

A quick note on ‘cold’ feet. Cold is a relative term. 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 feels quite cold. It’s similar with ‘cold feet’ or ‘cold 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 notice a head to toe difference of less than 15-20°c but most people will begin to notice differences greater than that though sometimes we’ll not really think about it. Many people have said that they don’t have ‘cold feet’ in their sauna but then after raising their benches and fixing their ventilation 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.

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 above the top of the stones and in to 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
  6. Reducing direct radiant heat from the heater and stones.
  7. Reduce energy loss/costs.

 

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, Glenn Auerbach and many others for their ideas and patience in answering my endless questions and their perseverance in making sure that I not only built a proper sauna but understood sauna.

Hot Room Space – 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).  Larger (like 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 needed to maintain healthy air quality and löyly. 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.

Reduce Harsh Radiant Heat – Space between bathers and the stove, about 1 – 2m (5′) or more, is a good thing. 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: Feet Above The Stones and Staying Out Of The Cold – Bench and ceiling height above the floor is critical for a good sauna and generally 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.

The sitting bench (highest bench) should usually be about 120cm (48”) below the ceiling as this is in the best heat, comfortable and allows using a vihta. However, 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 106cm (42”) 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.

“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 four 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) since at least 2006.

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 + 135cm (54”) Sitting. Then the ceiling 112 – 120cm (44-48”) above the sitting bench.

Bench and Ceiling Heights Method 4 – Physics To The Rescue – 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 perhaps 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’ll have taller people then perhaps 50” might be OK.

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. 

If A Low Ceiling Can’t Be Avoided, Maybe Get A Huum – 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. 

One downside is that these may produce more direct radiant heat than an enclosed style heater. Here the Drop likely produces the greatest direct radiant, the Hive-Mini next and the Hive the least. However, in many cases the Drop can be furthest away from bathers to reduce direct radiant, the Hive-Mini next and the Hive likely the closest. And then of course is stone capacity – the more the better.

So, a few trade-offs to juggle. The good news though is that I think that with a lower ceiling that any of the Huums should likely result in a better sauna experience than any alternatives that I’m aware of. 

A higher ceiling and benches is still much better but if a low ceiling can’t be avoided then a Huum (and proper high to low ventilation) may help considerably.

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.

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.

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 löyly that are above the door opening largely remain in the sauna (think of a cup of air inverted in a tub of water). The larger and higher this 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 air lost. The higher the ceiling is above the door opening the less air pressure there is at the opening and so the less heat escaping.

A 7’ sauna with a 6’6” door will loose 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 loose 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 upper sitting bench should always be within 40-50” (ideally 44-48”) of the ceiling and the next 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.

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. 

A low 7’ Ceiling Does Not Save 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.

Bench Widths – In Europe the recommendation is 60cm (24”) of sitting bench AND foot bench per person. 26-30” may be more comfortable for the average overweight American. 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 Board Gaps – A minimum of about 1/6 of the seating surface should be evenly air permeable so about 5:1 maximum wood to air gap. 4:1 or 3:1 can work better. Gaps should be wide enough for good air circulation (minimum 3/8”) but not so wide (7/8” maximum?) as to be uncomfortable to sit on. Too little air permeability results in less even heat on bathers bodies and increased mold and bacteria growth.

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 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.

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.

Foot Bench vs Platform – Many saunas in Scandinavia do not really have a ‘foot bench’. Rather you go up 2 or 3 steps to the platform and then there are sitting benches around the platform. 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

Wood – The use of wood is functional more than aesthetic. First, wood is hygroscopic which helps to even out temp and humidity extremes to provide a more comfortable experience. Some wood remains cool to the touch, and importantly, softer woods absorb some noise to make for a quieter and more peaceful environment. Ideally you want about 75% of the wall surface area and all of the ceiling and benches to be soft wood.

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 better than others. Softer woods are generally 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.

Cedar is popular in the U.S. but can be too fragrant for many people and the oil in some cedar and other woods can be toxic though I’ve no idea how this plays out in a hot sauna. Personally I like the smell of cedar but find that in a sauna it detracts from the sauna experience. Some ‘cedar’ products such as fence posts and pickets are actually made from Juniper which you may want to avoid due to toxicity.

Spruce, Fir and Aspen are the most popular in Finland, Sweden and elsewhere. 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 OK though.

Abachi 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

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). 

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 but it can be. Horizontal T&G or Shiplap done properly should avoid this.

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.

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.

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.

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.

Stove Sizing – Bigger is not necessarily better. 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. Too large of a stove, particularly a cast iron or similar, may also result in too much radiant heat on bathers that can feel harsh and uncomfortable vs heat from stones and löyly. 

Too high of kW may also result in shorter heating cycles, faster heating swings and less comfort. A properly sized heater will have longer run times which results in less noticeable temp changes and greater comfort.

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 wood and a proper mass of stones in the heater. For windows add 1kW per m² (per 11 sq ft) of window surface. For stone or similar wall surfaces add 0.7kW per m² (8 sq ft). 

And in general follow the recommendations from manufacturers. 

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. 

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.

Avoid steeply vaulted, coved or similar ceilings – In theory and in experience a flat or near flat (no more than about 15cm or 6” difference at any point) ceiling is best as it results in the most even temps. A slight rounded cove 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 and reduce fresh air movement. A-Frames, Barrel saunas and similar shapes should be avoided.

If you’ll have a sloped ceiling then the higher side should be above the benches, not above the stove. As above no more than about 6” height difference from one side to the other. 

 

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.

A good sauna is heated by the stones and the stones by the heater. Nothing makes up for proper stones.

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 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.

Rough Is Good – Stones should ideally have a rough surface to help hold tiny pools of water to make better steam. Smoother or rounder stones do not do this so well (though some people do prefer the much milder steam that these create). If you want the look of rounder stones then maybe use them only for the top layers. 

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.

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 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.

 

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 – Sauna (verb) 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 is just as important an element of each round as sweating. A sauna (noun) needs to be located where bathers can quickly and easily cool off each round. Ideally with a shower in an adjacent changing room and a 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. 

With these in mind. A sauna in a bathroom is totally fine and can work well for many people but that can 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 might be better and a sauna building outside best.    

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 sauna 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

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.

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. Sauna designers in Scandinavia shoot for this to be at least 1.5x the size of the hot room if possible. So an 8’x8’ hot room for 4 people will have about 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.

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.

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.

Wood, Gas or Electric Heater?  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 which is good. 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.

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. 

Some Perspective On Temps – The Finnish Sauna Society and International Sauna Association recommend temps of 80-105°c (176-221°f) or 85-100°c at bathers heads and shoulders (thermometer or probe placed 1m (39”) above the middle of the longest bench). Others 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 95-105°c with occasional warmer or cooler sessions. 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 95-100°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 10 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 or 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.

 

Thermometers and Other Measurement Devices

Moved to: Notes on Thermometers and Other Measurements

 

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 might know a little bit about physics). 

Vapor Barrier – A solid 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. 

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. We want to minimize it as much as practically possible. Well insulated walls provide for a better, more even and more comfortable löyly 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 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 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. 

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.

BSI10 Figure 03a sauna

 

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.

Mind The Gap – If you use radiant foil or foil faced polyiso / PIR (that meets temp requirements) 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. 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 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). So:

  • Best – Exterior cladding + rain screen + exterior mineral wool (1-2” minimum) + studs and insulation + foil vapor barrier + air gap + interior T&G.
  • Better – Cladding + rain screen + studs and insulation + foil faced polyiso or PIR + 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

Some perspective – Some people in the U.S. will say that the gap is not necessary. And they’re relatively correct. The gap provides some benefit to the sauna experience and to the longevity of the sauna but these benefits are somewhat minimal and not having the gap is probably not a huge negative in most cases. However, including the gap is inexpensive, quick and easy to do. There’s really no good reason to not do it.

 

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.

 

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.

 

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.

 

EVERYONE building a sauna should read Lassi Liikkanen’s ‘Secrets of Finnish Sauna Design’. I’ve read over 20 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. (and elsewhere?) 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 principles in this e-book are for an Americanized sauna, not a Finnish Sauna.

 

Other Worthwhile Books:

The Opposite Of Cold (Nordskog & Hautala)

Cathedrals Of The Flesh (Brue)

The Sauna Is (Hillala)

Sweat (Aaland)

Sauna Magic (Conover)

 

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.

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* 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.