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

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

Everything we found in English – books, forums, Reddit, Facebook, local sauna builders, and websites – consistently recommended 7’ ceilings, benches too low, ventilation that doesn’t ventilate, high radiant heavy steel stoves, cedar a requirement, 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 and elsewhere. But when we searched in Swedish, Finnish and German we saw much different information and information that does make good sense – like for ceilings ‘higher is better’.

Why such a difference? Why do folks in the U.S. build saunas so much different from those in Finland? And why are the saunas in Finland consistently so much better?

We in the english speaking world are good at building things that we think look kind of like saunas, just not so good at building things that really are saunas. 

Something you’ll not see in Finland – a psuedo-sauna with only 2 low benches down in the cold zone when there is obviously space to have done it correctly with a higher ceiling and benches. Bathers in this sauna are down in the cold zone and not up in the good heat and steam of a löyly cavity – they will have the dreaded ‘cold feet’. There are no signs of proper (or any?) ventilation, the door is too high so valuable heat will be lost when it’s opened. Bathers in this sauna will not experience löyly. A friend in Finland called this sauna ‘anemic’.

This similarly small ≈5’ x 5.5’ x 8.4′ sauna has proper higher benches so bathers feet are above the Cold Zone – they will have more even temperatures and steam, and will not have ‘cold feet’. Note the fresh air supply for downdraft ventilation in the ceiling above the heater and thermostat sensor at head height on the back wall rather than over the heater. There is also mechanical exhaust on the back wall below the platform. This will have good löyly and be a much more comfortable and enjoyable sauna than the anemic one. The steps can be pulled out for cleaning underneath.

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

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

A typical 8’x8’x9’ 4-person family sauna with proper high benches that get bathers up in the löyly cavity. More photos here.

If you want a sauna in Finland then you hire a sauna builder and they’ll build a sauna as described here. Good sauna builders in Finland know all of this. That’s not currently the case in North America where you’re more likely to get something like the anemic sauna pictured above.

My goal is for people in North America and elsewhere to be able to experience the same things that we’ve experienced in Finland. And hopefully the best of what we’ve experienced.

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

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. Owner/builder is responsible for meeting any applicable building or related codes. I strongly encourage everyone reading this to do further research on all of these topics and in particular to read Lassi Liikkanen’s ‘Secrets of Finnish Sauna Design’ – the book that I really wish existed when we built our sauna. If you find any disagreement between what’s here and what Lassi says then Lassi’s book and website is the safer path. I also highly recommend Lassi as a consultant. 

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

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

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

What Is A Sauna?

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

There are two critical elements here;

1. What is the experience that we want to have in a sauna?
2. How do we achieve that experience?

Bather Experience. The essence of sauna is an experience where in the sauna hot room bathers are heated very evenly from head to toe, front to back and minute to minute by convective heat – hot air. Sauna is officially called a ‘hot air bath’ – see Official Definition of a Sauna in the end notes. When steam is created this steam similarly envelopes us evenly from head to toe and front to back. Here though not minute to minute as we want the steam to be temporal. We want to feel an intense steam and then have it exhausted so that we can do it again.

And we want to be able to cool off between rounds in the sauna hot room.

Our ultimate goals that we want to achieve are:

• We are heated very evenly head to toes, front to back and minute to minute by soft convective heat (and steam when created). Every inch of our body feels this heat and steam evenly.
• We can choose any temperature from 75-105°c (ideally ± 15°c) for the convective heat we feel on our bodies.
• We feel no point source, uneven or harsh radiant heat.
• We have fresh air to breath without high levels of CO2, VOC’s or odors.
• We feel no, extremely minimal, unnoticeable, air movement.
• We feel no cool drafts, even when someone opens the door.
• Humidity is returned to normal quickly after creating steam (so that we can do it again!).
• We are in a quiet, calm and relaxing environment.
• We can easily and comfortably cool off outside, with a shower, bucket of water, in a lake or in a cold plunge.

This is the thermal experience that people in Finland have told me that they have found the most enjoyable and it’s also the one that I’ve found to be the most enjoyable.

How do we achieve that bather experience? Traditionally the best and perhaps only way to meet these goals has been a well ventilated, wood lined room, heated by a mass of stones, with bathers seated well above the top of the stones in what people in Finland call the Löyly Cavity. This gives us the 7 laws of löyly.

That is still the best way to meet our sauna goals and results in the most enjoyable experience.

The high ceiling (typically 255cm or 100”) required for this is not always possible though so newer options, including open-sided heaters and air circulators, have been introduced to provide us as good an experience as possible when proper higher ceilings and benches are not possible.

These notes are focused on achieving those goals – experiencing sauna in the same way as it’s enjoyed in Finland – By creating a löyly cavity in the sauna and placing bathers in the löyly cavity. 

—–

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

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

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

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

Neither of these two nor sauna are necessarily any better or worse than the others. Saunas, IR Cabins, Sweat Cabins, Russian Banyas, Sweat Lodges, Turkish Baths and Onsens are simply different thermal experiences. For more on these different experiences see Into To Sauna in the menu above.

Löyly and Sisu

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

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

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

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

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

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

That, all of the above combined, is Löyly.

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

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

—-

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

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

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

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

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

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

How do we know if we’ve got löyly? There are two ways to get an idea of what a good sauna is; 1) Experience and Observation and 2) Physics and Measurements. 

One huge advantage that people in Finland have is a country full of good saunas to experience and a bunch of builders who know sauna. We don’t have either. Some things to try though might be:

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

A second element is Objective Measurements and while not very romantic, they are the best way to take some of the guess work out of knowing what a good sauna is and so how to build good saunas. Measurements allow us to know objectively how a sauna performs and if it at least has the capability for quality heat and löyly. Some targets to aim for might be:

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

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

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

Lost In Translation: Why Are American Saunas So Bad

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

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

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

Why the difference? Just a few decades ago:

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

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

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

We also missed when people in Finland realized that even with feet above the stones a low ceiling sauna does not work well and traditional higher ceilings and benches are needed.

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

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

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

Sauna Temperature

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

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

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

A Brief History Of Our Understanding Of Sauna Physics

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

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

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

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

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

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

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

How A Sauna Works and the 3 Heat Zones

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Steam is less dense than air and so more buoyant. When we throw water on the stones the steam will follow the convective loop but has a stronger desire to stay up high which means that it will not descend down around us as easily as air. This is perhaps the core of why Sakari Pälsi said that ‘feet above the stones’ is so critical and Lassi Liikkanen says that it’s critical, at least with closed sided heaters, for the foot bench to be well above the top of the stones. A good example of this can be found in Lassi’s article Saunologist and the boogeyman of Lakeus.

This convective loop, steam and the löyly cavity they create are critical and one of the key things that distinguishes sauna from other forms of induced sweating.

So, we have three critical zones:

1. The Cold Zone is about the lower third of the volume (and thus height in a cabin sauna) of the space – dense heavy colder air will sink (stratification) and it needs a space to sink to …and this is it. This area also has greater temperature stratification. We want to avoid any of our body being in this area. Imagine the room is a third full of cold water and you want to stay dry. This BTW is not just saunas but almost any enclosed space. In warehouses the lower third (often called ‘the cold meters’ since in a typical 6m high warehouse this is 2 meters) is dramatically colder than the upper 2/3.
2. Sauna builders in Finland call the area where the primary convective loop is Löyly Onkalo, the Löyly Cavity or Löylytasku, the Löyly Pocket. This is above the Cold Zone and often well above the top of the stones. This is where we want to be – every bit of us. It is this convective loop that warms us evenly and thanks to the convective loop there is less stratification in the Löyly Cavity than below it and steam rarely descends below this zone.
3. The area above the door opening is the Heat Cavity. This is an area that contains and preserves heat when the door is opened. Generally the larger the better. More on this later.

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

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

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

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

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

Radiant Heat (Sidebar)

For those interested in a brief deeper dive…

Radiant heat is wonderful. Feeling the sun on a cool day or a campfire at night. Dinner outside on a cool night with radiant heaters overhead makes for a very enjoyable evening.

Everything around us emits radiant heat and anything that’s colder than any received radiant heat will be heated by it.

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

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

Radiant heat declines as it spreads out so the further you are from the source the less you will be affected by it. Both of your arms may feel radiant heat from a heater but the arm closest to the heater will feel much more than the arm further away. This decline follows the inverse square law so for example, doubling the distance from the source can actually reduce radiant by ≈4x.

Key radiant sources to consider in a sauna:

Heater – This can, depending on the design, emit a lot of radiant heat, is relatively point-source and so very directional on bathers bodies and can vary considerably from one place on our body to another. There are two related issues with radiant heat from the stove; 1) it’s directly uncomfortable and perhaps more important 2) the more radiant the stove emits the less convective heat it produces.

We’ll feel radiant on our side facing the heater and not so much on any parts of our body facing away. This unevenness is uncomfortable and undesirable so we want to minimize radiant heat from the heater as much as we can with good heater design and placement far enough away from bathers (3x-8x the distance to combustibles).

Heat transfer is a zero sum game. If you increase one, you decrease others. If there’s an increase in radiant, there’s a decrease in convective/advective and conductive, and it’s this convective heat that provides the fairly even head to toes and front to back heat and steam that we want to experience in a sauna.

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

Since this radiant is so minimal, mass source and felt more evenly on our bodies, a minimal amount of this from the wall on our backs could be desirable, especially on a cold day with lower sauna temperatures.  One stated benefit to thicker log or timber construction is that the greater thermal mass of the walls produces more of this lighter and more even radiant heat on bathers backs. 

Stone, Steel or Similar. A steel or stone wall can emit a lot more radiant than a wood wall and sometimes an uncomfortable amount. In most cases these are not a huge problem but they can be so some caution is warranted.

Sauna walls need radiant heat from the stove so that they can be heated ? – This false bit of info resurfaces every few months. The walls in a sauna are heated almost exclusively by convective heat. We can see this with IR images of saunas that indicate the radiant heat from the walls track the stratification of the ambient (convective heat) temperatures – hotter near the ceiling, cooler near the floor.

If radiant was a practical factor in heating the walls then they would be quite even from floor to ceiling. Or actually the walls would be hotter at heater level that is much closer to the source – radiant heat from the heater, and cooler near the ceiling that is further away and receives much less radiant heat. If you take the ceiling off of this sauna so that the convective heat can escape then you’d begin to see the walls heated more by radiant and they would be hotter at heater level and cooler near the tops of the walls.

Radiant and Conductive heat travel equally in all directions, Convective heat stratifies (e.g., hot air rises) in fluids such as air or water.

Enough radiant to heat the walls would cause severe burns on bathers who are closer to the source. In the image above there are two small places where radiant is a significant factor in heating a surface. First is the front edge of the sitting (upper) bench on the left of the image where it is near and above the heater. The second is the front edge of the foot bench at the bottom of the image where the foot bench is only a few inches from the heater. With the latter you can also see on the foot bench how quickly radiant heat declines as it spreads out. If you place your hand between either of these and the heater it will begin to burn almost immediately. 

The really hot spot in the lower left corner is the heater guard that is being heated by radiant.

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

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

Cold Feet

If you read much about sauna or talk to people in Finland about it then you’ll hear the term Cold Feet or Cold Toes. This is how people in Finland refer to a sauna with too much heat stratification – too great a temperature difference from heat to toes. Typically more than about a 20°c head to toes (head to foot bench) temp difference is cold feet. Sometimes, even with very little stratification, if their feet on the foot bench aren’t up in the steam they might complain about Cold Toes as well.

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 our head is in perhaps 90-110°c (194-230°f) temps it can feel, relatively, ‘cold’. It’s similar with ‘cold feet’ or ‘chilly backs’. Temps at the foot bench might be 50°c (122°f) which in a normal room would be hot but if our head is in 94°c (200°f) air then our feet can feel ‘cold’ at 50°c. We won’t much notice a head to toe difference of less than about 15-20°c (27-36°f) but most people will begin to notice differences greater than that.

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

Cold feet can also make our head, and so us, seem much hotter than it really is. In a sauna with a lot of stratification resulting in cold feet we might think that 140°f at our head is really hot. But then in the same sauna but with higher benches and ceiling and so no cold feet we find that 180-220°f is much more comfortable and enjoyable than our old low bench sauna at 140°f. A core element of this is that our bodies want to always be in homeostasis – a stable equilibrium. When our head is much hotter than our toes our body prioritizes this and reacts to this heat differential rather than just heat. That is not what we want in a sauna.

One doctor also mentioned to me that such high differentials between head and toes may not be healthy for us though I’ve not been able to find any research to substantiate that.

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

If you feel like you need to wear a sauna hat then that is likely because of too much stratification – a hot head and colder feet. This is perhaps why you don’t often see people wearing sauna hats in Finland where saunas aim to have very even heat.

The Notes – Some Important Details We’ve Learned

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

Stove Room Space – Finnish official recommendation (RT Card) is overall minimum volume of 3 m³ (105 cubic feet) per person or larger. Builders have told me that if necessary a minimum of 2 m³ (70 cf) per person plus one m³ (35 cf) for the elf (technically for the heater so yes, it is important) is also possible though not ideal.  Larger (4-5 m³ per person) is better, smaller not, though beyond 4-5 m³ we start to get in to diminishing returns and wasting energy to heat not-so-necessary space. 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.

Minimum Size – About 5’ x 5’ is about the absolute minimum interior footprint you’d want. Any smaller will result in a not very good sauna that will seem great for the first year or two that it’s a novelty but is likely to not get used beyond that. I would not waste money building anything smaller. Every additional inch beyond that is valuable.

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

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

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

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