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 U.S. and North American sauna vendors, and that there are a lot of details that are critical to get right and easy to get wrong. Forums, Reddit, Facebook groups and websites in english consistently recommended ceilings no higher than 7’, ventilation that the laws of physics say won’t ventilate and other things that just didn’t make sense.
We traveled to Finland and Sweden to experience sauna and learn from experts. Like Glenn Auerbach did more recently, we’d 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.
This is largely my notes from 2017-2019 as we were designing and building our sauna. Unfortunately there were a few things we didn’t learn until after our sauna was finished. These have been added here and whenever possible to our sauna.
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’s book (listed at the end). And if there is anything inaccurate below please let me know so that it can be corrected.
For a semi-brief intro of Finnish Sauna and how to properly take a sauna: Intro To Sauna
What Is Sauna?
One important thing I’ve learned is that you won’t find much of a detailed definition of what is or is not 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.
What you will find is that saunas considered to be good in Finland all include certain details like a lot of stones (a sauna is heated by stones and the stones by a heater) that you throw water on to control humidity, foot benches above the stones, good 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 and the other elements 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 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 and they very often hit it.
Occasionally though, constraints force compromises – and 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 the ideal and so may be a lessor or different experience.
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.
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.
Form, Function and Löyly
What follows is 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 an 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 for a sauna though.
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.
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 sauna in Finland that I realized just how bad saunas in the U.S. are. Glenn Auerbach thought he knew good sauna 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 with bad ventilation and low benches that result in 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 later learn that it’s inadequate, it’s not the Finnish sauna that we thought, that there are much better options. 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 and a love of mediocrity and don’t-sweat-the-details. 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 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 a desire for fresh air is normal for sauna, but it’s not. It’s 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 a lot of 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 nearly always above the stones. 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 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
Whether buying a kit, building from scratch or having someone build it for you, here are some critical things to look for (somewhat in order of importance). 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.
– 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.
– Vestibule, often a changing room and shower, to provide an air lock.
– Proper vapor barrier and insulation.
The first two are nearly non-negotiable items – falling short on these may have a dramatic affect on your sauna experience. All however can be cheated somewhat if necessary but may result in a lessor experience.
If buying a pre-cut also be cautious of people capacity. Some manufacturers will advertise a greater capacity than the sauna can actually accommodate.
I’d strongly recommend against any barrel less then 8-10’ in diameter. More below.
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, Christopher Wegscheid 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 or cubic meters (70 cf) per person plus one (35 cf) for the elf. Larger is better, smaller not. A good recommendation from Lassi Liikkanen for a four person sauna is an 8’x8’ interior floor space and 8’4” interior height resulting in 130cf/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 and provides a good heat cavity.
Reduce Harsh Radiant Heat – Space between bathers and the stove, perhaps about 1.5 – 2m (5′) or more, is a good thing. While the radiant heat from a ceiling heater will feel quite nice when it’s 45°f outside, or IR in a 120°f IR cabin, radiant can feel harsh in the higher temps of a sauna. Many people find it more comfortable to be warmed evenly by soft löyly than unevenly by harsh heat radiating directly off the heater. This is also one reason why having the foot bench above the stones is important.
Bench and Ceiling Height Are First Determined By The Stones – Temps are much more consistent, stable and comfortable above the top of the stones than below.
START with setting the foot bench at or somewhat above the top of the stones. 4” (10cm) above is good, 12” (30cm) is better. The sitting bench then is 17-18” (45cm) above the foot bench and the ceiling 44-48” above the sitting bench. Someone sitting upright on the sitting bench with their feet on the foot bench then have their entire body above the stones and have room to use a vihta.
If you’re building outside then you’re good to go. If inside and this is higher than your maximum ceiling height then you can play with things to get the best outcome. A heater that can be lower perhaps or tighten up the heights.
There is a misguided belief in the U.S. that sauna ceilings should not be higher than 7’. This is quite incorrect. This chart shows one reason why Finns say it is so important to have the foot bench above the stones and why a higher ceiling is often better than a lower ceiling. The higher you are in the space the less of a head to toe difference you’ll experience and the more enjoyable your sauna will be.
Side Note: This 7’ myth likely originated with the energy crisis of the 1980’s, a misunderstanding of sauna and physics, and nobody ever questioning it. In a typical room where we stand on the floor and want to remain comfortable at body level then a higher ceiling requires considerable extra heat above our body to remain comfortable 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 body temp. A sauna is quite different. Since we are keeping our body in the upper part of the space then any additional space is added in the lower portion below us and so lower temps. Regardless of how high the ceiling is we’ll still want the ceiling to be about 200°f. And though much higher temps we are only heating a sauna for a portion of each day/month/year rather than constantly. While a 7’ sauna may cost $1.00 / hr to maintain temps, a 9’ sauna will cost about $1.07 / hr to maintain much more comfortable temps.
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 or outside temp…). If the temps are 100°c top and 40°c bottom in a 7’ high sauna then they will be the same in a 9’ or 10’ high sauna. Raising a 7′ ceiling by perhaps 16” to 8’4” for instance allows bathers to also be 16” higher and so not just have greater warmth but perhaps more importantly have less of a head to toe temperature differential and so less likely to have cold feet. That their feet are above the stones helps more.
Ventilation with fresh air supply below the heater that pulls colder air upwards will make stratification and cold feet worse. Good high to low ventilation with supply above the heater will lessen stratification resulting in a more pleasant and comfortable experience. Do not count on ventilation or air movement to overcome too low of benches though – it can help but rarely overcome stratification.
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?
Feet above the stones also increases heat felt from löyly and decreases radiant heat from the stove. The former can be more comfortable and less harsh than the latter. This is particularly important for bathers closer to the stove.
Sauna builders in Finland will sometimes refer to the area above the stones as löyly onkalo – ‘The Löyly Cavity’. It’s where löyly is and it’s where we want to be.
About the lower third of any sauna, regardless of ceiling height, will be rather cool so many builders will also try to keep the foot bench above this point if possible.
A ceiling at 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” would be good.
With open sided tower or mesh style heaters such as the Harvia Cilindro or Tylo Himalaya you can sort of get away with cheating the foot bench down a few inches if necessary, like maybe 4” below the top of the stones. It won’t be as comfortable as being above but with proper high to low ventilation it can be acceptable.
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.
Large Heat Cavity Above The Door – Every time the door is opened some heat and löyly escape and it’s the heat from the top of the door opening down where this happens. Heat and löyly that are above the door opening remain in the sauna. For the most part 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 lost.
A 7’ sauna with a 6’6” door will loose 190°f air while protecting only about 6% while a 9’ sauna with a 6’6” door will loose 181°f air 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. In saunas that will have many people coming and going the builder will try to have the foot bench and often the platform above the top of the door (so a platform at about 6’6” and ceiling at 12’ vs changing room floor).
Bench Widths – Provide 2’ or 60cm of sitting bench AND foot bench per person. 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 Bench to Lay On – At least one sitting bench should be at least 76-80” 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 to lay down on. Lassi recommends as deep at 40” for the top sitting bench which is a good idea if you have space. If you have an L shaped bench then one 28” and one 40” would likely be ideal. If space is tight then narrowing the foot bench is the place to start.
Bench Board Gaps – A minimum of about 1/5 of the seating surface should be 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. A larger gap of about 1-2” at the wall that allows better airflow behind bathers can help keep bathers backs warmer (though shouldn’t be so large that all of the air flows there and none to bathers front). If the faces of the benches are not open then these should be at least 30% permeable (3:1 wood to gap).
Wood – Wood is hygroscopic which helps to even out temp and humidity extremes to provide a more comfortable experience. It also remains cool to the touch and absorbs 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 to be soft wood.
Almost any wood can be used though some are better than others. 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. Spruce, Fir and Aspen seem the most popular in Finland and Sweden. Maybe avoid woods like pine with too much sap. Be careful of knots, especially for benches, platforms and backrests, as a knot with sap or pitch can get hotter to touch than surrounding wood and knots can pop out with hot/cold cycles.
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 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.
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.
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 be deposited on to the foot bench. Duckboards actually seemed quite rare in Europe 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 smaller stove 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. The EU recommendations from EU manufacturers should be good. Pay attention though to things like very large windows that may result in excess heat loss and require a larger stove. The more stones the better and more stones help to smooth out the temp swings. A rough guesstimation is about 1.0 – 1.5kW per m³.
Hygiene requires higher temps – It’s critical that after a sauna has completed its duties for the day that heat is used to kill off 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 many people place the thermostat much lower 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 elsewhere 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 two problems;
1) Actual temps for bathers, at their heads and shoulders, are then only about 60-80°c which is well below the 85-100°c temps recommended by the Finnish and Int’l Sauna Societies.
2) Most critically this results in much too low of temps at the sitting and foot benches for good hygiene. Higher temps are needed to kill bacteria and prevent mold growth and UL labs prevents this.
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. 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 heater, HL probe or both 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 probe 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.
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.
Maybe avoid vaulted, coved or similar ceilings – In theory and in experience a flat or near flat 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 usually be avoided though.
More Is Better. MINIMUM 6kg / m³ (1/3 lb / cf) of space is a good starting target though several sauna builders I’ve talked with say 8kg / m³ is the minimum. More is better so 10-12kg / m³ is better. Beyond about 16kg / m³ is 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. A good sauna is heated by the stones and the stones by the heater. Nothing makes up for proper stones.
For perspective, a smoke sauna in Finland will have about 90kg of stones per m³.
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.
Harvia has a good discussion on stones here.
“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
The primary goal of ventilation in sauna is removal of exhaled CO2 (and to supply combustion air for a wood stove if needed). High levels of CO2, common in U.S. saunas, make saunas seem stuffy and cause bathers to exit because they need fresh (low CO2) air rather than because they’ve received the benefits of heat and löyly. High CO2 levels can also significantly reduce the benefits of muscle recovery after a workout. As well, we need to remove other impurities (both gaseous and particulate matter from throwing water on the stones) from the air and circulate air to achieve as even and comfortable of heat as possible. Lassi Liikkanen also points out that it’s critical to remove excess humidity during the sauna – a proper sauna round includes cycles of high humidity followed by high heat and without ventilation we just get ever increasing humidity.
Our goal with ventilation is Löyly. Steam added to bad stale air is just that, steam added to bad stale air, not löyly.
Beyond make-up-air for the stale CO2 laden air that’s being removed, there is no specific need to bring in oxygen, there is plenty of oxygen in the air – it would be nearly impossible to run out of oxygen in a sauna even with high sauna temps. The problem in saunas is high levels of CO2 that result in feelings of fatigue, suffocation, brain fog or dizziness.
The following applies to electrically heated saunas and most wood stove saunas that are loaded and receive combustion air from outside of the sauna hot room (typically the changing room). Wood stove saunas with loading and combustion from within the hot room have some slightly different criteria.
Convection doesn’t work – If you have an electrically heated sauna you need mechanical ventilation using an electrically powered duct blower or similar. 1) Convection, even when working well, rarely produces sufficient airflow for a healthy environment and often zero airflow. 2) Convection relies on colder fresh air entering near the floor and being pulled upwards towards bathers as hot air exits higher up which results in greater temperature stratification, cold feet and cool legs. 3) Convection is unreliable and changes with wind direction and speed, temp, humidity and barometric pressure.
Powered exhaust has a number of benefits including downward airflow that helps to reduce stratification and cold feet and does a better job of removing CO2 than upward airflow, consistent and predictable airflow regardless of outside conditions, and the ability to have it freshen the air once a day when the sauna is not in use.
15-25 CFM per person – We want to keep CO2 levels for bathers below 700 ppm and ideally below 550 ppm (or no more than 300 ppm above outside ambient levels and ideally no more than 150 ppm above outside ambient). A general recommendation is a minimum of 15 CFM (25 m³ / hr) per person though DIN1946 says that 18 CFM are needed. Due to higher levels of CO2 in exhaled breath gyms often require 20-25 CFM per person however so sauna’s may similarly need somewhat more than 15-18 CFM but this needs more study. It’s important to note that too much ventilation can lessen löyly by removing too much moisture so the amount may need careful balancing.
Fresh air supply ABOVE the heater – A fresh air supply below an electric heater results in this cooler air flowing across the floor (hot air rises, cold air sinks) and making a direct path to the exhaust vent rather than providing much or any fresh air benefit to bathers. The exhaust vent is exhausting the fresh air rather than the stale CO2 laden air. This also results in greater temperature stratification and colder feet.
Air needs to enter in a way that it sufficiently mixes with the hot air and circulates up to bathers heads. A single vent above the stones should usually work fairly well. Better would be several smaller vents that result in a lower cold-air to hot-air ratio and so better mixing.
However… If the lower part of the heater is in a bit of a chamber that has the fresh air supply as well as a somewhat restricted room supply so that the fresh supply air cannot so easily flow across the floor to the exhaust vent and is forced to mix with the room air and rise up through the heater, then a lower supply vent might work.
Note: Heaters sold in North America may require a lower vent to keep the overheat sensor (high limit switch) from nuisance tripping. In this case a dual vent system of a lower supply vent for the heater and an upper supply vent for ventilation may be required. The key will be to have only enough air to the heater for it to function properly and the rest from above the heater for bather ventilation. It’s not currently known why this is a problem for North American heaters and not for those sold elsewhere.
We are modifying the vent system in our electrically heated sauna so that we can try various combinations of supply and exhaust over the next few months (winter 2021-2022) to see what works best for maintaining good air quality (low CO2), comfortable temps from head to toe and so … Good Löyly. My guess is that a T shaped duct over the stove with a few small vents in the upright portion and a few in the top cross portion will be the winner. And similarly a horizontal duct below the foot bench with several small exhaust vents spread across it.
Mechanical exhaust below the foot bench – This has two benefits. 1) The rising flow from the fresh air supply above the sauna stones will then flow downwards across bathers faces and carry exhaled CO2 down (CO2 is heavy so wants to sink) to the exhaust vent. 2) This helps to pull warmer air downwards to lessen temp stratification and keep legs and feet warmer and more comfortable.
This exhaust vent should usually be on the wall opposite the stove and supply vent. Multiple smaller vents spread out along the wall below the bench may improve mixing and thus bather comfort.
Information in English is Wrong. Unfortunately, information available in English regarding saunas is very often bad advice. Bringing in supply air below the heater is, from a bather ventilation perspective, a bad idea.
Because physics doesn’t work like that. In reality the majority of the air will not flow up, but down across the floor – it’s cold air, it likes being down low. Cold air is kind of like pouring BB’s or pellets out of the vent. Or think about how a dry ice machine works. A very tiny amount may flow in to the heater and some will rise up to envelope feet with cool air (and that’s not comfortable during sauna nor does it provide for proper heat afterwards to reduce bacteria and mold growth).
This is likely worse, if it can be, with open sided heaters as the convective loop for these is likely higher up resulting in even less air entering below the heater being entrained and rising up through the heater.
There are also recommendations to place the exhaust vent up higher, near or in the ceiling, particularly when using natural convection, but this doesn’t work well for ventilation either and, as Lassi Liikkanen correctly points out “we’ll be losing some or all of the precious löyly”.
Information in Finnish (and Swedish and German) says to do this (for good reason):
EVERY sauna builder I’ve talked to in Finland and Sweden says to always have the supply above the heater and mechanical exhaust below the foot bench.
The Technical Research Centre of Finland (VTT) conducted research on sauna ventilation in 1991-1992. They confirmed what many sauna builders in Scandinavia already knew and practiced. The following graphic from their research does a good job of summing up how different supply vents function.
T4 is the ONLY location that resulted in proper ventilation. T3 worked semi well but also caused somewhat cold feet. T2 and T1 provided near zero ventilation and resulted in cold feet.
Further research (CFD simulation on the air flow in a sauna – thanks Lassi Liikkanen) provided both confirmation as well as greater detail on what’s happening including the effects on temperature stratification within the sauna.
I’ve done some testing in our sauna. A fresh supply above the heater with mechanical exhaust low on the opposite wall results in acceptable CO2 levels. Supply below the heater with exhaust 24” above the floor as recommended by the manufacturer or our heater results in levels of over 1000 ppm with just me alone in a six person sauna and over 3000 ppm with six people. Acceptable is 550, 600 or 700 (depending on whose standard and down from prior maximums of 1000, 2500 and 10,000 as we’ve learned more about the harms of CO2).
So…, I know EVERYTHING in English says to put the supply vent below the heater (or behind the heater). Even TylöHelo, FinnLeo, Amerec, Huum, Harvia and other heater vendors say to do this in their North American manuals (when their other manuals get it right). But it doesn’t work. Physics says it won’t work and research proves it doesn’t work. The result is unhealthy and potentially dangerous levels of CO2 in North American saunas (besides experiences that fall far short of a proper sauna experience).
Also, CO2 is heavier than air and so naturally wants to sink. Ventilation flowing from high to low works efficiently with this but ventilation flowing from floor to ceiling is going against this and may not result in as much CO2 being removed per volume of airflow.
Note for North America: Electric heater manufacturers in North America recommend supply air from below the heater rather than above as the rest of the world recommends. As this is known to provide poor ventilation and result in high levels of CO2 for occupants the only reasonable reason for recommending this is to cool the heater itself. There are only two reasons I can think of why this would be; 1) A UL requirement that the heater surfaces not get over some maximum temp or 2) The heaters or some portion of them are lower quality than those sold in Europe (that do not appear to need this cooling air) and so need the air to cool them.
Electric sauna heaters sold in North America often include a High Limit Switch or Over Heat Protection switch with a quite low trip temp. Perhaps another UL requirement though I have been unable to verify that. A supply of cool air may be needed to cool the probe (typically on the back side of the heater) or the heater itself to keep the HL switch from tripping. Comments from installers and problems they’ve seen also elude to U.S. heaters being made to lower quality standards than those elsewhere.
A possible solution is a dual supply vent; one low to cool the heater/probe and one high to provide ventilation for occupants (and this is likely the best for wood fired as well but more on that later). Ideally you want most coming from the upper vent to provide ventilation for bathers so the lower vent should be adjustable so that it provides just enough for what the heater needs to keep from overheating with the rest coming from the upper vent.
5 Effective CFM is better than 20 Ineffective CFM – If your heater has difficulty keeping up with cold fresh supply air then reduce the flow rather than move the vent lower. 5 CFM entering above the heater that helps to reduce CO2 is much more beneficial than 20 CFM entering below the heater that does not reduce CO2 levels and adds to cold feet.
Multiple Smaller Vents Might Be Best – Ten 2” supply vents above the stones might work better than a single 6” as it will result in better mixing (better hot:cold ratio so less of the colder supply air will sink to the floor). Similarly four 3” exhaust vents spread out below the foot bench might do a better job of removing excess CO2 for all bathers than a single 6” that might work well for those directly above but not so well for those further away.
Maybe Pre-Warm The Supply Air – Bringing supply air in to the hot room below the heater and then having a metal duct run up the wall behind the heater to nearer the ceiling where it is exhausted will pre-warm the supply air and may result in better mixing, better CO2 removal and a more comfortable sauna experience. This isn’t critical but something to consider. It may also be possible to capture some rising heated air from the heater to mix with this colder supply air before it’s supplied to the room which would result in better overall mixing.
Clearing Exhaust in/near Ceiling? – (Maybe) Include a clearing exhaust in or near the ceiling opposite from the stove and supply air. After the day is done this exhaust can be opened (with the stove still heating) with the blower running for about 15-30 minutes to clear the sauna of accumulated moisture which will help with keeping mold and bacteria at bay. HOWEVER, I’m not sure that this extra vent is necessary. If you have a supply vent over the heater and mechanical exhaust on the opposite side below the foot bench then when you’re done with your sauna for the day you should be able to simply leave it going (heat and ventilation) for 15-30 minutes and get the same result. It should effectively exhaust excess moisture and, assuming at least 55°c at the foot bench, kill most bacteria and mold. This needs more study.
Duct Design – The smaller the duct the more noise from airflow (and the more static pressure) so somewhat larger is better. 4” round for up to 50 CFM, 6” for up to 140 is good. Make sure that the blower and nearby duct is mounted w/ perf strap or isolation hangers and that neither the duct nor blower contact any framing to prevent vibration noise from coming inside your sauna. A silencer (such as from Fantech) installed between your vents and blower can reduce noise a bit more.
Use Hard Duct – Flex duct results in high static pressure, often cannot tolerate the heat of a sauna and should not be used for exhaust ducts. Stretched tight and installed properly, it’s OK for supply air (and may be a good idea in really cold environments as insulated flex doesn’t have the condensation problems that metal duct does).
Blower Size – Blowers are typically rated for how many CFM or m³/hr they deliver. The marketing material will typically only publish what is called free air flow which is the blower without any ducting or wall caps that can reduce airflow. These things that reduce airflow are called Static Pressure. If you are familiar with Static Pressure then you should calculate the static pressure losses in your fresh air supply and exhaust ducts and choose a fan that delivers the desired CFM @ xx” Wg. However, in most cases you’d likely be safe to simply choose a blower that has 150-200% of your desired airflow. A variable speed blower and controller are highly recommended.
Control – Controlling the exhaust blower with something like a Lutron Casetta provides a number of benefits. First is that it will allow for speed control of the blower so that ventilation can be adjusted to balance CO2 removal and heat. It can have a timer to automatically turn ventilation off a certain amount of time (20 minutes?) after sauna is done for the day. It can be programmed to turn on for some bit of time each day (20 minutes every morning at 7a ?) to prevent musty stale air when the sauna is not in use. On the latter it’s best if this can be done with a smart system of some sort so that it only does this automatic ventilation when supply air humidity will not be too high.
Measurement – Ideally we want to keep CO2 at bathers faces below 700 parts per million and ideally below 550 ppm. Measuring is difficult because CO2 meters don’t work well at sauna temps. We are working with a company on a solution but it’s still a ways out. In the interim there are a couple of options if you want to see how your own sauna does. First is to place a CO2 meter somewhere that temps remain below the max temp for the device (typically 60°c). In our sauna that’s on the platform below the foot bench. Many or most home devices do not provide accurate readings however those from CO2meter.com, IQ Air, and Awair (version 2 or later) have proven reliable. Avoid Foobot. Note that the actual CO2 level at bathers faces is generally a bit higher so adding 10-20% to readings might be good.
The second alternative is to heat your sauna to a temp that is safe for your CO2 meter to be closer in proximity to bathers faces and then doing three rounds of ‘cool sauna’. It’s important to find friends willing to do this so that you have as many people as you’d normally have. CO2 is multiplicative so 4 people exhale about 4x as much in to the room as one person.
For some discussion on why U.S. heaters are different/inferior: Why Does Tylö-Helo Recommend Different Ventilation In The U.S. Than Anywhere Else?
A Shower Is Important – A shower directly adjacent and without having to go outside in cold weather is almost critical. It’s important to shower (and dry off) before first entering sauna and a cool shower is often a good way to cool down after each round. Rinsing sweat off before going outside in cold weather is not a luxury. 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 non-winter environs something as simple as a hose (or pre-made hose shower) on an outside wall can work.
A Window on the world – Being able to see outside while in sauna is quite enjoyable.
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.
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.
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.
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). We want to minimize this as much as 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.
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 (0.5 – 0.75”) 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, 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 perhaps 5-20° warmer and provide a more comfortable experience. Details on this in Lassi’s book below.
- Best – 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
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 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 such as a shower or going outside in winter. Ideal is a door to outside directly from the changing room.
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. 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 75°c for 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 70-90°c.
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.
Avoid Barrels Under 8-10’ In Diameter – Like really. Do not buy or build one. They do not provide a good sauna experience. Bathers often do not experience sauna temps (consistent 80-100°c at head and shoulders, no more than 15°c cooler at feet) and uncomfortably cold feet and chilly backs are difficult or impossible to avoid. This is very much a bigger is better thing. 6’ is awful, 7’ is minimally better, 8’ is better still, 9’ doable and 10’ can almost or maybe even provide an experience similar to a proper sauna.
In theory the shape is supposed to make the heat roll evenly around bathers, but physics, stratification of heat, doesn’t actually work like that. I’ve a growing collection of photos of people’s attempts at alleviating some of the problems of barrels with boxes on top benches, intricate fan arrangements, tents as vestibules and other stuff. Some of the problems include;
Too Low Of Benches – The primary problem is cold feet but often also cold legs and not so hot bodies. The good heat and löyly is above bathers heads in barrels and bathers are down in the cold area (see heat stratification chart above). Feet should ideally be no more than 15°c / 27°f cooler than our head and generally never cooler than about 60°c / 140°f and these are impossible in most barrels. Good sauna builders will try to avoid having the foot bench, and so any part of bathers bodies, in the lower 1/3 of the space since this area is always too cool.
Too Little Volume Per Bather – The volume of space per person in a barrel is typically much less than recommended which when the door is closed can result in CO2 levels increasing quickly and to quite high levels. High ventilation rates (assuming good dispersion/mixing) can help with this but in such a small space, even with an adequate heater, often results in cold airflow on bathers, particularly around their feet. Each time the door opens helps alleviate the high CO2 but also brings in colder air and allows heat and löyly to escape.
One popular 4 person barrel that is 6’ (71”) in diameter by 5.3’ (64”) in length has 37 cubic feet per person which is less than half the minimum of +70 cubic feet per person recommended and about a quarter the +120 cubic feet considered ideal so CO2 levels will increase two to four times as fast in a barrel.
No Heat Cavity Above The Door – A good sauna has a large cavity above the door to store heat and löyly. Without this a significant amount of heat and löyly escapes each time the door is opened which is uncomfortable for bathers, takes considerable time to reheat and is a waste of energy. This is worse in barrels as the shape effectively channels the heat up and out of the door. It’s like someone designed it to evacuate valuable heat and löyly as quickly and efficiently as possible.
Direct Radiant Heat – Ideally in a sauna you don’t want any direct radiant heat, you want to be heated evenly all over by soft löyly. This is one reason for ‘Feet above the stones’. Bathers in a barrel experience significant direct radiant heat that leaves them hot on the side facing the heater and cooler on the side away from it (which is also the side that get’s blasted with cool air when the door opens). Glenn Aurbach describes it as like being a hot dog – you roast one side and then move over to the other bench to roast the other.
No Vestibule – The lack of a vestibule is much more critical with barrels than with proper saunas since a door opening to outside cold air in a barrel results in much greater and faster heat loss. And similarly, since heat is escaping so much more quickly, cold air is entering down below much more quickly.
No Insulation – The lack of insulation not only results in greater energy loss and higher electrical costs but also to less bather comfort. The walls of barrels are sucking heat out much faster than saunas with proper insulation which results in chilly backs for bathers. The few barrels I’ve seen in Scandinavia have been built from heavier 4-7” thick timbers which provide much better insulation.
Bacteria, Mold & Other Fungi – The foot bench (floor in a barrel) in barrel saunas rarely (actually never) maintains the 65°c / 150°f temps necessary to kill most mold and bacteria so growth of these is a common problem in barrels.
Low Cost May Be Misleading – Many barrel owners find that they need to build roofs over their barrels to prevent leaking, insulate them, add a vestibule, build new higher benches, add better ventilation and other stuff to try to mitigate some of the problems inherent in barrels. Uninsulated electric barrels likely cost a bit more in electricity due to lack of insulation and air sealing. Some or many barrels may not have a very long lifespan and will need to be replaced (or possibly just rebuilt) sooner. These additional costs could quickly make a barrel as expensive as a quite nice proper sauna building.
A large barrel, 3m or 10’ in diameter, that has the foot benches above the stones, a heat cavity above the door, bathers heads not too far below the ceiling and good ventilation can work but barrels in North America are not built like this.
More: 10 Things About Barrel Saunas and Why We Don’t Build Barrel Saunas.
Why Do So Many People Like Barrels? One is attractiveness – they’re cute. Second is perceived lower cost. Barrels can sometimes be placed nearer to property lines than a stick built sauna which is a significant advantage in smaller yards. A stick built might require a permit while a barrel not and a barrel might be considered temporary rather than a permanent structure.
The big one though is that we don’t know what we don’t know. We lack a good reference for what good sauna is. We think the cold feet and poor air quality in a barrel is normal. It’s not. People who have barrels and never experience anything better are often quite happy with their barrel. Especially if they live in a more temperate climate. And that’s rather good. And some people are just not very picky so a poor barrel is just as good for them as the best sauna in Finland. And that’s good for them. I’ve a friend who thinks freeze dried coffee is just as good as fresh ground drip and I’m happy for him. However, many barrel owners, once they experience real sauna, realize what they’ve been missing, what real sauna is like, quickly become dissatisfied and want a real sauna.
Improving Barrels – If you already have a Barrel. Raise the benches as much as possible and then raise the floor (that’s acting as a foot bench) to 16-18” below the benches. Moving the benches in towards the center a bit may allow a bit more head height for raising them. Normally you’d want the foot bench above the stones and sitting benches 18” above that but here that will be difficult or impossible so as high as practicable for each is the best we can do. Going a bit less then 18” bench heights will bring the floor and feet up higher and in this case the benefit of that will likely outweigh the slightly less comfortable shorter bench height. Making some 4” or so foot stools for people to use might help as well. Be careful about floor to heater clearances though.
In electrically heated saunas a fresh supply vent below the heater and exhaust higher up is a recipe for a not so good experience – it results in colder feet and does little to remove the CO2 that people are breathing. Instead, try 4 small 2″ – 2.5” fresh air supply holes above the heater (aligned vertical or slightly staggered, beginning about 2/3 of the way from the top of the stones to the ceiling and evenly spaced). And then powered exhaust below the benches (or ideally below the floor/foot bench) on the opposite end. Figure about 15-20 CFM per person (so 60-80 CFM for a 4 person sauna). This should help to even the temps out a bit, lessen cold feet and will do a better job of removing CO2.
A bit of wood across the top of the door opening that creates a bit of a löyly cavity above the door opening may help to preserve heat and löyly when the door opens. And since heat isn’t escaping as fast out of the top, then hopefully less cold air will come in near bathers feet. This might be particularly useful if people are coming and going at different times. Personally I don’t know that I’d do this (I probably would if I had a barrel) out of concern for people bumping their heads but thought I’d mention it.
Adding a vestibule of some sort to act as an air lock would prove quite beneficial, though at some point it’s just best to jump straight to building a proper sauna.
Sauna Ventilation – Finding Good Pure Air
Sauna Times Blog
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).
Other Worthwhile Books:
The Opposite Of Cold (Nordskog & Hautala)
Cathedrals Of The Flesh (Brue)
The Sauna Is (Hillala)
Sauna Magic (Conover)
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.