Guest post series continues. Please welcome Walker to Saunatimes. Walker has invested in his own custom sauna build as part of his new home build. As you’ll see, Walker has put a lot of thought into sauna ventilation and finding good pure air. This article is in continuation of our ventilation series. You can read more about the many opinions and drawing for venting sauna here. As well as installing vents here. Welcome Walker:
Note: While much of this is applicable to all saunas regardless of heat source, some of the following is focused on electric heated saunas. Wood fire saunas function a bit differently and need somewhat different and potentially greater ventilation. Designing ventilation to attain a healthy CO2 level for bathers is applicable to all saunas.
IMPORTANT: I am not a doctor nor do I have any specialist medical or scientific training. I am a journalist. My job is to try to make complicated things more quickly and easily digestible – distill dozens or hundreds of hours of reading, research and interviews in to a 5-minute read.
“The human body is designed to discharge 70% of its toxins through breathing … If your breathing is not operating at peak efficiency, you are not ridding yourself of toxins properly.”
– Gay Hendricks, PhD
“Badly constructed houses do for the healthy what badly constructed hospitals do for the sick. Once insure that the air in a house is stagnant, and sickness is certain to follow.”
– Florence Nightingale, 1859 Notes on Nursing
If you put a plastic bag on your head and pull it tight around your neck, what will happen? After a few seconds you’ll start to feel a bit of confusion, then you’ll find it difficult to concentrate, you may feel dizzy or lightheaded, you’ll begin feeling like you’re suffocating (you are), you’ll pass out and eventually die.
What about something a bit larger? Perhaps being in a sealed up 6’x4’x3’ clear plastic box with a good book. Same as above except it will take a bit longer for our trip to heaven.
And a poorly ventilated sauna?
Those things we experienced above; the loss of our ability to think clearly, the feeling of suffocation, dizziness – are not due to lack of oxygen but too much CO2. It is too much CO2 in our blood that signals our autonomic nerve system to cause us to take a breath. We do need oxygen as well, but there is a huge amount of oxygen in the air so we nearly always get an abundance of oxygen with every breath. It’s getting rid of excess CO2 that’s the driver. (People in a sealed room will die from too high of CO2 levels before low O2 levels will begin to cause any negative effect.)
When we breathe in the partial pressure of O2 (pO2) in the air is greater than the pO2 in our blood and so O2 is transferred from the air to our blood. Similarly, going the other way, the partial pressure of CO2 (pCO2) in our blood is greater than that of the air so a bunch of CO2 passes from our blood to the air and is exhaled. The greater the pressure differential, the greater the amount of CO2 transferred with each breath and the lower the pressure differential, the less CO2 transferred per breath. Similarly, at high altitudes the percentage of oxygen is actually the same as at lower altitudes, it’s the lower partial pressure of it that causes us to get less per breath.
Our body likes to maintain a blood CO2 level of about 23 to 29 mEq/L (the actual amount isn’t important here but I wanted to provide some context). As CO2 exceeds that range we begin to experience physical and mental problems such as loss of our ability to concentrate or control our muscles and eventually we’ll begin to feel light-headed or nauseous. So, we want it to stay in that range.
Fresh outside air is comprised of 20.96% O2 and just 0.038% CO2 which is often expressed as 380 parts per million or ppm. (CO2 was once below 300 ppm based on analysis of glacial air pockets.)
We breathe in, a bunch of CO2 gets transferred from our blood to the air in our lungs and exhaled (and dispersed, taken up by plants, converted to O2, lather rinse repeat – a very very cool system). If all is going well the CO2 in our exhaled breath is 40,000-60,000 ppm – about 100x that fresh air we sucked in. The air we exhale then is now about 15.4% oxygen, but 5.6% CO2 (my actual). We’re getting rid of a lot of CO2 with each breath. And that CO2 is going in to the sauna… And then where?
If the air we breathe in has more than 380 ppm of CO2 (higher partial pressure) then there is less of a pressure differential to our blood so less CO2 from our blood gets transferred with each breath and our blood CO2 level rises. If the CO2 levels in a sauna (or gym, classroom, bedroom or wherever) are too high then we have more difficulty eliminating excess CO2 from our blood resulting in our brain and body not functioning so well. The following chart provides some idea of what we generally experience with what levels of CO2 in the air for lower levels of CO2.
Our physical and mental abilities likely begin to decline somewhat with any ambient level over 400 ppm but we don’t really see noticeable or measurable effects until higher levels. We begin to notice cognitive impairment at about 550 ppm, decreased athletic performance at about 600 and declining motor skills at 800. Sleep quality may begin to measurably decline at 500. If you have a typical poorly ventilated U.S. sauna then at the end of your last round try threading a needle as soon as you get out (this usually becomes more difficult at about 2400 ppm).
We are still learning about CO2. For a long time it was not considered worthy of study. Twenty years ago it was of no concern for levels as high as 10,000 ppm. Six years ago we began getting some of the first research data on cognitive impairment. Today we know that an increase to just 550 ppm has a negative affect on us. And knowing how CO2 affects cognitive ability, companies with knowledge workers are investing considerable amounts to provide fresher air.
Except for most cases of death, these effects appear temporary and so perhaps of little concern other than discomfort during our sauna. However, we do not know what the long term affects of higher CO2 exposure may really be. (We do however know that people who sauna often in well ventilated saunas are healthier, less likely to suffer cardio problems or dementia and live longer.)
A poorly ventilated sauna is in this respect no different than a bag tied around our neck. If stale exhaled CO2 laden air is not removed and replenished by fresh outside air then CO2 levels (and potentially the levels of a variety of VOC’s) will rise and our blood CO2 will rise along with it. Our sauna needs to breath just as we do. It is in effect an extension of our own respiratory system.
That 6’x4’x3’ clear plastic box we’re sealed up in above? That’s how much space each person has in an average sauna.
Our first and primary ventilation goal then is removal of excess CO2.
Löyly (pronounced low-lou) is not just the steam rising from the rocks as many in North America believe but is a combination of several critical elements.
“There is no shortcut to perfect löyly, it is always about stones and proper ventilation.”
– Jesse Hämäläinen, Narvi Sauna Heaters, Finland, 2012
“Löyly is the Purity, Temperature and Moisture Content of the air contained inside the sauna as well as its thermal radiation.”
– 1988 paper on sauna health benefits
For Finn’s, and anyone in Scandinavia, Löyly is sauna. And proper ventilation is a core element of Löyly. No proper ventilation, no Löyly, not sauna. And as we saw above, for good reason.
That 1988 paper went on to say “The purity of the sauna air is above all a factor contributing to the enjoyment of the bathing experience. The sauna air must not contain any obnoxious extent gaseous impurities, particles, or micro-organisms. The purity of the sauna air is ensured primarily by effective ventilation.”
Today we know a lot more about CO2 and why proper ventilation is so critical to the enjoyment of sauna. But we also know that proper ventilation may be much more important than just the enjoyment part of sauna. And this not just for CO2 but for other impurities such as mold or bacteria that need to be dealt with. Proper ventilation is key to good sauna and this is exceedingly obvious if you read much about sauna in Finland or talk to anyone in Finland about sauna – they will always mention proper ventilation.
Steam added to stale air is just that – steam added to stale air. It is not löyly. However, if you have a foundation of good fresh air that is not stale from too much CO2, that is free of perfumes, perspiration odors and other impurities and is of the proper temperature then when you add steam produced from ladling water on the stones you have löyly.
When people in a sauna shout LÖYLY as the steam is produced it is not for the steam itself but because the steam added to the other elements of fresh pure air of proper temperature creates löyly.
U.S. Saunas Are Not Sauna
What most people in North America experience is not sauna but a warm room with bad air.
“90% of saunas in North America are bad. The other 10% are worse.”
– Board Members, Finnish Sauna Society– Mikkel Aaland
There are three areas where U.S. sauna’s seem to consistently fall short:
Ventilation: My guess is that most people in the U.S. leave a sauna not because they’ve had enough heat and löyly nor benefited from heat and löyly, but because they’re beginning to feel like they’re suffocating from high CO2 levels – and mistaking that for heat exhaustion. Little or none of the enjoyable or health benefits, no löyly, and all of the bad of being closed up in a tiny room of stale air. That is not a very enjoyable experience.
Overall Temperature: The Finnish Sauna Society recommends temps of 85-100°c at the bathers heads and at least 65°c at their feet – for health, enjoyment and hygiene. To insure this they often place thermostats for electric sauna heaters where they will best approximate the temp at users heads – typically on a wall 12” below the ceiling and 12-20” to the right or left of the heater.
Someone at Underwriter’s Laboratories in the U.S. appears to have decided that the highest temp for U.S. saunas is to be 90°c – and not at bathers heads but at the hottest point (3” below the ceiling directly over the heater). The result is maximums of 70-80°c at bather’s heads, 50-75°c at the sitting bench and 35-70°c at the foot (lower) bench. These temps are much lower than recommended for health and enjoyment. Most concerning though is that these temps are too low to provide for very critical hygiene. Saunas are breading grounds for mold and bacteria, the high heat and resulting low humidity is needed to kill them. The bad air we experience in the U.S. is not just suffocatingly high CO2 but mold and other stuff allowed to grow because of limitations on proper temperatures. Lower temps may also cause some people to stay in longer and so exacerbate our CO2 problems. In their ignorance of sauna, this person at UL is likely making our saunas actually less safe and less healthy.
It’s also important that we remember to fully warm up our sauna’s because not doing this means that we miss out on important heat radiation.
Temperature Stratification: Sauna should ideally be about the same temp from head to toe. Traditionally this has been partially accomplished by having the foot bench at or slightly above the top of the rocks. The temperature differentials are much less above the top of the rocks (12°c in mine) than below (39°c in mine). New ventilation strategies such as mechanized exhaust below the benches are also improving on this.
A major sauna manufacturer wanted me to build just such a warm room with bad air. I and my dealer (thanks Devin and Jey) had to fight them every step of the way as they wanted the benches too low, overall temps too low and a ventilation system that doesn’t ventilate well resulting in high levels of CO2.
But it’s not just this one manufacturer. These problems are pervasive throughout North America. Many people in North America build saunas with zero ventilation and most of the rest with poor ventilation.
Do we want a Finnish Sauna or American warm room with bad air? I think that for most people in the US experiencing real lõyly, good pure sauna air, for the first time will be a revelation.
A Reference Point
One problem we face in North America is that good information on sauna is difficult to come by. Not just how to build a sauna or how to take a sauna but what sauna is and what sauna should be or feel like.
Much of the knowledge of how to build a good sauna, how it should feel and how to ventilate it is contained in the heads and experience of people who build saunas – in Europe. For many it’s knowledge passed down to them from those who came before and it’s a lifetime of experiencing proper sauna embedded deep within them. They know bad sauna air when they breath it. There is little written about how to do it properly and much of that is in Finnish, Swedish and German (I’ve become intimately familiar w/ translate apps). Unless you talk to these knowledgeable people directly and really, unless they build your sauna for you (because there are a lot of details to get it right), it is extremely difficult to do it right here.
This is compounded by changes to how saunas are built. Historically sauna’s were quite leaky and breathed well on their own. Just like with our houses though, sauna’s today are much more tightly sealed up than ever before and with that we’ve sealed out a lot of natural ventilation and löyly. Planned ventilation is more important than ever before. And this is perhaps compounded further by the U.S. being behind on building/space ventilation in general.
And made worse by diagrams on the internet that show hoped for airflows that are simply impossible with actual physics.
And worse yet – our being unfamiliar with what true sauna should feel like. Our lack of a reference point embedded deep in us from a lifetime of real sauna. We think we’re experiencing sauna… Until we do.
“I took 50 saunas in 12 days in Finland, and I was never dizzy.”
Glenn Auerbach, saunatimes.com, 2019.
Those who’ve only experienced sauna in North America think that’s it. That’s their reference point. They think that the feeling of suffocation they experienced is the heat. They think it’s normal. But it’s not. It’s not normal and it’s not the heat. And they don’t know that until they experience proper sauna with proper ventilation.
Even for those of us who travel to and from Europe frequently (Ex-Covid) I think it is impossible to have a real reference point, a gauge of a sauna here vs one in Finland. After saunas there we spend hours on a plane with not so great air, we live in houses and work in offices that are poorly ventilated and have too high levels of CO2 and then we get in a sauna and the feeling of suffocation seems normal. It’s not. Glenn’s comment is about as good and accurate as we can get and he’s been experiencing and building saunas for 30 years.
Sauna in a properly ventilated sauna gets better and more refreshing with each round. That’s different than what many people in America have told me that they experience.
In considering ventilation it occurred to me that the problem is not how to ventilate, but what we’re aiming for. What are we trying to accomplish? What’s our goal?
Without objective measures I don’t believe it is possible for us to build saunas in North America properly.
What follows is not a proscription for how to ventilate a sauna. Because right now I don’t know what that is. I’ve searched and, at least in English, there is a lot more inaccurate and bad information than good. I also don’t believe there is necessarily a single best way to accomplish it. It can successfully vary from one sauna to the next and this is part of why people say that each sauna has its own soul.
Rather, this is focused on better understanding what we are trying to accomplish – what is good air? What is the purity element of löyly? And how can we know. How can we have a reference point so that we get it right? So that we don’t build a sauna and use it for five years and then experience a proper sauna and realize how bad ours had been. How can we do it correctly from the start?
Some aspects of löyly cannot be measured. Some bits come from the soul of each individual sauna. One element, moisture content, is perhaps purely personal preference. But two elements, temperature and air purity, can and should be measured. They are definitive and air quality much more so than temperature. There is considerable personal preference in temperature but air quality is either good or bad – our body doesn’t care what our personal preference or opinion is. How we accomplish them is a separate discussion. Let’s first figure out what we are trying to accomplish.
Some CO2 Measurements
Over the past 6 months I’ve done informal testing in our sauna with my GasLab meter. This is a 6 person sauna of 12.6 m3 (444 cubic ft) or about 2 m3 (74 cubic ft) per person. It’s heated by a 9kw Tylö-Helo Himalaya. This image and the one at the beginning should give you a rough idea of the setup. My meter is at 1m above the sitting bench, approximately where a friend would be sitting and what is the generally accepted standard for temp measurements. (A friend would be much more enjoyable but with Covid we must make do and the meter is it.) Measurements from the floor (black tile) are 19” to the standing platform + 18” to the foot bench + 18” to the sitting bench + 43” to the ceiling. Due to height constraints we were unable to get the foot bench to above the rocks as we’d wanted. You can see the exhaust vent in the ceiling.
1) With my current venting (traditional Finnish/European venting of 4” gap below the door, convection vent to outside in the opposite corner ceiling) and just me in the sauna – the CO2 rises from an initial 397ppm to 651 ppm after 15 minutes. At 20 minutes it was 704 and 30 minutes 783.
That was just me alone and measured at the seating positions with the best airflow. With six people in there for 20 minutes the increase in CO2 would be about 1842 ppm resulting in an ambient of 2239 ppm (and higher for seating positions farther away from the exhaust vent).
2) When I repeated this with a venting solution recommended by Tylö-Helo U.S. in their manual the CO2 levels rose faster. From 400 at the start to 873 after 20 minutes and 1082 after 30 minutes. With six people then we’d expect the rise in CO2 after 20 minutes to be 2838 for an ambient CO2 level of 3235 ppm. This by the way is nearly the same result as when I closed off venting entirely, which isn’t surprising as the design of their ventilation solution would be expected to result in no effective ventilation. Again, that is with just me in a 6 person sauna.
These levels are all much too high and will have many bathers feeling suffocated and lightheaded.
It’s also important to note that the CO2 levels will often decline slowly. If you do rounds of 15 minutes in + 15 minutes out then the CO2 level will likely not return to 400 ppm before you start your next round. It will likely be higher when you start round 2 than when you started round 1. And a bit higher yet starting round 3. And so on.
With just me alone I saw cumulative increases of about 80 ppm per round. Four of us would be an increase of about 320 ppm per round or 960 after 3 rounds. Better ventilation will significantly reduce or hopefully eliminate this cumulative effect.
For comparison, a sauna of 2 cubic meters per person, 6 air changes per hour and good air distribution/mixing (E.G., a good Finnish sauna) with bathers spending 15 minutes in and 15 minutes out for 3 rounds calculates to CO2 levels of very roughly 582, 594 and 606 ppm. I say very roughly because I don’t know what the actual mix rate would be. I assume it is very high but we need a lot more tests to know for sure. Given my experience in Finnish saunas this seems about right. I felt better in Finnish saunas (≈600 ppm CO2) than in my current sauna alone (≈700-800 ppm CO2) and both of these better than our current sauna w/ multiple people.
Do we want a Finnish Sauna or American warm room with bad air?
Goals of Sauna Ventilation
1) First and foremost, removal of excess CO2 and replacement with fresh outside air.
2) Removal of all or as much of air impurities as possible. This includes a variety of VOCs, mold & bacteria and perspiration / body odor.
3) Heat distribution within the sauna. Air is actually a poor conductor of heat and then there is the issue of convection and hot air rising. Air movement is needed to conduct the heat around the sauna and well planned ventilation can greatly lessen cold feet.
4) Removal of humidity and perspiration after the end of the days sauna.
5) Maintain fresh air during periods of non-use.
How Much Ventilation Is Needed?
Some? A lot? 15 CFM per person? 6 changes per hour?
If nobody ever told you what temp a sauna should be but only said ‘hot’, what is the likelihood you’d get it right? Or they said that you need 9kw of heat? Turn on that 9kw and leave it on? Air freshness is much more difficult to gauge than temp. I’m still surprised how often someone is told to keep windows open to reduce the CO2 levels in their bedroom at night and how surprised they are with how much better they sleep and how much better they feel the next day. Just as with our saunas, they’d no idea how bad their air was.
In addition to the amount of ventilation we also need to pay attention to the dispersion or mixing within the sauna to insure that all bathers get sufficient fresh air. We could have great air for people in a direct line between the supply and exhaust vents but poor air for those farther away. The ventilation plan recommended by my manufacturer would have resulted in much or all of the limited fresh air flowing across the floor to the exhaust vent without benefiting bathers, even with powered exhaust – plenty of air but largely useless.
An Objective Standard: 500-700 ppm CO2 For All Bathers.
How can you know if you’re being affected by healthy heat or by bad air? If you’re leaving the sauna because you’ve received max benefit from heat and löyly or because your blood is suffocating from too much CO2?
Our body’s response to bad air is fairly objective. X level of ambient CO2 results in Y level of blood CO2 and produces Z effect on our cognitive and physical systems.
What we want is fresh air to breath – so let’s measure that.
Based on what we currently know, if CO2 at bathers faces is below 500 ppm then you are likely experiencing the full benefits of sauna heat and löyly. If CO2 is above 700 ppm then you are likely experiencing bad air, not heat and löyly. It’s as simple as that.
Good ventilation that maintains healthy low levels of CO2 applies to our homes (particularly bedrooms at night), offices, gyms, restaurants and other indoor spaces as well. It may be more important in a sauna because of the heat stress our bodies are going through but it’s still important elsewhere.
Why Measure CO2?
1) CO2 is itself undesirable at levels above 500 ppm. We want to know what it is so that we can keep it low.
2) CO2 is an indicator gas for ventilation in occupied spaces. If CO2 levels are kept low then we know that we are getting relatively good ventilation, that any VOCs or impurities present are likely being exhausted and kept at healthy levels.
3) It takes the guesswork out of ventilation. Good ventilation requires exhausting stale air, supplying fresh make up air and insuring that the air throughout the sauna is being mixed well so that all bathers have good pure air Löyly. It can be complicated and easy to miss judge. We can debate this or that ventilation scheme until the cows come home but until we measure it we really have no idea. Measuring CO2 makes this much simpler, it’s absolute, no guessing – If under 500 ppm then we’re good, if over 700 ppm then we’ve work to do.
4) It’s affordable and easy to measure. (Be careful to observe the maximum operational temperatures of various measuring devices).
How Much Ventilation Is Needed?
Enough to insure that we do not get excessive build up of CO2 in our blood and experience negative effects from it. Enough to insure that other air impurities are exhausted.
Ideally then a sauna should have ventilation that results in CO2 remaining below 500 ppm but up to perhaps 700 is likely acceptable. Above 700 means that bathers are being negatively affected by bad air and the higher the CO2 level the greater the negative affect on bathers so over 700 should be avoided if possible.
Focusing on CO2 gets us to where we want to be – fresh pure healthy air for bathers – Löyly. It eliminates problems of too little ventilation or poor dispersion. If we’re maintaining proper low levels of CO2 and proper heat then the rest should be taking care of itself. There’s no need to question if this ventilation scheme or that ventilation scheme does or does not work (except for those of us who enjoy exploring those things) – if it results in maintaining appropriate levels of CO2, which in turn should be exhausting other impurities, then it works.
Realistically, staying below 500 ppm may prove difficult. We may have to find a balance between air purity and temperature.
Aiming for 6 air changes per hour or 8 CFM’s per person can likely work in many cases so long as people use the proper vent configuration to insure good dispersion. However, a clear problem we have in North America is a lot of diagrams of vent configurations that even with proper airflow will not provide sufficient fresh air to bathers. A CO2 measurement eliminates this problem.
The Analyzed Sauna
I’ve studied the effects of indoor air quality and CO2 enough to know that getting this right in our sauna will make for a much more enjoyable and beneficial experience for me and guests and be well worth the investment of time and money. To better understand what’s happening I’m hoping to embark on a bit of further analysis. The goal is to better understand heat, fresh air ventilation and how they interact in a sauna. This will involve several elements.
Temp logger and probes. I have a two port logger and probes but I’d ideally want 6 or 8 probes so that I can consistently track temps across many tests and routine sauna sessions. Test points might include; 3” below ceiling directly over heater (UL’s U.S. thermostat location), 12” below ceiling & 12-20” to side of heater (EU thermostat location), 2 or more seating locations at bather’s head height (1m above seat), foot bench, some lower point. Ventilation has the potential to create temperature problems but it can also help to distribute heat more evenly and reduce cold feet so it’s critical to monitor temps with various ventilation schemes.
Ideally I’ll have enough that they can be consistent through numerous tests and routine sauna sessions without moving any. Even so I’ll do some sessions with probes all along seating areas to better understand these differences and arrayed from ceiling to floor to better understand stratification.
These loggers are accurate to ±0.5°c which is much better than typical thermometers and thermostats that can be off by ±7°c or much more.
CO2 Logger and sensors. I’m currently using my handheld GasLab meter. It’s quite accurate but cannot log data when temps are over 60°c. And even though GasLab told me that it’s OK to use it in a sauna up to 99°c I’m a bit leary. And it’s only one measuring point.
I’m working with them on a setup of three sensors that will send data back to a logger located in a cooler environment such as the floor of the sauna or in the changing room. This will allow us to compare CO2 levels and thus fresh air ventilation at multiple locations. Mostly at head height along the seating areas but at some point also other parts of the room.
Smoke. There’s no better way to understand airflow (and lack of airflow) than smoke. I’m somewhat concerned about polluting my sauna though. That’s kind of funny when you think about the popularity of Smoke Sauna but airflow testing smoke is usually chemical based and I want to make sure I don’t create any long term odor/chemical problems so want to find a way to do this that does not leave any unpleasant or unhealthy after smell. Any ideas on this would be very welcomed. Dry Ice (CO2) sort of works but not very well.
Energy Use. This will come from IotaWatt and be correlated with various ventilation schemes. How much in energy use will better air quality cost?
CFM’s. I would like to know with some accuracy the actual CFM’s of exhaust. Measuring or logging this accurately on an ongoing basis may prove difficult given my duct layout but hopefully I can figure out something that is somewhat accurate.
Humidity. My industrial hygrometer can only work to 50°c. I may see if I can find budget to include humidity on one of the temp loggers. All of the inexpensive sauna hygrometers I’ve tried so far have been grossly inaccurate. This is, I think, very non-critical and more for curiosity. Two or three scoops of water on the rocks every so often produces a very comfortable level of humidity and I think that’s all we’re after.
Air Pressure. This is another fun to have for my own curiosity but not really necessary. Using a digital pressure differential manometer to measure pressure changes at various levels.
Maybe after Covid is over we can see about hauling some of this to other saunas to see how they perform.
Conclusion / Next Steps
Now that I’ve a good, measurable and attainable goal rather than something nebulous I can focus on doing it.
My goal is 500-600 ppm CO2 for all bathers which will make for a much more pleasant experience than the higher levels we currently have and be more similar to a properly ventilated sauna in Finland. I want to achieve this while maintaining sufficient and even temps throughout and with as little energy use as possible. So for me the next steps are some analysis to better understand sauna temps, air purity, and air movement with various ventilation schemes. Is keeping CO2 below 500 ppm a good and reasonable goal? Or is staying below 800 ppm difficult enough? What is a good goal for CO2 levels in a sauna?
I also want to better understand our physiology and how respiration works at 96°c vs 20°c and in a dry vs moist environment. Do our lungs transfer O2 and CO2 as effectively in a hot sauna? Informal sampling with an Oximeter indicates my blood oxygen is slightly lower than average after I’ve been in the sauna and slightly higher than average after cooling down. We know that this is not likely due to any changes in the amount of ambient oxygen in the air so is likely a change in how it’s transferred in my lungs. Dr. Jari Laukkanen is an expert on sauna health and I’ve a friend who’s an anesthesiologist at Mayo Clinic so hopefully more to come on this soon.
I would also like to better understand, besides bather comfort and enjoyment, what affects poor ventilation and high CO2 have specific to sauna. Does higher blood CO2 lower the affect of sauna on relaxing muscles after a workout? Or lower the affect on diminishing heart disease or dementia? Higher CO2 levels are likely to cause bathers to spend less time in the sauna due to a feeling of suffocation or dizziness, what affect does that have on health and other benefits? Or increased blood pressure, heart rate or heart rate variability?
In a sauna there is a bit greater air pressure near the ceiling than at the floor (stack effect, heat rises, etc.). Is it enough to make a difference in respiration? In theory we’d take in more O2 and eliminate less CO2, but my own blood oxygen indicates just the opposite. Does vent placement matter with this?
I want to better understand löyly. Some I can do here but mostly I require a trip to Finland this next winter! 🙂
Finally, thanks to numerous people whose knowledge, insight and good humor have improved my understanding and this article including Dr David Johnson, Risto Elomaa, Jarmo Lehtola, Kimmo Raitio, and several others.
Respiration (PDF): https://www.draeger.com/Products/Content/co2-measurement-bk-9097450-en.pdf
24 thoughts on “Sauna ventilation and Finding Good Pure Air”
Very good article! And I look forward to how you will change the ventilation to reduce CO2.
I just installed a sauna in the time of Covid and have taken into consideration ventilation.
Leaving nothing to chance in addition to vents above the heater and below the bench on the opposite wall I also installed a ‘register boost fan’ in the vent bellow the bench blowing out to control the passage of air. I am drawing air from an unheated Attic open at the eves. I am monitoring CO2 with an Amprobe CO2 meter though I hesitate leaving it in the sauna for long because of the temperature. It’s a work in progress so I don’t have any results to report yet.
Great article. I can’t wait to see what the data says for ideal ventilation setup. It really makes me wonder if I have adequate ventilation in my home sauna. For my 5×8′ ft sauna I have a 4×11″ intake vent under the heater, and another 4×11″ in the opposite corner under the top bench. There is an similar exhaust vent up top above for when I’m done, perhaps I should try opening that a little while in the sauna to see if it makes a difference.
Would you provide me with the information of the CO2 sensor you are using and where you purchased? I’d love to be sure my the CO2 levels are at/near 500ppm before I sell another sauna!
Great article!! Thanks for helping us understand!
This is certainly thought provoking. In thinking about my sauna, my perception is that there’s actually pretty good airflow in the hot room without a vent. It looks like the author might have an electric stove in an interior sauna. Mine is outdoors and wood fired. My perception is that there’s significant air flow due to stove combustion – the stove breathes too & draws air in from the hot room, expelling it up the chimney. There’s certainly a noticeable draft under the hot room door. Do I really need a dedicated vent sending hot room air outside?
My primary takeaway is around my exterior door. It’s currently a very rough, ill fitting door cobbled together from plywood & doesn’t seal at all. I think when I replace that with a ‘real’ door my fresh air intake will be reduced, which may contribute to poor stove pergormance and/or co2 buildup as described above. I’m thinking I’ll need to be mindful of allowing for enough fresh air intake – either leaving the exterior door ajar or cracking open the window.
Thanks so much for this article. Working on my first outdoor wood fired sauna, and have to say I am very confused! Vents under the seat? above the head in the opposite corner from the stove? both? How big? Finding it quite tough to find any consensus here or on the web. Is this a thing you just have to tweak after construction? We are building about a 7′ cube with the smaller finnleo wood fire stove right now, thinking about a 6″ round dryer vent size hole near the stove and again in the upper corner of the sauna – but i am now seeing drawings with the vent under the seat opposite from the stove to hopefully gain more heat in the headspace…
Any ideas most appreciated!
Nearly 20 years ago, there was a website called saunasite.com with a range of information about sauna design and building ideas. The owners then developed a related commerce site called saunanet.com .
I read through their pages and was intrigued by their discussion of issues with in-house sauna installations. Particularly with respect to electric stove heated basement saunas. Their commerce side offered a product called SaunAir a sauna ventilator. i found some pages from this defunct site on archive.org, the internet archive. aka wayback machine. while the crawls available now are incomplete, the ventilation info or saunair product presented are not currently available on any sites I know of. Their concepts seem sound.
Saunair ventilator was a modest system, temperature triggered (turns on at 50C) outside air delivery via low volume forced air ventilator – fan that would deliver approx 30 cfm from -ideally- outside the building envelope (that fig from my -possibly incorrect- multiple SI to english units conversion) piped to a ceiling vent in front of bathers faces on top laude. with the positive pressure pushing air out of the sauna room via the gap under the door. a few years ago, i went so far as to try and contact the people named/manufacturer listed in Finland for this product… to no avail.
This ventilation issue has increasing importance to me in 2021. I am preparing a basement site for an electric heated sauna in my home. I have a poured concrete floor with embedded hydronic radiant heat tubing installed and connected to my main house boiler on a separate zone. Currently waiting on delivery of a slab sensor for control, such that the floor is not cold to bathers feet, but otherwise minimally affects the air temp in the sauna/ shower locations. in the meantime, I am working up a construction materials list.
If my calculations are correct, fans designed for cooling gaming computers would provide the appropriate airflow. DIY tinkerers should be able to adapt such a fan to 3 or 4 inch dia ducting and some form of Dampered air inlet. When the sauna is hot, this positive pressure fresh outside air delivery would reduce in-sauna air stratification for bathers, making legs and feet less cool than with conventional convection patterns AND would reduce any concerns about CO2 buildup in the sauna.
I can share the screengrabs I have from this site if you can provide an email or method to post those to blog comments… or you can look for yourself on archive.org – with initial search strings: saunasite.com and saunanet.com Begin digging through pages that were grabbed around 2002. the saunair banner and pages disappeared in later years. I may have missed crawl snapshots that discussed their design concepts more fully…ones that I remembered originally reading years ago.
Walker, your article on sauna ventilation is very interesting and I eagerly await your empirical studies!
For what it may be worth:
A couple years ago, I purchased a short book entitled “The Art of Sauna Building” by Pertti Olavi Jalasjaa.
Mr. Jalasjaa recommends a home sauna with an electric heater use two 3″x10″ or 4″x12″ vents, with the inlet vent located under the heater and the exhaust vent located as far from the intake vent as possible. The “normal” height of he exhaust vent is 28″ above the intake vent. Plus, the sauna door has a 1/2″ gap at the bottom.
Food for thought!!
More on measuring…
I am working with a company on an option for getting accurate measurements in a sauna at 105°c. Hopefully more on this in a week or two.
In the meantime for those interested one alternative might be: https://www.co2meter.com/collections/data-logging-sensors/products/k33-environmental-logger-co2-sensor?variant=1075451789332 (plus flow director, pump kit, tube, filters, etc).
The trick with that is to mount the inlet of the inlet tube at head height (and about 24″ out from the wall) and then down the wall to the floor. Then have it run across the floor for a bit to give the air in the tube time to cool down. The primary issue w/ something like the CM501 is the electronics, not so much the sensor itself (though there are bounds on the sensor and temp is a component of accurate CO2 measurement). NDIR sensors are also sensitive to condensate. The K33 includes temp & RH so you can play with how hot you can get your sauna and keep the sensor itself within accurate operating range.
There are other NDIR sensor alternatives that would likely be a bit less expensive to do the same thing.
Brian, wood fired saunas do need ventilation. In Finland and elsewhere the standard is a 3″ gap under the door and then a convection exhaust vent high on the opposite wall or ceiling.
Without that vent most of the fresh air flows from wherever it enters (typically under the door but could be another supply vent) straight to the stove and out the chimney without providing much or any benefit to bathers. You need the convection vent high on the wall to exhaust stale CO2 laden air and to pull fresh air up.
My guess is that the final venting for our sauna is likely be all mini vents. A problem we’ve seen with larger supply and exhaust vents is poor mixing of the fresh incoming air and poor distribution to all bathers. A single large supply vent is just too much cooler air entering in one single place that doesn’t mix well so a lot of this air drops down to the floor and provides no benefit to bathers. Similar on the exhaust side where bathers further away from the exhaust vent don’t get as much fresh air (and so higher CO2).
Supply then will likely be a T shaped duct running up the wall behind the heater. Starting with a 2”x16” black duct from the inlet near the floor to the ceiling and then a 2×6 top of the T. The flat wide shape and color primarily so that the heater can somewhat pre-heat the fresh incoming air and help it to mix better w/ the hot air in the sauna rather than sink to the floor.
Above the rocks on the vertical portion and across the T top will be a bunch of small vent openings. Maybe 1” round, maybe slots, maybe some fun design. The goal is for the cooler fresh air to spread out as it enters the room so that not too much enters in one place. This should result in much better mixing.
The exhaust will likely be a single duct about 1’ below the foot bench running the full length with perhaps four 4” adjustable mini vents. The idea here is to kind of pull air out towards all bathers so we’ll adjust these to accomplish that.
The exhaust has a 120 CFM inline blower controlled by a home automation system. A single button press turns it on for 30 minutes, 2 presses 60, etc. It is also programmed to turn on for 20 minutes any time it has been off for 48 hrs just to keep the air in the sauna fresh when we’re not using it regularly. We’ll determine the speed/CFM’s later but I’m guessing 20 CFM’s for 1 or 2 people and 40 for 3 to 6 people.
Michael, is your lower vent mechanically powered with a blower? If not it is likely not providing any benefit. Even if it is powered the benefit is likely marginal as fresh supply air is likely flowing across your floor and then up to the vent. I would close it and use only the upper vent.
Very interesting read! I agree that the feel of the sauna and the proper ventilation are a must for a relaxing experience. Sauna sessions help in lowering the levels of cortisol in the body and reducing stress and anxiety.
Martina, can you expand on the Cortisol effect more?
I finally finished my sauna after dragging my feet for 8 months. Fired it up for the first time yesterday and all seems well…then I ran across this post and the stressing over details started :^)
My sauna is 4′ wide x 6′ long x 7′ tall. I have the heater in a bottom corner with a 5″x5″ vent directly beneath it.
The vent for outflow is in the opposite corner about 6″ below the ceiling and is a 4″ diameter circle… There is also a half inch gap along the bottom of the door to let air in.
Obviously every sauna has it’s own details – but do the sizes of the inlet and the outlet mentioned above seem sufficient for a 168 square foot sauna?
Do both if you want. If you are building it, you can unbuild it or take it away.
I have a 6×8 OD exterior sauna with electric heater. 2″ gap under the door, and 3″x10″ louvered vent behind the heater. Depending on wind direction or ambient air temperature I will open one or the other as I heat the sauna. Under my top bench in the opposite corner from the heater I have an exhuast vent, about 3×7″. Slightly above head level, also in this corner, is a higher exhaust 3″ diameter exhaust. I close the top one to initially heat, and open the lower one to get some air circulation going.
When I get into the sauna, I usually make sure both floor intakes are open unless it’s particularly windy and cold (single digits and 15mph+) then I’ll close one. The air flows from both the door and the vent to the heater. There is no cold air coming in from the under-bench vent. If I go outside I can feel the hot air coming out, but it’s not forceful, but I know air is moving. When I settle inI slide closed the lower exhaust, and then open the 3″ wide hole I have up high. Again if I go outside, the air really starts pumping out of this one, there is strong currrent.
There is a noticeable difference in my experience in the sauna if I don’t open the upper vent and only use the lower one. At some point I will feel a sudden “I need to get outta here!” suffocation thing, but with the upper vent open and the air circulation at head level, I do not get this feeling. And the temperatures stay the same.
Don’t worry to trial and error it. It doesn’t have to perfect right away. You can always change something downline. Just make sure you are getting air into it, and there is a place for air to get out of it. I made a bunch of sliding louvers so I can easily open and close and adust the openings. Options are nice! If I’m solo, maybe the top one is open 1/2. 4 people? Everything is open.
!!! For Electric Heat Sauna Builders: The mention above about Underwrite Labs and their misguided temperature limits in the North American market on the thermostat is real. I have my bulb down almost a meter down from the ceiling to get the temperatures I want at head level (95c). Keep this in mind and give yourself the room you need when placing the controller to get the thermostat where you might want it, especially if it’s distance is limited by copper tubing.
Very much on your page Christophe!
Agree with all, especially “if I don’t open the upper vent and only use the lower one. At some point I will feel a sudden “I need to get outta here!” suffocation thing”.
And also: “Don’t worry to trial and error it. It doesn’t have to perfect right away. You can always change something downline.”.
This is perhaps the best sauna building advice, for ventilation as well as other aspects, such as “should I insulate my changing room?” And very timely as I am working on a post along the lines of “Live it. Feel it. Know it. Three ways to help make your perfect sauna.”.
And i’ll go on a spiritual rampage: when we sauna and allow ourselves to sit quiet on the bench and listen to the soft voices that may present themselves, what can happen is that we aren’t the ones making the decisions. The sauna decides for us. For example, I have heard a voice something like.. ‘put a vent here you dumb ass!’
My new sauna will be placed next to my house, not touching, but close — 2-5 inches — on both sides of where the heater will be placed. The sauna will be on a suspended deck. Is there any problem with venting directly below the electric heater? (I will have an outlet vent across the room, half way up the wall.)
Great article! How are you enjoying your Himalaya heater? Do you have it hooked up with the sauna logic 2 control so you can heat up the sauna when driving home? Seems like an awesome feature to have!
Wow, Walker, what a fascinating article! I’m half way through building a backyard sauna and about ready to place vents so this is a hot topic for me. I love the direction you are going and hope you’ll keep us apprised of your findings. I think you have enough data here to do some very rough, but interesting, estimations.
At 50°C this barely counts as a sauna, so lets ignore the heating aspect and just imagine a guy in a closed room with one pipe bringing air in and another out. Looking at your blue curves, we can see the CO2 level looks like it will plateau after a while. Let’s imagine it’ll be at 1000 ppm at steady state. At that point, the fresh air coming in will have 400 ppm CO2, the stale air leaving will have 1000 ppm. We are increasing the CO2 by 600 ppm as air passes through the sauna.
You also said your actual exhale concentration is about 60,000 ppm. That tells us that the flow rate coming out of your mouth is 60,000/600 = 100 times the flow rate through the sauna. Human exhale rate is about 0.2 cfm, so air flow through the sauna is 20 cfm. If you had a 3″x10″ exhaust vent you would experience an air speed through it of 100 ft/min or 1 mph. That’s a very light breeze. That’s a lot of _really_ rough estimating but the end result is at least in the right ball park.
I think you could confirm those numbers by looking at the shape of the three curves in combination with the known volume of your sauna. It’s interesting that the first curve looks like it is going to plateau at 800, the second at 900 and the third at 1000. There’s something interesting going on there — like as CO2 builds up in your blood the amount in your breath increases.
If your goal was to keep the CO2 to 600 ppm instead of 1000, then the air passing through the sauna would pick up only 200 ppm. That means the air flow through the sauna would have to be 60,000/200 = 300 times human exhale rate or 60 cfm. You need to go from 20 cfm to 60 cfm. It’d be fairly easy to calculate how many Watts it would take to raise the extra 40 cfm from room temp to your actual sauna temp and check if your heater has the capacity and what it would cost in cents/hr. Generally speaking I think you’d need to triple the sizes of the vent openings.
Hi Glen! I know you’re super busy about to begin the Sauna Forum, something you love and I’m asking about something you hate : barrels. 1 out of 2 ain’t bad ?? Anyway. I’ve got an electric Saaku (Helo) cup 9KW.
What do you recommend being the best way to vent my the 8×7 barrel?
I’ve seen examples where two 2″ holes are drilled under the electric stove (always open) as the intake and an outtake on the high opposite wall with same holes cut but using a slider cover to control the opening.
Perhaps this will prevent the stove from shutting off at 75’C, cool, and then kick on only to have it continue this.
So….intake under the stove might stop the internal thermostat protector from this so I can get to 80-90’C.
Copy this method perhaps ??
I really appreciate any insights you have.
Cheers to Loyly!
Yea, I’m a bit hard on barrels, mainly the cheap ass versions sold online. I’ve been in barrel saunas from Russia that are great! But the insta-barrels that many order with a couple mouse clicks and “build” with a screwdriver onsite often serve a great purpose. They are a gateway drug to good sauna, as you know.
Electric will benefit from air flow next to the heater and encouragement for out vent opposite wall up higher. As standard behavior.
The best electric heated sauna set up is one where the heater is on, and elements red no more than 30% of the time. The cycling on and off should heat the rocks to a good temperature, allowing for good löyly, without the stingy toaster oven feeling in the air (and on your ears and face).
Ventilation is something that can affect air temperature, yes, but think of it like standing on a frozen Minnesota lake in January:
– Sunny, 0°f and no wind and you can be ok in a sweater.
– Sunny, 0°f and even just a 5 mile hour wind and the cold will blow right through you (and you’ll be begging for a sauna close by).
So think of ventilation (air flow) as a way for you to help enjoy the heat. Eternally even.
You can type “holy trinity” in search bar above for more. (Heat, Steam, Ventilation).
Hope this helps Kevin.
Hope you are doing great-I’d imagine so judging by your excitement in the podcasts (love them by the way!). I have left you in peace and quiet for several years now, but am finally tackling the problem spots in my sauna. Chief among them is ventilation-I’ve had condensation problems in my changing room (just tore down the ceiling, took out the insulation, going to add some ventilation (previously none) and adjust the insulation or perhaps forgo it in the changing room ceiling?).
Primarily I’m writing to you about my hot room, however. I think I should be getting better heat in my hot room, as I get very little air movement. At the moment I often leave my window (see picture) cracked when in use, but still don’t think this creates the proper air flow (often it seems to pull in air). I plan on creating a vent low in the sauna, and one perhaps at bench height and another near ceiling height (following the advice from the guest poster on saunatimes a year ago). My question is, does it matter where I create the floor vent? I realistically only have the back wall, the wall with the oven, and the wall across from the oven (which is the easiest to access, but I’m concerned about because it exits to a small niche between my house and the sauna, and I’m not sure if air needs to be moving outdoors at the location of the vent for this to function properly?). Should the vents be presumably at different spots in the room to facilitate air movement?
Thanks for any feedback or thoughts! Always greatly appreciated.
I’m sensing that you have an electric heater. I sense this as most everyone with wood fired typically has pretty good ventilation by default, with the standard crack along hot room door, air intake down low, and exhaust about a food from the ceiling.
But electric heated saunas suffer a bit as with no combustion, there is no natural draw, with exception of the “heat rises” convection action, which most often is just subtle air movement. So the “bigger hammer” option is mechanical ventilation.
For this, a simple bathroom fan installed as part of an exhaust vent below the upper bench opposite wall of the heater. And here’s the kicker: a fresh air intake vent above the heater to draw air in and pass around the heater.
This system is done with electric heater saunas in Finland.
Basically, simply, practically, and logically, a lower exhaust vent only works if integrated with mechanical ventilation.
Hey Walker an (Glenn?),
I’m taking a hybrid approach to venting and would like to discuss it with you. I’ve read your extensive post on local mile and much of what is on this site. I bought a sauna on alibaba with the intention of upgrading the heater to 10.5 kw Harvia Verta combi. It should arrive in December. I’m gathering my equipment for mechanical venting for both exhaust and inflow above the heater. I’m planning on having them both active throughout the session(warming/sauna/cool down). I’m also planning on circulating air through the bottom of the heater using USB fans on a digital controller(no outside vent at the bottom). I was hoping to get your thought on this approach?
I was thinking that pulling in colder air from the floor and pushing it through the heater (10-30 CFM?) would decrease the lower stratification and keep the heater from overheating when trying to get to higher temps +180F.
The rocks sit at 34″ with the ceiling 74″. Probably should have gone with Huum/Himalaya but I wanted more control over the steam. I’m pushing air in over the heater to get more mixing and hoping to bring the higher temps below the the rocks. To pull the air down more effectively, I was planning on using two 1″ slot plenums 48″ long about 12 inches off the floor opposite the heater. Any input from the vent mavens would be appreciated.