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What is the best venting system for your sauna building?

Bob's sauna loft, showing ridge vent for good ventilation

Creating the best venting system for your sauna building is critical. We are happy to visit with a professional builder who is finely tuned to air circulation and proper venting. Enter Bob:

Bob Daigle, Casco Maine

We’ve recently completed the construction of our outdoor / stand alone Sauna.  In the process of thanking Glenn for his book & insights, he asked me to explain a bit about our construction technique. In particular, what the air circulation strategy we used.

Full disclosure, we built our own home a few years ago. I have worked professionally as a builder / carpenter, so I went into this with a “build a very tight small house” mindset.  Our own home is ICF construction and is incredibly tight.  This saves us on heating, but also has serious ventilation consequences.  To address this in our home we use a whole house ERV to swap indoor air with outdoor air, replacing the air in our home roughly 8x per day.  We monitor our indoor house environment with a Netatmo device (highly recommend). This device measures humidity, temperature, noise, and C02.  

Bob's well vented authentic wood burning sauna venting system for your sauna building
Bob’s completed sauna (with optimal ventilation)

I’ll admit, I’m a bit consumed with indoor air quality and ventilation :), and here is the best venting system for your sauna building

Given this experience, from the start of this sauna project, we were thinking quite a lot about air flow & ventilation.  This would be a small building with one extremely tight room (Sauna Room) and one adjoining room with less need for “tightness” (Changing Room). The Sauna room changes temperature radically. Further, it introduces large volumes of moisture into the entire building envelope.  To me, this combination spelled potential problems, unless proper ventilation was built into the design.

Interior of Bob's sauna with a venting system for your sauna building
Bob’s sauna room, lots of heat. Lots of moisture. But well designed and capable of handling it all.

Our solution was to build the structure using the same ventilation techniques we’ve used on houses

The first step was to insulate the walls & ceilings using a proper moisture barrier. In the Sauna room we used batt insulation between the 2×4 studs, foam sealed the windows. We used the Reflectix vapor barrier on walls & ceiling (taped), as Glenn suggested in his book. We also insulated the Changing Room walls. The exterior plywood sheathing was covered in standard exterior house wrap. The entire structure’s interior & exterior was then covered with T&G Northern White Cedar (Maine grown!).
Cathedral ceiling from the changing room venting system for your sauna building
a “cathedral ceiling” effect in the changing room

The best venting system for your sauna building consists of a few parts

First, the door leading into the Sauna from the Changing Room has a 1/2” gap at the bottom. The sides & top of the door are sealed using a foam strip in the door casement. The gap at the bottom of the door allows the influx of fresh air, particularly when the stove is operating. This has the benefit of supplying oxygen for the fire. Also, the air gap keeps the floor rather cool & dry, as the air washes over during its’ travel to the stove. (I really notice and appreciate this cool draft as I’m kneeling in front of the stove, mid-sauna, to add more wood).
Secondly we used the same strategy as we did in our own home. The roof was built to provide a 12” overhang on all four sides of our 8×12 structure. A 12″ overhang not only looks pleasing, but it also provides a place to install two 12” soffit vents in both the front & back side eaves. These are simple / cheap metal one’s from the Home Depot. In addition, we installed a Ridge Vent on the ridge of the roof. Again, the ridge vent was simple to install and is inexpensive.  
Finally, instead of putting a ceiling on the changing room, we left it open. An open ceiling allows the cool down room to fully participate in the movement of air supplied by the soffit vents & ridge vents. This provides a “cathedral ceiling” effect in the changing room, and also gives direct access to the wood stove chimney pipe going up thru the attic. This access to the attic chimney helped a lot in the chimney install. Also, the open ceiling is easy to inspect. I was thinking about adding a vent on each Gable end, but decided against this as overkill.
The combination of the soffit vents & ridge vents are sufficient.  
soffit vents for installing a venting system for your sauna building
two 12” soffit vents in both the front & back side eaves

The result seems to be working quite well

The stove clearly has a strong supply of air coming from under the door. I suspect the stove gains much of its air volume from the soffit & ridge vents when it is really cranking & requiring oxygen.  Also, I feel comfortable that any moisture that makes its way out of the Sauna Room is being dispersed thru the vents.  
By the way, we are using the “Bake & Breathe” technique Glenn has recommended. We leave the Sauna Room door closed for the night while the Sauna Room is still quite hot. Then we open the hot room door in the morning. We are finding the Sauna Room bone dry in the AM. Then we just crack open the Sauna Room door and let the entire structure breathe until the next Sauna session.    
One final point, due to our (my) focus, maybe an obsession?, on indoor air quality / circulation – we’ve installed a Carbon Monoxide alarm in the Sauna Room.  We find it reassuring that the feeling of deep relaxation is coming from the heat & steam – not from some bad gas vapor…
Again, a big Thank You to Glenn for his guidance.  I stumbled upon his website when thinking about building this Sauna and it has made an enormous positive difference in our ability to do this right. Merry Christmas – – looking forward to the Maine Winter!  

Below are a few photos showing the completed structure as well as the cathedral ceiling in the changing room and the soffit vents under the eaves.  You can see the roof ridge vent as well.  

Bob helping us design the best venting system for your sauna building
Bob outside his well ventilated, insulated sauna

Thanks to Bob for sharing with us the best venting system for your sauna building!

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21 thoughts on “What is the best venting system for your sauna building?”

  1. Hi Mike, Bob here – – I did not put any additional ventilation ports in the Sauna Room other than the 1/2″ gap under the door. Originally I was planning to do so, but after reading Glenn’s book and his comments about ventilation, I decided to follow his guidance. My “fallback” if needed was to just crack one of the windows we have in our Sauna Room – but to date I’ve found this is not necessary. I agree with him that the gap under the door provides plenty of oxygen for the stove. And, the occasional opening / closing of the Sauna Room door provides fresh air into the Sauna Room. In our sauna the draft in the chimney is excellent, good evidence it is getting plenty of oxygen. Hope this helps…

  2. Hi All,

    Love the website and I’m super keen on building my own sauna this year. I’ve got a spot in mind on our back patio but i have a quick question before I go to the next stage of design. Is it possible to build the sauna up against an external brick wall? So one of the sides is built against the external wall with potentially a sloping roof to the other side?

    Thanks,

    Greg.

  3. Hi Greg:

    Others with more thermal dynamical experience may be able to explain it better, but stone is an excellent conductor of heat, which sounds like good news but it’s not. Stone sucks heat out of a sauna like Dementors sucking out the soul in Harry Potter.

    So, I’d not be thinking about building a sauna up against a brick wall, unless you created a thermal break to the brick. (rigid insulation).

    Stone and heavy brick work in commercial Banyas and big saunas because of two main reasons:
    1. The masonry is isolated. Once heated, the heat is retained within the material (this is when stone being a good conductor of heat is a good thing).
    2. Commercial saunas and banyas are heated almost 24/7. So a massive amount of lämpömassa is a desired/good thing.

  4. Bob, sometime can you stick your Netatmo in your sauna in a seating position (so that its CO2 sensor is at about the same height as someone’s mouth), heat it up to 48-50°c (upper limit for Netatmo to accurately measure CO2), then have one or two people take a warm sauna round or three. It’d be interesting to see what the levels are.

  5. Hi Greg,

    I starting my Sauna build in Aspen Co. I’m planning on an electric heater. The sauna will be 5x7x6.5tall. 166 cubic feet 4.7 cubic meters. I have found a unit called Huum from Estonia. Would a 4.5 kw be the right size for this sauna? The only place I can find this unit is on EBay so I can’t talk with anyone about it.

  6. Greg, IIRC a masonry wall eats up about 2kw per 10 square feet. That’s a lot. Unlike the rocks in a sauna stove that radiate their heat out in to the sauna, a wall like that will radiate a lot of the heat to other than inside the sauna. It will act as a thermal bridge to pull heat from your sauna. There is also a question of what’s on the other side of that and what affect 100°c of heat against it will have.
    As Glenn recommended, you need a break between your sauna and that wall and you also need to be cautious about handling moisture.

    Dennis, the general rule of thumb is 1kw / m3 so you’d likely be OK. I’ve not heard of Huum though, why that? Estonia does have some good sauna.

  7. Hi Glen,

    Thanks for the info. The reason I ask about putting it up against a brick wall is more due to space issue than anything else. I have a narrow run of patio against a small external side wall to the side of the house. so if I can butt one side of the sauna up against this wall this will give me a space of around 2.5 meters wide by 2.7 meters long. So if I understand correctly, I can build up against the wall if i build that side in the same way as the other walls (insulated etc.)

    The max height of the external wall is around 2.7 meters, so I’m thinking if I can build up against this wall i could also use this to build a flat pitched roof to the other end of the sauna.

    I’ve uploaded a couple of pictures of the proposed area below (please excuse the crappy red pen). This is sort of what I’m proposing with the door on the left hand side.

    Let me know what you guys think.

    Thanks,

    Greg (from the U.K)

    https://ibb.co/2Z7Psjd
    https://ibb.co/mJwnsvW

  8. yes. I saw your drawing. (Crappy red pen was just fine!).

    2×2 firing strips, 16″ on center. glue and screw into the brick (sparingly is fine). Then rigid foam between the joist cavities. Then foil. Then fir that out or just t&g over that. The Sauna Twins are also in the UK and they can steer you towards a free range organic version of poly-iso (Finn Foam), which is a thermal break vapor barrier material.

  9. Hi Glenn,

    I’m planning a backyard sauna build this summer and in spite of all that is good, I’ve decided to go electric. I’m a big fan of Bob’s design above (beautiful sauna, Bob!), especially the vaulted ceiling (I’m 6’7 so the ceiling height for me is may bueno). I’m wondering what adjustments would be made in Bob’s design if the stove was swapped out for an electric one. There should be no need for a gap under the sauna door in this case, should there be? Anything else I should consider? Also, at my height, what would you recommend for hot room height? I’m worried 7′ might be a little tight but I don’t want to lose efficiency if I don’t have to.

    Lastly, I’ve looked at the Kuuma stoves and am strongly considering one. That said, I’m having some trouble identifying 2 or 3 electric options that are high quality and great for loyly. Wondering if you can recommend any options that you feel confident in?

    Thanks for all the great content, including your book which I purchased and am cruising through. Looking forward to some time on the bench hopefully next winter.

    Scott

  10. Hi, Bob here.

    Subject – Sauna Room Co2 Measurement using our Netatmo Device.

    This note is to respond to Walkers request to put our Netatmo measurement device in our sauna to measure C02 (Carbon Dioxide). This will give us a good idea of the quality of the air / and ventilation inside the Sauna Room. I did this today – the results were quite interesting.

    First, a bit of background on Co2. Co2 is Carbon Dioxide, NOT Carbon Monoxide. Carbon Dioxide is an important “marker” for air quality, plus in high concentrations makes you stupid (er) & groggy. Carbon Monoxide, (I’m not talking about his here), can kill you.

    Carbon Dioxide, Co2, is produced by several things – the burning of fossil fuels at the top of the list. But also, it is caused by people breathing. We humans emit Carbon Dioxide every day with every breath we exhale. In a tightly closed room (like a sauna, or a tightly built house), a group of people can bring Co2 to very high levels, very quickly. Good ventilation is the key to resolving this.

    In our home we use an air measurement device (Netatmo) to measure a bunch of things – temperature, humidity, noise level, barometric pressure, and most importantly – Co2. The reason for this is our house is super tight – and even though we have an ERV constantly circulating the air – it is amazing how quickly the Co2 can go up – either by cooking without putting the hood fan on, or having a large group of people in the house, or both if the chef starts drinking the cooking wine.

    For reference, average outside Co2 is roughly 400 ppm the world over. This, by the way, has climbed significantly from pre-industrial revolution levels. But, 400 ppm is still OK for us to breathe – “clean outside air”. Inside our home, we typically see 450 – 500ppm over the course of the day, going up a bit when we cook dinner.

    As an aside, Co2 at 400ppm the world over is not so good for our planet… but I digress…

    When we start seeing 1,200ppm or higher in our home, it’s usually because we burned something on the stove, or have a few people over. When we see Co2 over 1,200-1,500ppm we just crack a window for a while to bring it back down.

    With Walkers request in mind (good one by the way), I put our Netatmo device in our Sauna and measured the result.

    The results – with two of us in the Sauna & Netatmo at “nose height”:

    – Round 1: At 150 degrees we had a Co2 level of 1,325ppm at end of round. This round was appx. 20 minutes; we took a break for 10 minutes.

    – Round 2: The sauna temp had climbed to 160 degrees. At end of round 2 (20 minutes), we had a Co2 level of 1,652ppm. At this point I took the Netatmo out of the sauna – it was getting too hot for the Netatmo device to work properly.

    – Round 3: 180 degrees for 20 minutes. No measurement, but I expect Co2 likely a bit higher. The air felt, smelled – and seemed just fine.

    My conclusion is this – – I think our ventilation strategy is working fine. The draw on the stove is strong. The air feels clean, not stuffy. The Co2 is climbing during our rounds, which I expected for such a small tight space – but not to a dangerous level. However, when we have more than the two of us in the Sauna I will crack a window to introduce a bit more air ventilation – – not imperative, but I think sensible.

    Thanks to Walker for asking this question – – it really shined a light on the actual air quality in our wood fired sauna.

  11. Hi Bob, thanks for doing that. Great info. Your and my sauna seem very similar (assuming yours is about 8 m3?).

    For comparison, a sauna of 2 cubic meters per person, 6 air changes per hour (based on fresh air supply rate (CFM)) 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 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.

    My own sauna w/ just me so 12 m3 per person results in 633, 721 and 842 or w/ 6 people (2 m3 / person) calculates to about 1798, 2326 and 3052. I’ve currently got about 9 CFM or 1.2 changes per hour which largely agrees w/ my calculations above. So far so good.

    More: https://bamasotan.us/2020/12/sauna-ventilation/

  12. Hi Glenn,

    Thanks so much for this initial information. I’m 100% building the sauna in the next 3 months or so. I’m just going to lay out my new decking area then concentrate on this bigger project. I’m going to download and read your book. Sure I’ll be back in the not so distant future with some more advice!

    all the best,

    Greg.

  13. I’m writing this with a confession of sorts – – maybe a “learned experience confess”. My opinions & strategy on ventilation have changed…

    I am, by the way, the person who wrote the original Guest Blog Post above – all of 3 weeks ago. As before, I am still & surely a Sauna neophyte – but endeavoring to keep learning all the time…

    My post on Sauna ventilation / construction was fun to write and I’ve really enjoyed reading the responses – particularly Walker’s, who seems to be a mix between a sauna enthusiast and nuclear scientist – impressive. His request to measure the Co2 really got me thinking. As you may have read, when we measured the inside air, the Co2 climbed pretty substantially the one time I measured it with my Netatmo device. Not to dangerous levels, but enough to get my attention.

    Then, I read an earlier blog post from Glenn’s site, which talked about how Finnish Saunas have a 4” gap under the Sauna room door – vs. the 1/2’ gap I had in ours. Also, Glenn’s comment that he took dozens of saunas on a trip to Finland, with no feeling of dizziness / fatigue. Then, the real kicker, was a quote from the Finnish Sauna Society thru a link Walker sent me in one of his blog post reply’s – (Bamasotan – https://bamasotan.us/2020/12/sauna-ventilation/). I found this article, by the way, an excellent discussion of sauna ventilation and the negative impacts of excessive Co2.

    Here’s the quote:

    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

    This quote really hit me, a bit of an insult – but also some serious experience based wisdom. The reason behind this commentary is, of course, insufficient ventilation in the Sauna Room.

    So, with all of 3 weeks of Sauna experience, a burgeoning non-expert who is rapidly becoming an addict – – I went into change mode. I cut off the bottom of the Sauna Room door to allow a 4” Gap, and when using the Sauna I opened the window inside the Sauna room (across from the stove & in front of & to the side of the upper bench) roughly 4″ – 6” (it’s a casement window and slides side to side). This is the window you can see on the front of the building in my photos.

    By the way, depending upon the wind & pressure outside the building, I’ve found that “tuning” the window opening is required. Sometimes open 6”, sometimes 3” – but always open when we are using the Sauna.

    With this increased ventilation strategy the difference in the Sauna experience was, in a word, dramatic. In our very first Sauna Session, with window open 6″ & 4″ door gap:

    1. I could feel, with my hand, a fair amount of very hot air moving out of the window. Yet, despite this, the temperature in the Sauna actually climbed! We had hotter temperature inside the Sauna than ever before, reaching nearly 200f on round 3! Despite managing the fire in the wood stove similarly, it seemed it was burning hotter? The chimney draw, as before, was excellent, but I suspect it was just getting more substantial and purer oxygen to fuel the combustion. In subsequent sauna sessions, we have reduced the amount of wood we put into the stove a bit to manage the temperature to about 170-180f.

    2. The difference in the air “feel” while sitting on the top bench was a game changer. Really, a game changer. My wife Maggie put it best – – it reminded her of snorkeling / scuba diving. Her metaphor: Before, she was having a good time, but her mask was fogged, obscuring her vision substantially. After these changes, the mask cleared and she could see dramatically better, no more foggy goggles, plenty of pretty fish. Indeed, when sitting in our Sauna now, the air just seems to have more clarity.

    3. I totally concur with her assessment. It’s hard to put a finger right on it – especially given our lack of experience – – but it just feels so much more fresher, and the heat feels purer / hotter.

    My sense is that we have improved our Sauna experience substantially by increasing the ventilation. I do understand every Sauna is different, but ours is a pretty standard configuration in an 8′ x 12′ building – (88″H x 88″L x 88″W). And, at least for our Sauna, we’ve discovered that increasing the ventilation does not negatively effect the temperature, in fact the opposite.

    So, if I were to re-write the article on Sauna Ventilation – – it would be quite different. I’d stick with our overall building design – ridge vent, soffit vents, cathedral ceiling in changing room. But, my advice would be to go overboard with the ventilation in the Sauna room, whether it is with door gaps, windows, vents – – keep it “tunable” so you can dial it up or down. And, have some confidence with heat – – even with all this fresh air zooming around, it still stays plenty hot in this super-well insulated small room.

    As I learn about this wonderful subject & lifestyle, I’m convinced good ventilation is perhaps the most important ingredient to having a positive and enduring experience.

  14. My name is Larry Bounds from southern Utah at 6,000 ft.

    I built a 6’x10’ Sauna on an existing wooden deck with 3/8” gaps between boards,
    And a 1” under door gap and a Vent inside the sauna room at ceiling height that can be adjusted for air flow, this works quiet well,
    I tiled the floor and two walls for my wood stove heat barrier
    And the air flow in through the floor and out the vent is amazing,
    The steam level is lacking for my liking with the ceiling vent wide open, so we adjust as necessary for a meaningful sauna experience,

  15. I love this post and the comments!! I just wrapped up my exterior and installed my chimney from from ceiling through roof. I’m so pumped!!!!!!!!!!!!

  16. Spot on Larry. As you know, every sauna as its own soul. The stove is the heart of your sauna. Ventilation is its lungs. Happy to hear that yours is breathing well.

  17. Bob. It looks like your sauna door opens in. Am I seeing that right. From all my research having the door open outward for safety purposes seems to to be the standard.

  18. Informative article Bob! A well-ventilated sauna is essential for making the experience more comfortable and relaxing. The original Finnish saunas were pits dug into the ground in which a fire was created to heat stones. Steam was generated by throwing water over the stones, to increase the temperature in the pit.

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