It makes sense to think that you want to keep your sauna hot room tight, with a minimal gap along the bottom of the hot room door and very well sealed walls. And this is partially true. We want to contain heat. We want the heat to resonate in the hot room with good, dense lämpömassa (think heavy stone wood fired pizza oven vs lame ass thin metal toaster oven).
But with venting, I’ll be the first guy to admit that I used to build my saunas tight, trying to contain heat within the hot room with minimal air gaps. Then I went back to Finland. In Finland, the pros call for about a 10-15cm gap along the hot room door. You crazy? I was always thinking of a gap for a mouse, tops. But 10-15 cm? My cat can fit under that door gap. These generous gaps along the hot room door work only if the room adjacent to your hot room is ambient. A gap along the hot room door does not work, for example, in saunas where you exit the hot room to the outdoors.
And what about wall vents? It makes complete sense to install them and have chutes for opening and closing, but truth be told, I wouldn’t open the wall vents much. My thinking was, to really crank the heat, shouldn’t we keep the wall vents closed? No, no, no. Once again, as with many things in life, the opposite is true, and the Finns are right. (Insert political argument about education, health care, public transportation and/or salmon soup here).
Any idea? Need a clue? Ok, try this: ever stood outside on a frozen lake on a sub zero calm day? Not so bad, is it? Now, compare: ever stood outside on a frozen lake on a sub zero windy day? You don’t last too long, do you? “The wind blows right through you, it’s no place for the old.” – Fairytale from New York, The Pogues. Wind chill will kick our ass.
A well ventilated sauna creates air flow (circulating good heat). Instead of wind chill, a well ventilated sauna creates a hard to notice yet gentle breeze of heat. You won’t necessarily feel it, but a well ventilated sauna will feel hotter than a poorly ventilated sauna as warm air is passing over our bodies on the sauna bench. A very subtle yet slightly hot… breeze.
You can probably sit in a poorly ventilated sauna for a longer time than a well ventilated sauna. What’s worse is that in a bad sauna, you will leave the hot room wiped out, exhausted because you’ve been breathing stale, poorly oxygenated air.
Good venting actually makes our sauna feel hotter. And good venting helps keep our saunas fresh and smelling clean. When we apply the Bake and Breathe method, along with good venting, our saunas don’t seem to be needing to be cleaned much, if at all.
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:
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 recommended). This device measures humidity, temperature, noise, and C02.
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.
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!).
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 on 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 through the attic. This access to the attic chimney helped a lot in the chimney installation. 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.
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 through 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!
- Locate your vent location. I like a vent about ear level when sitting on the upper bench.
- Find and mark your studs. Be sure to be clear of studs and safely between joist cavities.
- Make sure there’s no wires. Let’s not drill into where there’s any wiring.
- Use hole saw to drill through sauna paneling.
- Continue drilling into exterior sheathing and paneling until center drill goes through to the outside.
- Pull out and go outside to find the hole saw drill hole.
- Drill out the outside with hole saw, creating a clear channel from the hot room to outside.
- Dry fit 4″ Hinged Louvered Vent Hood.
- Cut back metal duct length on vent hood so that it is flush to paneling, or recessed 1/8″ ish.
- Silicone the back of the louvre vent and install from the outside.
- Install vent cover chute. Two screws or 3-4 finish nails should do it.
A better way to vent your sauna is to allow for about a 1/2″ or so crack along the bottom of your sauna door.
Why is this a good idea?
- No extra engineering of having to put an air intake vent in one of your walls.
- Air coming in through a crack in the bottom of your door will help keep your floor dry, the incoming air acts as a gentle blow dryer, running along the floor to feed air your sauna.
Two things we can do:
- Let’s pop in a couple wall vents. Borrow or dig out a simple hole saw and drill out a couple holes. Set in dryer vents and flash around the inside with chutes. The mobile sauna shown here illustrates wall vents..
- Let’s make a wider gap along our hot room door. Unpin your hot room door, lay it on saw horses, and cut off a couple inches along the bottom with a skill saw.
How many hours have you spent enjoying yourself chilling out between sauna rounds in your cool down room? For many of us, if we were to put a pencil to it, I think the number could be greater than many other things in our lives. Could anything be better than our time spent cooling down? For many of us, the cool down is as important as the heat up. A good cool down space is valued and important for equalizing our body temperature and enjoying the benefits from the rubber band theory without jackrabbiting our sauna rounds.
In English, the cool down room is also often called the changing room or poorly translated from German: the antechamber. With saunas built along masculine lines, this room may also be referred to as the man cave, dog house, or party room.
For many, we are able to “infinitely control” the temperature in our cool down rooms by simply cracking open the door to the outside and / or cracking open the door to our hot rooms. Yet this action, generally, only controls our cool down room temperatures. Humidity, on the other hand, tends to linger. Especially when it’s very cold outside. On cold sauna nights, we can see a massive amount of steam leaving our cool down rooms, but as our cool down room temperature goes down, moisture collects on surfaces (glass, metal, towels, beer cans). Many love the climate of cool and moisture. It’s akin to a hike in a rainforest. Magical. However, excessive, lingering moisture is not great for our sauna buildings.
Is your sauna cool down room begging for mechanical ventilation? If yes, where is the best place to locate a cool down room vent? Up high makes some sense, as bathroom mechanical vents are on the ceiling, but as warm air cools, it releases moisture as water. Maybe a mechanical vent is best lower on the wall in the cool down room?
A SaunaTimes reader asks: “Do I have to vent (exhaust) the opposite side from the heater? I am thinking about having a small gap on the bottom of the door (intake) and then having a vent on the upper end of the door for exhaust. Something tells me that this is not a robust design. Oh yeah, my door and heater will be on the same side of the sauna. Door will be centered and the heater will be in the corner.”
I say: Good move on venting with a crack along the bottom of the door. It’s easy and functional. As far as an exhaust vent, the first thing to note is that an exhaust vent is really only to allow stale air to escape the sauna, say, after a sauna session. Some purists like to vent during a sauna, but if you are using wood heat, there is enough fresh air pull just by wood combustion.
But an exhaust vent is a good idea. You build a sauna once, so may as well put one in. Your instincts are correct. Put your vent on the opposite wall to your door, and I like about a foot from the ceiling. They’re easy to install. Drill or jig saw a 3 1/2″ hole in your wall sheeting. You can buy a vent with the exterior grill already in. Screw it to a stud and insulate around it. When you do your t&g cedar, just cut around the round metal pipe. I like to make a little vent control slot thingy using some t&g cedar, with a little wooden handle.
PS. If you have already built a sauna without an exhaust vent, no worries, just open the door when you’re done.