How would you build a sauna in climate zone 6?
I’ll be building a standalone structure that is a sauna. Only heated when in use but obviously the temp is very high. It’ll also be a wet sauna so the absolute humidity can be quite high when in use.
I feel like there’s two trains of thought: 1) use a vapor barrier on the interior and when you’re done with sauna session, leave a bath fan on for an hour and cool/dry the interior wood out (cedar). 2) No vapor barrier, and highly vapor permeable materials like rockwool insulation and plywood and just left the building “breathe” and it’ll dry itself out.
I do want it to be relatively air tight so it’s not super hard to heat and doesn’t become home to rodents. So if I go with option 2, it’s not going to dry out super fast.
Lastly what glass should I use? Just buy tempered shower glass or upgrade to tempered triple pane? It’d be cool to see outside, and single pane is going to condense like crazy.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Here's how I'm thinking about it: you have an area that you intentionally take up to nearly 100F and 100% relative humidity. You want it to be safe under those conditions when it is quite cold outside. When you're done you want to safely take it back to something safe.
You want this building to be very well insulated, condensation is your enemy. You want a vapor barrier on the inside of the walls, and you want the walls to be able to dry to the exterior. So you want the permeable insulation and plywood sheathing. You're never going to prevent condensation on the inside, so you want materials that won't be damaged and won't hold the moisture.
When you're done, you want to vent the moisture to the outside, but while doing so you want to keep the temperature inside high enough to minimize condensation. I would recommend looking into an energy-recovery ventilator, that will keep more of your heat in. You may also have to keep applying heat until all of the moisture is out to keep the inside from cooling too fast, leading to condensation. This may require some sort of controller that tracks humidity and temperature, or it may be as simple as putting the HRV on a timer and the heat on a timer and letting them both run for a set time.
In cold weather a building like that with heat being applied should dry well.
My dad build one about 25 years ago and it’s wood fired so when he’s done, he drains the water and stokes the fire. I was originally leaning towards electric heater but wood fired might be better for longevity of the interior finishes. Using his, the interior gets to roughly 150 degrees and 40% - 50% humidity. I’m sure mine will be in the vicinity of that.
I was actually thinking less insulation would facilitate drying faster. It’ll be harder to heat but it’s only heated a maximum of an hour per day, plus it’s a small space so the energy penalty is not that significant.
Having recently been to a lot of remote off-grid backcountry ski huts with outdoor wood-fired saunas, can confirm: You could keep this thing pretty darn simple and have it work just fine. Probably not worth the brain damage of overthinking it.
The ones I've been in have a wood stove with an attached hopper for heating water (ie, melting snow to boiling), a spigot on the bottom of the hopper for draining, rocks on top of the wood stove for ladling hot water on to, and just a wood shell of a building with a bench. Studs, cedar siding (not even sheathing), a site-built wood door, some of those rubber grip mats on the floor like you see in dish rooms, and that's it. The last one I was in heated up to about 170°F on a -30°F day in about 10, 15 minutes. We just drained the hopper and let the fire die out after each use; didn't seem likely to cause any problems.
If you wanted to go deeper down the rabbit hole though, there's a great article by Joe Lstiburek on BSI's site about wine cellars (which are similar to sauna assemblies, scroll down to the bottom of the article):
https://buildingscience.com/documents/insights/bsi-010-wine-cellars
Keep us posted, I'd love to see where you wind up on this (I'm thinking about doing something similar at my house, Z6/southwest Montana).
Will do. I’m in Bigfork, where are you?
Hah, right on! I'm in Belgrade/Bozeman
I would build it as walk in freezer. This means foil faced rigid with taped seams in the inside over all the surfaces. Strapping/sleepers over this and attach the interior finish to it. I would detail the interior finish (walls, floor, ceiling) the same as a rain screen, so intake and exhaust gaps at the top bottom of walls and edges of the ceiling and floor.
The rigid doesn't care about moisture and with the facer it works a vapor barrier to keep the moisture out of the structure. The vented air gap behind the interior finish allows for air movement to quickly dry it out once you are done.
With the amount of time you'll use something like this, not sure it makes sense to even insulate cavities if the rigid is about 1.5".
The important part with any outdoor structure like this is the critter barrier. These occasional use structures are a magnet for critters so make sure this is solid. The outside of the structure should be a solid wood box (the floors are part of this box) with no gaps and all seams taped. I would use a regular exterior entrance door with full glass so it seals well. You can push some steel wool into the spray foam around the door jambs before it sets so critters can't chew through it.
For humidity control once you are done, probably the simplest is an exhaust fan on timer.
P.S. You can build the same thing without rigid with batts in the studs+interior poly as long as the poly is air tight. Still need the interior strapping and air gap all around.
Steel wool in the spray foam is a new one for me. Great tip!
I wouldn't insulate it at all, or include anything to slow moisture from moving through the building. The spaces are typically tiny and have a wood stove that will easily heat an uninsulated space. I would use a high performance, vapor-open WRB and air control layer on the exterior.
I think a small amount of insulation might be nice so as to keep the temperature fluctuations to a minimum while using a wood stove. But I get your point for sure.
I have been involved with several sauna builds in the last couple years in zone 7A, with several different approaches. I also plan on building my own starting this summer. I plan on having space for about 8 people in mine.
I would size the wood stove so that it can heat the space quickly, and provide enough energy to dry the interior quickly when done in the sauna. If the stove is big enough it will overwhelm any lack of insulation in the build.
I would use 2x4 framing with roxul insulation and a well detailed interior VB/AB. The wall can then dry to both sides of the VB, in and out. Best practice would be a rain screen at the exterior wall.
Floor should be water resistant/proof and slope to a drain for when people pour ladles or buckets of water on their heads.
Build in several vent hatches, both high and low, that you can open and close as desired. Make them rodent proof and you can keep them open when the sauna is not in use for extra drying.
Having an entrance room with a door is a nice touch.
Good luck!
Good call on the entrance room + door, plumb_bob. Definitely makes it more comfortable when you're walking from a heated house to an outdoor sauna in the middle of winter, even if it's just big enough to hang a couple towels and maybe a coat/bathrobe/etc.
Yeah mine will have two rooms plus a covered patio. Super nice to go sit outside in the winter to cool off and not step in snow. Also a great place to keep the firewood dry.
Banyas can be heated into the 200°F neighborhood. 140 seems cool.
What about building with AAC blocks, sealed tight and cedar batten on the floor, walls, and benches?
When your done for the evening, ventilate the moisture out and let it cool overnight.
I built a small sauna in our basement about 15 years ago with walls done very much like Joe has detailed here (and Paul already linked to in this thread) https://buildingscience.com/documents/insights/bsi-010-wine-cellars in the steam room section. More than enough heat is provided by a "wet" rated 6kW electric stove. A local saw mill provided all the cedar to clad the walls and ceiling.
I did this in our basement, so there is a drain in the sauna, concrete floor. Other than making the middle bench slide back (for more room if showering), the best feature is the floor. I epoxy coated the concrete, then built two low profile cedar floor sections, like a deck with wide gaps, on cedar sleepers. Not only is it great on the feet, but it's also easy to lift/remove to clean the epoxy coated floor. 15 plus years and counting and it's been a great feature in our home. I do believe we also have the largest population of Finlanders, outside Finland here...ha.
A 12'x6' space includes both a 3 piece bath and enclosed (around 6x6) sauna. Perhaps one of my favourite designed spaces ever with respect to efficiency, it uses a neo angle shower, vanity and pocket door to maximize space in the bath room. The bath area incorporates two cedar benches so doubles as a sauna rest area. The sauna itself is set up to allow showering inside as well with a fixture on the wall. The middle sauna bench slides back to create a pretty luxurious sized space. The bath and sauna are both 6'x6' ..very compact spaces.
Awesome space!
First of all, a good sauna should be able to hit 200 degrees F.
Second, if doesn't matter how much löyly (that's the Finnish word for the water/steam you create by ladling water on the rocks) you add -- saunas are dry. Moisture is not a problem.
My experience is with wood stoves. Don't overthink the construction. Everything will stay dry.
So I guess what I want is actually called a “Banya” which is pretty high humidity. “Sauna” is dry. I learned something! https://gobanya.co.uk/russian-banyas-vs-saunas/amp/
I would want the structure to handle 160F at 40% humidity which has a dewpoint of 125F. Dry sauna at 200F and 5% humidity has a dewpoint of only 85F.
Pooling everyone’s input, 2x4 walls with fluffy insulation, interior VB & AB, interior rain screen, plenty of heat coupled with a bath fan will adequately dry the interior out after use.
What’s everyone’s thought on what glass to use? Double pane would probably not condense in a sauna but definitely does in a banya. I’m guessing triple pane would also condense but I don’t know for sure.