How to handle moisture/insulation for a backyard sauna
I’m building a sauna for a client in Philadelphia (Marine zone 4, I believe). It’s wood-burning and the client will mostly be using it as a dry sauna, though they’d like the option to increase the humidity.
I’m trying to figure out insulation to handle the moisture.
My plan was to use 2×4 walls with R13 fiberglass between the studs, and 1″ R3 rigid foam on the exterior. As I understand it, the rigid foam will keep moisture on the outside.
Do I also need a vapor barrier on the inside to protect against possible humidity from the sauna entering the walls? Or does that possibility mean that I should *not* also use exterior foam?
Also, do I want a vapor barrier under the subfloor/above the ceiling?
Thanks in advance!
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There have been many discussions about saunas here; you might start by reviewing those: https://www.google.com/search?q=greenbuildingadvisor.com%3A+%22sauna%22&rlz=1C1ONGR_enUS1048US1050&oq=greenbuildingadvisor.com%3A+%22sauna%22&gs_lcrp=EgZjaHJvbWUyBggAEEUYOdIBCDc4NDhqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8
Another article worth reading:
https://buildingscience.com/documents/insights/bsi-010-wine-cellars
It's about wine cellars, but he addresses saunas and provides some assemblies near the end.
A vapor barrier is critical to prevent moisture from being driven in to the wall cavity (saunas can have considerable pressure when water is thrown on the stones). A typical sauna wall construction is interior cladding, air gap, foil faced polyiso, 2x structure w/ mineral wool, sheathing, WRB, air gap, exterior cladding.
Be careful to not have a second vapor barrier.
There are a lot of details that are important for a sauna to function correctly and not rot. Trumpkin's Notes on saunas and Secrets of Finnish Sauna Design are good reading for anyone building a sauna.