I have a sauna kit (thin log walls, 2 inches thick). It works pretty good but alot cooler in the winters since the log walls have little insulation obviously. Wondering what would be the best way to go about adding extra insulation. Vapor drive is a big concern in the sauna with a lot of hot steamy air. I always put another load of wood when I’m done to sort of “bake” the room dry. I haven’t noticed any mold or mildew in the structure.

I was thinking, Rockwool rigid insulation (comfort board) on the exterior. Then strapping and some sort of siding like a cheap T&G or board and batten. Hot moist air will no doubt get through the log walls but I’m hoping the comfortboard is vapor open enough to keep anything from mold and rot. Would that hot air make the rockwool offgas and put a smell or toxic air into the sauna?

Another option is Johns manville foil faced foam board (it’s rated for hot sauna temps) on the interior, then strapping, then T&G. I think this could also work but it would squeeze the interior space and require a redesign of all the benches.

Any insight on this would be much appreciated!