I'm building a sauna into a house in the Pacific Northwest. One wall is external, three walls are internal, and there is an attic overtop where I can vent to the outside.
All walls and ceiling of the sauna will have insulation covered by aluminum vapor barrier with tape that ought to implement a complete seal. This reflects moisture and heat back into the sauna.
I've heard that you want permeability between the outside of a structure and the inside so water vapor (not droplets) can pass through, and that humidity is always trying to get to the less humid side.
Based on these principles and the fact that there will generally be moisture on the inside of the room and on the outside of the house, what practical steps can I take to make sure humidity doesn't damage the walls?