mitigate mold concerns from humidity in sauna in PNW

Question

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?

Answer 1

If you're building a proper sauna (Finnish sauna) and using it correctly, there should be no moisture problem in it. The air you feed it initially has the same humidity as the surrounding air, then the stove raises the air temperature leading to a decrease in relative humidity (same water content by weight + higher temperature = lower relative humidity, RH%).

The only two things that can add water vapor to the air in your sauna are 1. multiple people sweating in there for a while and 2. somebody pouring unnecessary amounts of water over the heated rocks, thinking a sauna should look like the steam baths (Turkish baths) they've seen in the movies, where people can barely see eachother through the thick clouds of vapor.

1. People in a sauna still need oxygen replenishment, so you should build it with a fresh air intake under the stove or somewhere close and low, and an air exhaust on the opposite wall and at half height or higher (sometimes it's a small sliding wooden window on the ceiling). This should deal with the humidity from sweat just fine.

2. Stop it. A sauna is not a steam bath. The air in there should be (very) dry. The only time you pour 1-2 ladles of water over the rocks is maybe if you want to "kickstart" your sweat response by generating a sudden wave of extra heat, or if you're looking for the supposed antioxidant or other benefits of löyly. Other than that, adding too much water just makes the high heat unbearable, shortens your sauna session to way under the normal 15 minutes, plus it introduces unecessary risks of germ growth, which is normally inhibited in a sauna by both the very high temperature and the dry air.