ᐅ Building a Sauna – Many Questions

Hello everyone,

Since my parents are a bit hesitant to consult an expert and I don’t really have much knowledge on this topic, I’m just going to ask you all—maybe someone knows something 🙂

My parents have been planning for some time to build an extension to their private home and install a sauna in this extension. Now, a sauna exposes the walls to a significantly different type of moisture than a regular bathroom (it’s warmer, of course) – how is the best way to insulate the walls to prevent mold from forming on the inside?

I’ve tried to read up on this via Google, but I haven’t really found a clear answer 🙁 For individual products like mortar, plaster, concrete, etc., there is always mention of capillary water absorption/water absorption coefficient and something about water vapor – but I don’t really understand what that means or whether a high or low value is good or bad.

Logic tells me that exposure of a wall to moisture and air will lead to mold over time, but maybe it works in a completely different way...

I also asked about this once at a home improvement store, but the salesperson looked at me as if I was crazy...

Could someone please explain this to me? :/

Hmm, I’m not an expert, but what exactly does heat have to do with moisture?

No, no, no.

The sauna heater itself does not release moisture, so where would it come from?

However, moisture can be introduced through a water pour (löyly). Additionally, people might have wet towels, wet hair, or even a shower in the room.

In general, your question is correct: fresh air is indeed important in a sauna room. And any moisture (if introduced) needs to be ventilated out again.

The heat in the sauna (produced by the stove) is initially dry heat; the humidity, as mentioned before, is introduced either through pouring water on the stove or through sweating. The amount of moisture generated by pouring water primarily depends on the users, and not everyone wants to have steam sessions at all.

There is a lime-cement plaster available for bathrooms. It is less sensitive to moisture and would also be suitable for a sauna room.

For ventilation, I recommend a large window and possibly a decentralized exhaust system that removes moisture from the room and allows fresh air to flow in. Whether with or without heat recovery is a matter of personal preference. After a sauna session, I personally prefer cooler air. 😉

My sister has had a sauna in her basement for about 10 years, which is used (more or less) regularly.
After using the sauna, cross-ventilation is done through a window (possibly including the basement exterior door).
Around the year 2000, ventilation systems were not really common yet – so none were installed.

As far as I know, she has never had any mold problems.