Building a Sauna - Many Questions

I believe the mistake in reasoning here lies in confusing a (Finnish) sauna with a steam bath.

I don't believe that. ;)

The conditions have changed over the years, as have the materials.
Newly constructed buildings or extensions are significantly more “airtight” than older buildings.

For this reason, lime-cement plaster is still applied in bathrooms, even though this involves additional effort (both labor and costs) compared to gypsum plaster.
Despite exhaust air or ventilation systems.;)

but this is due to the fact that moisture is generated in the bathroom through showering etc. A Finnish sauna without large infusions produces significantly less humidity than taking a shower once

Since we still go to the public sauna at the moment, I cannot give a final statement on that. We will install an infrared heat cabin. The neighbor has a sauna in the bathroom. On the walls, the plaster I have already recommended. Also, two large floor-to-ceiling windows for cross-ventilation. She does this regularly after every sauna session.

Thanks for your answers :) I spoke with my parents again, apparently they don't want to do a steam infusion after all :)

which plaster do you mean?

Ah okay, ^^ I had overlooked that one side.