Build a sauna yourself

Good day,

as our house construction is slowly coming to an end, I am slowly but surely dealing with the question of how I should build my sauna. That is pretty much my only own contribution.

The situation is as follows: The room for the sauna is separated from the main bathroom (masonry). The door opening already exists, of course, and I would have a sauna door installed by the glazier here. The floor area measures 2.30 x 2.30 meters in the shell construction. The room is under the sloping roof by 80%. Our knee wall is 1 m high, the room height on the upper floor is 2.60 m. I think three benches should be feasible, two of which should be at least 2 meters long. It is clear that the wall construction should not add more than 15 cm.

Now I ask myself:

-Should I have the walls plastered from the inside?
-How thick should the insulation be, is 6 cm enough?
-Do I need an air layer, vapor barrier, or anything else in the walls?
-Are there any building physics principles I still need to consider?

My idea so far: wall will be plastered, on top lathing 60 mm lathing, mineral wool pressed in between, on top counter-lathing 40 mm, on top wooden cladding.

A built-in sauna must be ventilated mechanically, there is no other way. Just as important as a sauna is the anteroom. If you decorate it with care, the owners will use their sauna for a long time and with pleasure. Much joy with the project Gabriele