Building a Sauna - Many Questions

Install a sauna and ventilate properly. Hang up a humidity meter for 3 euros and observe. Possibly retrofit an exhaust fan. The moisture during the infusion mostly stays inside the sauna and dissipates over time as the stove slowly cools down. Otherwise, every sauna interior would mold. What helps: airing out and heating the room properly.

We are currently planning to include a sauna and would simply leave the door open after using the sauna and have the rest of the ventilation system operate with heat recovery. A sauna produces less moisture than taking a shower, so where is the problem. If, for physically impossible reasons, there were still too much moisture, you can always open the windows or set the ventilation system to full power.