Cabin ventilation and room ventilation are different
Cabin vents help moderate temperature and fresh-air exchange during use. The room around the cabin must also dissipate heat and moisture after a session. A small sealed closet may trap both even if the sauna itself has a roof vent.
Traditional saunas are a different design problem
Traditional heater rooms depend on correct inlet and exhaust placement for comfort, heater performance, and drying. Follow the heater and room-design instructions; do not copy infrared-cabin advice into a custom traditional build.
A simple ownership routine
After use, wipe moisture, open the cabin door and vents, and allow the surrounding room ventilation or dehumidification to carry moisture away. Persistent condensation is a building issue to solve, not normal sauna operation to ignore.
Sources and documentation
Specifications, claims, and installation requirements can change. Confirm the current manufacturer manual for the exact model and variant.