The walls and ceiling of the sauna room should have good heat insulation, especially if the room is to be heated by means of an electric heater. The wall materials of the sauna, such as concrete, logs, glass, tiles, brick, glass bricks, etc., require a relatively powerful heater and quite a long time to heat up.
The larger the sauna is in terms of capacity, the greater the heater power should be to heat up the room sufficiently well to take a sauna bath. When approaching a heater dealer you should know beforehand what the wall materials are in your sauna and the capacity/inside area of the sauna. Air-conditioning in the sauna room must be arranged in such a way that it does not cause the heat produced by the heater to be wasted.
Much research has been done on the proper air-conditioning system for saunas, and for many years this has been applied at the planning and implementation stages of sauna ventilation in any new construction. The manufacturers product brochure, available from the heater dealer, gives further detailed instructions relating to choice of heater power.