The sauna must have proper thermal insulation. As a rule of thumb one square meter of un-insulated ceiling or wall above the lower laude increases the effect of the heater needed by 1 kW.
The sauna must also have proper moisture insulation to prevent the moisture from spreading out to the other rooms. The main concern is the vapour transmission between the panelling, vapour barrier, and insulation.
The proper insulation must have the following properties from outside to inside:
1. The thermal insulation must be mineral wool, minimum thickness 50 mm on the walls and 100 mm on the ceiling.
2. The vapour barrier of plastic-paper-aluminum foil laminate, with the aluminum inside, must be properly sealed with generous overlapping and absolutely air tight without any pinholes. In other words, the vapour barrier must be inside the insulation and the bright, heat radiating aluminum foil facing inside to reflect the heat back to the sauna.
3. An air gap of minimum 20 mm between the vapour barrier and the interior panel.
4. The interior panel. There must be a gap between the wall panelling and the ceiling panelling so that the vapour can get out from behind the panel.
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