The wall build-up (inside → out)
- Panelling, 12–15 mm softwood (tongue and groove).
- Air gap, about 20 mm, made by vertical battens; it lets the back of the panel dry.
- Aluminium sauna foil (vapor barrier), shiny side towards the room, seams overlapped about 50 mm and taped with aluminium tape, also at corners, around the vent openings and at the ceiling joint.
- Insulation, typically 50–100 mm mineral wool in walls and more in the ceiling.
- The wall or framing behind it.
Why foil and not plastic
Foil withstands sauna temperatures and reflects radiant heat back into the room. Plastic sheeting is not made for them, and kraft-faced insulation must not face the hot room. Use products sold as sauna foil and follow the panel maker's instructions.
When no vapor barrier is needed
Solid log walls and barrel saunas are built from massive wood without insulation, so there is no insulation to protect: the wood itself buffers and dries the moisture. They need ventilation and time to dry after each use instead.
Typical mistakes
- Foil not taped (or taped with ordinary tape that fails in the heat).
- Panel nailed straight onto the foil without an air gap.
- Gaps around the vent ducts, light fittings and the heater's cable entry.