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Sauna Vapor Barrier: What It Is and Where It Goes
A sauna makes steam on purpose. Without a barrier, that moisture soaks into the wall and rots it from the inside. Here is how the foil stops it.
Every time you throw water on the stones you push hot, wet air into the walls. Framing and insulation that get repeatedly damp will grow mould and rot, so a sauna wall is built to keep that moisture in the room, not in the structure.
The barrier is aluminium foil (or foil-faced kraft), and it goes on the warm side of the insulation, directly behind the wood cladding, facing into the room. Order from inside out: cladding, a small air gap on the furring strips, foil, insulation, framing, exterior sheathing. The foil reflects radiant heat back into the room as a bonus, so the room heats faster and holds temperature.
The details that actually matter
- Overlap and tape every seam. Lap sheets 3–4 inches and seal with aluminium foil tape. An untaped seam is a moisture path.
- Run it floor to ceiling, including the ceiling. The ceiling sees the most heat and steam; skipping it is the most common failure.
- Leave the ventilation gap. The furring strips behind the cladding create a drainage and drying gap. Foil tight against the wood traps condensation.
- No plastic sheeting. Polyethylene melts and off-gasses at sauna temperatures. Use foil rated for the heat.
Get the barrier right and the wood on the inside can stay unfinished and breathe. That is also why you never seal or varnish sauna cladding, the finish traps heat and the wood needs to dry between sessions.
FAQ
Does the vapor barrier go behind or in front of the insulation?
On the warm side, between the insulation and the wood cladding, facing into the room, with a small air gap to the cladding.
Can I use plastic instead of foil?
No. Polyethylene sheeting is not rated for sauna heat and will soften and off-gas. Use aluminium foil or foil-faced barrier and tape the seams.