Sauna insulation is the part of a build that is invisible when finished and decides everything: how fast the room heats, what it costs to run, and whether the wall is dry in ten years. There is one correct order of layers, and the mistake that ruins saunas is always the same one.
The order of layers
From the room outward: cladding, then an air gap, then a foil faced vapour barrier, then insulation between the studs, then the outer sheathing. Every layer is doing a job and the sequence is not a preference. A build that puts insulation against cladding with no barrier will push moist air into the stud bay every session.
Why the vapour barrier is on the hot side
A sauna is warm and humid inside and cooler outside, so vapour is being driven outward. The barrier belongs on the warm, humid face, taped continuous, lapped at every joint and sealed around lights and fixings. Put it on the cold side, as some house details do, and you have built a moisture trap inside the wall.
The air gap nobody draws
A gap of roughly an inch between the foil and the cladding lets the small amount of moisture that gets past drain and dry, and it stops the cladding cooking against the foil. Battens over the studs create it. Free plans almost never show it and it is one of the two details that separate a wall that lasts from one that does not.
What insulation and how much
Mineral wool is the usual choice because it tolerates heat and does not off-gas, and it should fill the stud bay without being compressed. More depth in the ceiling than the walls is worth it, because that is where the heat sits. Do not use foam boards rated for lower temperatures against a hot wall, whatever the R value says.
Questions people ask about sauna insulation
Which side does the vapour barrier go on?
The hot, humid side, directly behind the cladding and in front of the insulation, taped continuous and sealed at every penetration.
Do I need an air gap?
Yes, roughly an inch between the foil and the cladding, made with battens. It lets the wall dry and stops cladding cooking against the foil.
What insulation should I use?
Mineral wool, filling the stud bay uncompressed, with more depth in the ceiling than the walls. Avoid foam boards not rated for the temperature.