Sauna insulation and vapor barrier

How much insulation

Finnleo says to plan to insulate sauna walls to at least R12 and the ceiling to R16.

Finnleo's custom-cut instructions call for R11 or better unfaced fiberglass batts between the wall studs, always referring to local building codes. For the ceiling, Finnleo specifies a single layer of 6-inch fiberglass or a double layer of 3-1/2-inch fiberglass.

The North American Sauna Society says insulation matters, with the ceiling the most critical place to insulate well, and that regular fiberglass plus an aluminum foil vapor barrier should be used.

The foil vapor barrier

Finnleo says the foil supplied with its custom-cut kits acts both as a vapor barrier and as a heat reflector.

Finnish Sauna Builders says the vapor barrier must go on the warm side of the insulation, directly behind the sauna wall cladding. Finnish Sauna Builders gives the typical wall assembly, from outside in, as exterior wall, insulation, vapor barrier, then interior cladding.

Finnleo says unbacked batts are best to avoid a double vapor barrier that could trap moisture; if only backed insulation is available, the paper or foil side faces the room.

Finnleo says to staple the foil to the inside framing over all walls and the ceiling, rolling it out horizontally from the bottom up with about 5 inches of overlap. Finnleo says the foil joints may be taped with foil tape if desired, and that tape is not provided.

Finnish Sauna Builders says ordinary duct tape will not withstand sauna conditions, and foil tape made for vapor barriers should seal the seams. Finnish Sauna Builders lists wall-to-wall corners, wall-to-ceiling joints, electrical boxes, door frames and wiring or vent penetrations as critical sealing points.

What a kit includes

Finnleo lists a foil vapor barrier among the parts included in its custom indoor sauna kits. Finnleo says framing, insulation and exterior finish are not furnished with its custom-cut kits unless specified. (Finnleo)

The North American Sauna Society says pre-cut sauna kits require carpentry skills and go either on existing framed walls or on walls framed to suit the design.

Read next

• Sauna ventilation
• Sauna doors and glass
• How to size a sauna heater