Sauna Foil Vapor Barrier
If you are building a framed, insulated sauna room, the foil barrier goes on the warm side of the insulation, under the wall boards. Here is how much to buy and what it costs.
By Caleb Whitaker Prices checked
A typical 6 x 8 x 7 foot sauna has about 244 square feet of wall and ceiling. Add 10 to 15 percent for overlaps and buy about 270 to 280 square feet. Foil rolls we saw cost $52.02 for 250 sq ft and $193.56 for 500 sq ft with tape.
How much foil you need
| Inside size (ft) | Wall and ceiling area | Buy at least |
|---|---|---|
| 4 x 6 x 7 | 164 sq ft | 184 sq ft |
| 5 x 7 x 7 | 203 sq ft | 227 sq ft |
| 6 x 8 x 7 | 244 sq ft | 273 sq ft |
| 7 x 9 x 7 | 287 sq ft | 321 sq ft |
| 8 x 10 x 7 | 332 sq ft | 372 sq ft |
Area is two long walls plus two short walls at full height, plus the ceiling. The floor is left out. Subtract nothing for the door, since the offcuts cover seams.
Prices seen
| Seller | Roll | Price seen | |
|---|---|---|---|
| Walmart sellers | 250 sq ft (4 x 62.5 ft), 8 mil, solid | $52.02 (was $61.56) | Visit website |
| Walmart sellers | 500 sq ft (4 x 125 ft), 8 mil, solid, with tape | $193.56 | Visit website |
| The Sauna Place | 269 sq ft (3.28 x 82 ft), single-sided | See seller | Visit website |
Why foil, not plastic
Sauna builders use aluminum foil because it handles high heat and moisture and reflects radiant heat back into the room. Standard plastic sheeting is made for ordinary house walls, not a room that runs near 200°F.
How it goes in
- Frame the walls and ceiling and fit the insulation.
- Staple the foil over the studs with the shiny side facing into the sauna.
- Overlap every seam and seal it with high-temperature foil tape.
- Seal around electrical boxes and the heater wiring.
- Leave a small air gap, then fix the furring strips and the wall boards on top.
Your heater and kit manuals come first. If they say something different, follow them.
Solid foil or perforated
Buy solid, non-perforated foil for a sauna. Perforated radiant barrier is made for attics and lets moisture through, which defeats the point.
Worked example
A 6 x 8 x 7 ft room: two walls of 8 x 7 (112 sq ft), two walls of 6 x 7 (84 sq ft) and a 6 x 8 ceiling (48 sq ft) make 244 sq ft. Add 12 percent for overlaps and you need about 273 sq ft. One 250 sq ft roll falls short, so buy a 500 sq ft roll or two smaller rolls.
What else you need
| Item | Why |
|---|---|
| High-temperature foil tape | Seals seams and cuts |
| Stapler and staples | Fixes foil to the studs |
| Furring strips | Hold the wall boards off the foil for an air gap |
| Insulation | Goes in the wall cavity behind the foil |
| Wall boards (cedar, spruce or similar) | The finished sauna surface |
Mistakes to avoid
- Facing the foil the wrong way. Shiny side goes toward the sauna.
- Leaving seams untaped. Moisture finds every gap.
- Using perforated attic foil.
- Covering the heater wiring or controls in a way the heater manual does not allow.
- Fitting boards tight to the foil with no gap.
Do kits need foil?
Most barrel and cube kits do not. Their solid wood walls are the whole wall. Foil is for framed rooms with insulation, such as a sauna built inside a shed, garage or basement. See how to build an outdoor sauna.
Building indoors instead? See sauna kits.
Common questions
How much vapor barrier do I need for a sauna?
Add up the wall and ceiling area and add 10 to 15 percent for overlaps. A 6 x 8 x 7 ft room needs about 270 to 280 sq ft.
Which side does the foil face?
Shiny side toward the inside of the sauna, so it reflects heat back into the room.
Can I use plastic vapor barrier in a sauna?
Sauna builders recommend aluminum foil because it holds up to the heat and moisture.
Do I need foil tape?
Yes. Tape every seam and penetration with high-temperature foil tape so moisture cannot get behind the boards.