BoulderSauna Insulation & Vapor Barrier
As with typical home construction, your sauna walls and ceiling must be insulated and protected with a vapor barrier. Typical vapor barriers are made of plastic, which would quickly degrade, outgas, and even melt from the heat of your sauna. It is vital, therefore, to use an aluminum foil vapor barrier instead of plastic.
Fiberglass insulation comes wrapped in plastic, backed with craft paper, or (less commonly) naked. If you install craft-backed insulation and plan to cover it with craft-backed aluminum foil, you'll end up with a double layer of paper. Extra layers of stuff encourage moisture trapping, so it's best to go naked. In the end there would be only three layers from the inside out: fiberglass, craft paper, and aluminum foil.
You should be starting to rough in your wiring at this time; you don't want to find yourself cutting up your vapor barrier later just to run cables behind it.
Fiberglass insulation is awful to work with. It sheds toxic glass splinters all over the place and you have to protect your skin from head to toe, wearing gloves and a dust mask. Then you have to quarantine yourself and your contaminated clothing to avoid spreading it all over your house. But there is an environmentally friendly alternative: recycled cotton insulation. Made primarily from shredded blue jeans and costing only slightly more than fiberglass, it's about as hazardous to handle as, well, a clean pair of pants. The framing shown here has cotton insulation in the ceiling (and the only reason fiberglass got in the walls is I discovered the alternative halfway through construction). The cotton product is called UltraTouch, manufactured by Bonded Logic. It's wonderful. And it isn't too hard to find an insulation distributor that carries it.
Standard 4-inch (R-13) insulation is fine for filling the wall cavities. For the ceiling (where most of the heat will go) use 6-inch (R-19) batts.
If you're using a kit, it should include the foil vapor barrier. It may be aluminum foil backed with craft paper or a thicker paperless foil. I find the paperless stuff easier to work with. The foil does two important things: it prevents water vapor from reaching the insulation and wall studs, and it reflects heat back into the sauna. You install starting at the floor, using a staple gun. Lap the higher sheets on top of the lower sheets. To seal all the seams and inevitable tears, use good quality high-temperature aluminum duct tape. During the installation, it helps to envision carefree water fights occurring inside the sauna, and what would happen if the vapor barrier took a bucket of water (Answer: it all ends up on the floor inside the sauna and not inside the walls).
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