Q:
Your recent article Does Fiberglass Insulation Still Make Sense? (FHB #160, pp. 50-55) was informative, but it left me wondering about the best way to insulate a crawlspace. I have R-19 batt insulation between the floor joists. Is there anything else I can do, such as install rigid foam over the batts, to keep out the cold?
Dean Stillings, via email, None
A:
Bruce Harley, author of Build Like a Pro: Insulate and Weatherize (The Taunton Press, 2002), replies: If you want a warmer floor above the crawlspace, you could install rigid-foam insulation, such as expanded polystyrene, continuously across the underside of the joists. If detailed carefully at the seams with foam or caulking, the insulation will add R-value and help to stop cold air from seeping in.
The only disadvantage in adding a well-sealed layer of foam to the underside of the floor joists is that plumbing leaks can go undetected for a long time.
It’s usually better to spend your money insulating the crawlspace walls, sills, and rim joist. You can use either rigid foam or spray-in foam (such as polyurethane). This approach quickly and effectively insulates and air-seals the entire crawlspace, bringing the area inside the thermal envelope.
Whether you insulate just the floor or the entire crawlspace, it’s important to install a good-quality vapor barrier over the ground. I recommend using Tu-Tuf (Sto-Cote Products; 800-435-2621) or Tenoarm (410-366-1146; www.conservationtechnology.com). Seal the seams of the vapor barrier with builder’s tape (Tyvek tape or 3M 8086 builder’s tape). It’s also important for you to seal the edges of the vapor barrier to the foundation wall with some roofer’s tar and to seal any perforations for the support columns.
