Q: What are rim joists, and how do you insulate them? Our home was recently inspected, and the buyer asked that we pay for the repairs. Is this something I can do myself?
A: Rim joists are the boards at the end of all floor joists on the first and second floor. They literally are a “rim” around the house.
Homes with a crawl space or a basement have rim joists on each level. Homes on a slab do not have rim joists on the first level. There also are the full-length outside joists on each level, which fall into the “need to be insulated” category. Any space or cavity exposed to the outside air needs to be insulated.
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Depending on your skills — and if working in small, dark areas is not a problem — then yes, it’s easy to insulate the rim joists. A typical floor joist is a 2- by 10-inch joist set 16 inches from center to center. That leaves a space of approximately 14½ inches by 9 7/8 inches between each joist.
The easiest method is to cut several pieces of 6-inch-thick fiberglass insulation to a length of 10 inches. The insulation will fit into the cavity of the rim joists and will be held in place by friction, because the insulation is slightly larger than the cavity.
The downside to using fiberglass insulation in a crawl space is humidity. Moisture can build up on the surface of the rim joists on the cooler sides of the foundation and decay can start to damage the wood floor joists and flooring.
Because of humidity problems, some builders either leave the rim joists uninsulated or use a piece of rigid foam board, cut to fit into each opening. Although the foam is more expensive, it is, in my opinion, the best solution to the problem.
Because not all the cavities are exactly alike, the precut foam boards may be loose in the cavity. I would use a can of spray foam to seal the foam board at the edges to ensure an airtight seal
Foam board or spray foam also is the best way to insulate the long rim joists on the outside of the foundation or second-floor area. It is difficult to secure and seal a long piece of fiberglass insulation to the outside rim joists without damaging the insulation.
Dwight Barnett is a certified master inspector. Contact him at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 268, Evansville, IN 47702 or firstname.lastname@example.org. Sorry, no personal replies. Always consult local codes and contractors.