What type of insulation is used for rim joists?

Rigid foam

Rigid foam is the best insulation for rim joists. We chose 2-in. -thick (R-10) “extruded polystyrene.” Don’t use “expanded polystyrene,” which is a less effective air and moisture barrier.

Do rim joists need to be insulated?





Rim joists are above grade, so it makes sense to insulate the joists to the same level as above-grade walls. At the rim joist area, many building components come together—the foundation wall, the mudsill, the rim joist, the subfloor—so it’s also important to seal all cracks against air leakage.

What is the R-value of 2 inches of spray foam?

R-Value In 2 Inches Of Spray Foam



The R-value ranges from R-3.5 to R-3.6 per inch. The filling of a 2×4 cavity yields about an R-13.