Rim Joist Insulation Calculator

Two inches of rigid foam cut to fit and sealed at the edges with canned foam, or closed-cell spray foam. Never use bare fiberglass — warm indoor air passes straight through it and condenses on the cold rim board.

Two inches of XPS or polyiso (R-10 to R-12) suits most of the US. In zones 6 to 8 go to three inches.

It is a thin band of wood separating heated space from outdoors, running the entire perimeter of the house, and it is very often bare. For its square footage it leaks more than almost any other surface, and it is accessible from the basement without opening anything up. Per hour of work it is usually the highest-value insulation job in a house.

It is common and it is the wrong material there. The rim joist is a cold surface with warm humid basement air reaching it, and a batt lets that air straight through to condense on the wood. Rigid foam cut to fit and sealed at the edges, or closed-cell spray foam, stops the air rather than just slowing the heat.

Cut rigid foam slightly undersize for each bay, push it against the rim, and seal the perimeter with canned foam so there is no air path around the edges. The sealing is the part that does the work. A piece of foam friction-fitted without sealing performs little better than the batt it replaced.

Divide the rim joist area by 32 sq ft per 4×8 sheet and round up. A 120 ft basement perimeter with 9.25 in rim joists is about 93 sq ft, so 3 sheets of 2 in foam, cut into 90 pieces of 14.5 × 9.25 in for 16 in joist spacing.