As an Amazon Associate I earn from qualifying purchases.
R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Rim joist area = basement perimeter × rim height: a 120 ft perimeter with 9.25 in joists is about 93 sq ft, or 90 bays at 16 in on center, each filled with a 14.5 × 9.25 in piece of 2 in foam (R-10) sealed with canned foam. Enter your basement below for bays, cut sizes, sheets and cost — rim joists leak more air per square foot than almost anywhere else in a house.
With these inputs:
| Result | Value |
|---|---|
| Rim joist area | 93 sq ft |
| Bays to fill | 90 bays |
| Foam cut size per bay | 15 in × 9.25 in |
| 4×8 sheets required | 3 sheets |
| Spray foam alternative | 185 bd ft |
| R-value achieved | R-10 |
| Material cost | $78 |
Rim joist area = perimeter × rim height in feet · Bays = perimeter in inches ÷ joist spacing · Board feet = area × thickness
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.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.