How-to
How to Insulate Rim Joists
The rim joist — the band board sitting on the foundation — is a few square feet of surface that can account for a tenth of a home's heat loss. It's also one of the cheapest and quickest fixes in the whole house.
Updated January 2026 · about 6 min read
Why fiberglass does not work here
Fiberglass is air permeable. Warm indoor air moves through it, reaches the cold band board, and condenses — which is why old fiberglass stuffed in rim bays is so often black and damp.
Rim joists need an air-impermeable insulation: rigid foam sealed at the edges, or closed-cell spray foam.
Cut-and-cobble with rigid foam
Measure each bay, cut 2-inch XPS or polyiso about half an inch small on each side, push it back against the band board, then seal all four edges with canned foam. Two inches of polyiso is about R-12; add a second layer in cold zones for R-20.
Foil-faced polyiso doubles as a vapor retarder and reflects heat back into the space. Keep foam away from any chimney or flue penetration.
Gear for this job
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Closed-cell spray foam
A two-part kit sprays 2–3 inches across all bays in an afternoon and seals itself. It costs more per square foot but handles irregular bays, wiring and pipes with no cutting. Wear a supplied-air or organic vapor respirator and ventilate.
Do it while you're already down there
Rim joist work pairs naturally with basement wall insulation, crawl space encapsulation or duct sealing. The tools and mess are the same.
Cut board or canned foam? Rigid foam vs spray foam compares the two.
Run the numbers
Frequently asked questions
- What R-value do rim joists need?
- R-10 to R-20 of air-impermeable insulation depending on climate zone — roughly 2 inches of closed-cell foam or 2–4 inches of rigid board.
- Can I use fiberglass in rim joists?
- Only over a sealed foam layer. Fiberglass alone lets moist air reach the cold band board and condense.
- How much can rim joist insulation save?
- In a leaky older home, sealing and insulating the rim commonly cuts 5–10% off heating costs for a few hundred dollars in materials.
Insulation gear worth owning
The tools and materials that show up on almost every insulation job.
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