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
Multi Purpose XPS Foam Board,2 Pack 15.6x11.7X 2in Polystyrene Craft Foam Sheets,Rigid and Easy to Shape,DIY Modeling Art Projects, Party Decoration, Cosplay Prop Production, Carve,Heat Insulation
Check price on Amazon
LuckyFoam Blue 4 Pack 15x12x2 inch Foam Sheets 2 Inch Thick | Polystyrene Rectangles XPS Foam Board Craft for Insulating DIY Modeling Art Supplies
Check price on Amazon
Fabbay 2 Pcs Pink XPS Foam Board 15" X 12" 2" Thick Rectangle Craft | Polystyrene Foam for Insulation, Modeling Art Supplies, Lightweight Pink Boards, Easy to Cut and Shape for DIY Projects
Check price on Amazon
F&L R6 Fiberglass Board Acoustic Insulation Panels – 1" to 2" Thick, 24"x24" and 24"x48" – Thermal Soundproofing for Walls, Ceilings, Studios & HVAC Duct Applications (1-1/2in x 24in x 48in)
Check price on Amazon
As an Amazon Associate we earn from qualifying purchases. Product links on this page may earn us a commission at no extra cost to you.
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.
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.
Multi Purpose XPS Foam Board,2 Pack 15.6x11.7X 2in Polystyrene Craft Foam Sheets,Rigid and Easy to Shape,DIY Modeling Art Projects, Party Decoration, Cosplay Prop Production, Carve,Heat Insulation
Check price on Amazon
Multi Purpose XPS Foam Board,2 Pack 15.6x11.7X 2in Polystyrene Craft Foam Sheets,Rigid and Easy to Shape,DIY Modeling Art Projects, Party Decoration, Cosplay Prop Production, Carve,Heat Insulation
Check price on Amazon
BEEST FullStop Spray Foam Insulation Kit - 240 Board ft/sq.ft. Fire Rated Spray Insulation Foam for Walls & Attics with Thermal & Soundproofing - Includes 12x Cans, 1 Cleaner, Pro X Gun, Safety Gear
Check price on Amazon
Loctite Tite Foam Gaps & Cracks Spray Foam Sealant, 12 fl oz Can, 1 Pack - Polyurethane Expanding Insulation
Check price on Amazon
As an Amazon Associate we earn from qualifying purchases. Product links on this page may earn us a commission at no extra cost to you.