Rim Joist Insulation Calculator
Rim joists leak more air per square foot than almost anywhere else in a house. This sizes the foam you need for every bay around the perimeter.
Your project
Your results
- 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
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
Gear for this job
Rigid Foam Board Insulation
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
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Double-Layer Insulated Foam Panels, 3-Pack Insulation Seal Panels for Portable AC Window Vent Seal Kit, Thick Adjustable Insulation Board, Air Conditioner Window Gap Filler
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GCGOODS Window Air Conditioner Insulated Foam Panels, Winter Surround Insulation Side Panel with Top Seal Strip for Window AC Unit Indoor, White
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Spray Foam Sealant Gaps And Cracks
Loctite Tite Foam Gaps & Cracks Spray Foam Sealant, 12 fl oz Can, 1 Pack - Polyurethane Expanding Insulation
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Red Devil 0913 Spray Foam & Fill Small Gaps & Cracks Expanding Insulation, Polyurethane Sealant, 12 oz, Off-White, Pack of 1
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Great Stuff Gaps and Cracks Insulating Foam Sealant,12 Ounce - Case of 12
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Insulation Knife Cutter
Insulation Knife with Measuring Scale, Stainless Steel Double-Sided Serrated Insulation Cutter Tool with Nylon Sheath for Rockwool, Foam, Fiberglass, Drywall, Lawn and Garden
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Goldblatt 8" Rockwool Insulation Knife, Sod Cutter Tool for Fiberglass | Sod Knife, Double-sided Wavy Serrated Stainless Saw, Hammer-end Grip, for Rockwool, Foam, Styrofoam & Pipe Insulation
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Insulation Knife - Sod Cutter Knife: stainless steel 11.14-Inch sharp blade, double side utility Lawn Repair Saw with Nylon Sheath
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How this calculator works
- Rim joist area = perimeter × rim height in feet
- Bays = perimeter in inches ÷ joist spacing
- Board feet = area × thickness
Frequently asked questions
What is the best insulation for rim joists?
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.
How thick should rim joist foam be?
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.
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Rim joist area
93 sq ft