Rim-joist insulation calculator
The rim joist is a top air-sealing win. Enter the perimeter and rim height and get the area, the spray-foam board-feet and kits, or the rigid pieces to cut to the bays.
Calculator
A 140 ft perimeter × 11" rim is 128.8 ft² — about 257.6 board-feet (2 kits) of foam, or 5 rigid 4×8 sheets cut to the bays. Rim joists are a top air-sealing priority.
Formula
rim_area = perimeter_ft × rim_height_ft
board_feet = rim_area × foam_thickness_in · kits = ceil( board_feet ÷ yield_per_set )
rigid_pieces = ceil( rim_area ÷ sheet_area_ft² )
Two ways to fill the same band. Spray foam is measured in board-feet (1 board-foot = 1 ft² at 1 inch) and rounded up to whole kits; rigid foam is measured as 4×8 sheets you cut into blocks for each bay. Either way, start from the rim area — perimeter times the joist depth.
Worked example
A 140 ft perimeter with an 11" (0.92 ft) rim, filled 2" with closed-cell foam from 200 bd-ft DIY kits, or with 4×8 rigid board:
Rim area: 140 × 0.92 = 128.8 ft². Board-feet: 128.8 × 2 = 257.6 bd-ft. Kits: ceil(257.6 ÷ 200) = 2 kits. Rigid: ceil(128.8 ÷ 32) = 5 sheets.
So it is 2 small foam kits, or 5 sheets of rigid board cut into rim blocks. The foam air-seals as it fills; if you go rigid, run a bead of canned foam around every block to seal the edges — that seal is the whole point of doing the rim.
Seal the band, then fill it
The rim joist is an air-sealing job first. The band where the floor framing sits on the foundation is one of the leakiest lines in the house — it is the reason you do this at all. Closed-cell spray foam or rigid board sealed at the edges both stop air; a bare fiberglass batt stuffed into the rim looks insulated but leaks air straight through and can trap condensation. Use foam here.
2 inches of closed-cell is the usual target. At ~6.5 R/inch, 2" gives about R-13 and stays above the dew point in most climates, so the rim does not sweat. Match or beat the R of the wall above it. Colder zones sometimes go to 3".
Cut rigid blocks a touch small, then seal the gap. Rigid foam cut slightly undersized and set into each bay, with canned spray foam around all four edges, is a clean DIY rim detail. The gap you leave for the foam bead is deliberate — a tight friction fit with no seal still leaks. That is why the sheet count assumes some cutting waste already.
What this is not. It is a quantity tool. It does not certify the ignition/thermal barrier, moisture or combustion-air detailing on foam — those are set by the product data sheet and local code. Keep foam away from heat sources like flues, and confirm the assembly with a pro.
Reference table
Closed-cell foam thickness at the rim and the R it delivers (at a ~6.5 R/inch labeled typical) — board-feet per ft² is just the thickness.
| Foam thickness | Approx R | Board-feet per ft² |
|---|---|---|
| 1 in | R-6.5 | 1 bd-ft |
| 2 in | R-13 | 2 bd-ft |
| 3 in | R-19.5 | 3 bd-ft |
Labeled typicals — DIY kits yield ~200 or ~600 bd-ft; confirm R and yield on the kit and data sheet.