Rim Joist Insulation Calculator

What this calculation does not cover

• Board thickness is not an input, so the count is the same for a 25 mm board as for a 100 mm one, and it covers a single layer. Nothing here tests the resulting R-value against what the rim joist is expected to achieve, and building up two layers to reach a target doubles the sheet count.
• Thickness is also what keeps the rim board dry, and that is not checked either. Foam too thin for the climate leaves the inner face of the rim board cold enough for indoor air to condense against it, so the penalty for getting thickness wrong here is damp timber rather than a slightly higher heating bill, and the thickness needed to hold that face above the dew point depends on the climate zone.
• The 10% allowance covers ordinary offcuts, not the yield lost to cutting a whole sheet down into bay-sized pieces. A 4x8 ft sheet cut into 9.25 in by 14.5 in rectangles — 2x10 joists, the default here, at 16 in centres — gives 30 pieces, about 87% of the sheet, with the rip and end strips discarded, so a long band worked out from area alone can come up a sheet short.
• Only the boards are counted. The canned foam that seals the perimeter of every cut piece, the sealant at the sill plate, and any covering the foam needs where it is left exposed are all outside the number — foam plastic inside a dwelling normally has to sit behind a thermal or ignition barrier, and whether the rim joist band is granted an exception is jurisdictional.
• Perimeter times joist height treats the band as an unbroken rectangle. Nothing is deducted for basement windows, walkout doors or beam pockets, and nothing is added where a cantilever, bay window or bump-out carries the band past the foundation line that the help text asks you to measure.
• The band is assumed to be exactly as tall as the joist. Many rim joist details are insulated from the underside of the subfloor down over the sill plate to the top of the foundation wall, which is taller than the joist depth entered here.