Rigid foam R-value & sheet calculator
Rigid foam is bought by the 4×8 sheet (32 ft²) and rated by the inch. Give it your thickness, the board’s R per inch and the area, and this returns the assembly R you get and the number of sheets to load in the truck.
Calculator
At 2.00" and 5.00 R/inch the board is R-10; 640 ft² takes 20 sheets. Polyiso R/inch drops in cold temperatures — a labeled caveat; confirm the rated R on the board.
Formula
R = thickness × R_per_inch · sheets = ceil(area ÷ sheet_area)
The R adds in series with the rest of your assembly, so continuous exterior foam over studs also cuts thermal bridging — a bonus the raw R does not show. Sheets always round up: you cannot buy two-thirds of a board.
Worked example
2 inches of XPS at 5.0 R/inch is 2 × 5.0 = R-10. Covering 640 ft² with 4×8 sheets is 640 ÷ 32 = 20 sheets exactly — and because there is no waste factor baked in, buy a sheet or two extra for cuts around openings and offcuts you cannot reuse. Swap to polyiso at 6.0 R/inch and the same 2 inches jumps to R-12.
EPS vs XPS vs polyiso, and the cuts
Pick the board by where it goes, then order sheets with a margin:
- EPS (~4.0 R/inch) is the cheapest and fine below grade and under slabs; lower R per inch means more thickness for a given R.
- XPS (~5.0) is the moisture-resistant middle ground — the pink/blue board for foundations and walls.
- Polyiso (~6.0) has the highest R per inch, but the rating drops in cold weather, so a labeled R-6 board delivers less on a January wall. For exterior continuous insulation in a cold climate, do not bank on the full number.
- Cutting waste is real. The sheet count is exact area ÷ 32; once you cut around windows, doors and out-of-square framing, offcuts pile up. Add ~10% on a cut-up wall.
Foam usually needs an ignition or thermal barrier when left exposed, and the vapor/moisture detailing is set by the product data sheet and local code — not by this sheet count. Confirm both before you close it up.
Reference table
| Rigid board | Labeled R per inch | R at 1" | R at 2" |
|---|---|---|---|
| Rigid EPS | R3.6–R4.2/in | R-3.9 | R-7.8 |
| Rigid XPS | R4.5–R5.0/in | R-4.75 | R-9.5 |
| Rigid polyiso | R5.6–R6.5/in | R-6.05 | R-12.1 |
A 4×8 sheet is 32 ft². Polyiso is rated warm and drops in the cold — a labeled caveat; use the rated R printed on the board.
Frequently asked questions
How many sheets of rigid foam do I need?
Divide the area by 32 (a 4×8 sheet) and round up: 640 ft² is 20 sheets. Add a sheet or two for cuts around openings and offcuts you cannot reuse.
What is the R-value of 2 inches of rigid foam?
It depends on the board: about R-8 for EPS (4.0/in), R-10 for XPS (5.0/in) or R-12 for polyiso (6.0/in). Use the rated R per inch printed on your board.
Which rigid foam has the highest R per inch?
Polyiso, at a labeled ~5.6–6.5 per inch — but it rates warm and drops in the cold, so for exterior use in a cold climate do not count on the full number.
Do the R-values add to my wall?
Yes — rigid foam R adds in series with the cavity insulation and sheathing, and continuous exterior foam also cuts thermal bridging through the studs. Total the layers with the R-value calculator.