Rigid-Foam Board / Sheet Calculator
How many 4×8 sheets, and what R does the board give? Sheets = area ÷ 32; R = thickness × R per inch.
Calculator
500 ft² takes 16 sheets of 4×8 board; at 2.0" and 6.00 R/inch the board is R-12. Sheet size 4×8 = 32 ft² labeled; foam R/inch is a labeled typical (EPS ~4.0, XPS ~5.0, polyiso ~6.0) — confirm on the board.
Rigid foam comes in 4×8 sheets, so ordering is a tiling problem: divide the wall or roof area by 32 ft² per sheet and round up. This tool also returns the R the board delivers, so you can size continuous exterior insulation — the layer that beats thermal bridging because no stud crosses it.
The default covers 500 ft² in 2″ polyiso (R-6.0/in).
Formula
sheets = ceil( area_sqft ÷ 32 ) (a 4×8 sheet = 32 ft²)
board_R = thickness_in × R_per_inch
Worked example
500 ft² wall, 2″ polyiso. Sheets = 500 ÷ 32 = 15.6 → 16 sheets. Board R = 2 × 6.0 = R-12.0. That R-12 is continuous — it insulates over the studs, so it lifts the whole-wall R far more than the same R buried in the cavity.
Background & practice
Sheets do not tile perfectly. The 16-sheet count assumes clean coverage; real walls have windows, corners and offcuts that both add waste and give you usable scraps. Order a sheet or two over on cut-up elevations.
Know your board. EPS ~R-4.0/in (cheapest, drains well), XPS ~R-5.0/in (moisture-tolerant, common below grade), polyiso ~R-6.0/in (highest R, but its rated R drops in cold weather — some builders derate it in winter climates). Foil-faced polyiso also acts as a radiant/vapor layer.
What to measure first: the net area, the fastener length you need to reach framing through the foam, and the code’s ignition/thermal-barrier rule if the foam is left exposed. For the R-value trade-off between the three boards, use the materials sheet calculator.
Reference table
| Board | 1" | 1.5" | 2" | 3" | Sheets / 1,000 ft² |
|---|---|---|---|---|---|
| Rigid EPS | R-3.9 | R-5.85 | R-7.8 | R-11.7 | 32 (4×8) |
| Rigid XPS | R-4.75 | R-7.13 | R-9.5 | R-14.25 | 32 (4×8) |
| Rigid polyiso | R-6.05 | R-9.08 | R-12.1 | R-18.15 | 32 (4×8) |
Board R at the labeled band midpoint; a 4×8 sheet = 32 ft². Polyiso’s rated R drops in cold weather — some builders derate it below freezing. Confirm the printed R on the board.