Open- vs closed-cell spray foam calculator
For the same target R, closed-cell hits it in fewer inches while open-cell needs more depth. This lays the two side by side — depth, board-feet and cost — so you buy the foam that actually fits the cavity and the budget.
Calculator
For R-13 over 500 ft², open-cell needs 3.61" (1,806 bd-ft) vs closed-cell 2.00" (1,000 bd-ft). Open-cell is cheaper per board-foot but needs more depth; closed-cell adds rigidity and a low perm rating — a labeled note, not a verdict; vapor/ignition detailing per data sheet & code.
Formula
depth = target_R ÷ R_per_inch · board_feet = area × depth · cost = board_feet × $/board-foot
Same target R, two R/inch values → two depths → two board-feet totals. Because closed-cell packs more R per inch, it always needs fewer board-feet for the same R.
Worked example
500 ft² of wall to R-13. Open-cell at 3.6 R/inch needs 13 ÷ 3.6 = 3.61 inches, so 500 × 3.61 = 1,806 board-feet. Closed-cell at 6.5 R/inch needs 13 ÷ 6.5 = 2.0 inches, so 500 × 2.0 = 1,000 board-feet. Closed-cell moves less than half the board-feet for the same R — but it costs more per board-foot, so plug your two prices in and let the cost columns settle it, not a rule of thumb.
Reading the result like a pro
Board-feet is not the whole story. Use these field notes with the numbers:
- Cavity depth caps you. A 2×4 wall gives ~3.5" of usable depth. Closed-cell can reach a high R inside that; open-cell may run out of room before it hits a big target. Check your framing depth against the depth the tool reports.
- Closed-cell adds rigidity and a low perm rating — useful on rim joists and below grade — while open-cell is a softer air-seal that is cheaper to fill a deep bay. That is a labeled trade-off, not a verdict.
- Overspray and trim. Sprayed foam is cut flush after it rises, so both columns run a little high in practice. Order a margin.
- Vapor, ignition and combustion-air detailing are set by the product data sheet and local code, not by this comparison — check with a professional before you leave foam exposed.
Reference table
| Foam | Labeled R per inch | Depth for R-13 | Depth for R-21 |
|---|---|---|---|
| Open-cell spray foam | R3.5–R3.7/in | 3.61" | 5.83" |
| Closed-cell spray foam | R6.0–R7.0/in | 2.00" | 3.23" |
R/inch is a labeled typical — confirm the rated value on the product data sheet.
Frequently asked questions
Open-cell or closed-cell — which do I need?
It depends on depth, moisture and budget. Closed-cell gives the most R per inch and a low perm rating in a shallow cavity or below grade; open-cell is cheaper to fill a deep, dry bay. Run your target R and prices through the tool and compare the cost columns — then confirm the assembly on the data sheet and local code.
How much more depth does open-cell need?
At the labeled typicals (open ~3.6 R/in, closed ~6.5 R/in), open-cell needs roughly 80% more thickness for the same R. For R-13 that is about 3.6 inches open vs 2.0 inches closed.
Why compare board-feet instead of square feet?
Because foam is sold and priced by the board-foot, and depth differs between the two foams. Comparing board-feet is the only way to line up an apples-to-apples material and cost figure.
Are the R/inch numbers exact?
No — they are labeled planning typicals. Real R depends on install quality and the specific product; confirm the rated R/inch on the data sheet and override the defaults if yours differ.