Spray Foam Insulation Calculator
Find how thick to spray foam insulation for rim joists, walls or foundation walls, how many board feet that takes, and how many kits to buy.
Spray foam is sold by the board foot, a square foot one inch thick, so the amount depends on how thick it has to go. Closed-cell foam reaches the same R-value in about half the thickness of open cell, and a wall cavity limits how deep either can go.
Where you're spraying
Each spot has a building-code insulation level, which sets how thick the foam goes.
Area to spray
For rim joists, the house's perimeter times about ¾ ft of joist height; for walls, length times height, less windows and doors.
Type of foam
Closed cell is the common choice for rim joists and foundation walls; open cell is cheaper per board foot.
Spray foam kits to buy
1 kit of 630 board feet
Closed-cell foam to match the R-13 code level for walls
A board foot is one square foot of foam one inch thick. Building codes usually require exposed spray foam to be covered with drywall or another ignition barrier, and colder climates call for more insulation than this.
💡About this calculator▼
Spray foam is sold by the board foot, one square foot of foam one inch thick, so how much you need depends as much on thickness as on area. Two inches of closed-cell foam reaches about the same insulation level as three and a half inches of open cell.
This works out the thickness and the amount. Choose where you're spraying (rim joists, 2×4 walls, 2×6 walls or basement and crawl space walls), enter the area, and pick closed or open cell. The calculator sets the thickness from the building-code insulation level for that spot, never deeper than a wall cavity holds, and gives you the board feet and, for closed cell, the kits to buy.
For pricing the whole job, our attic insulation cost calculator compares spray foam with blown-in insulation by the square foot.
The insulation level. The national energy code sets minimum R-values: R-13 for wood-frame walls in the warmest climates, R-20 in mixed ones (with more in colder zones), and R-10 of continuous insulation on basement and crawl space walls in zone 4. Rim joists, where the floor framing sits on the foundation, are usually insulated to the wall level, R-13.
Thickness. Closed-cell foam gives about R-6.1 per inch (one maker rates its kits at R-12.2 for 2 inches), and open cell about R-3.7. Thickness is the R-value divided by that, rounded up to the next quarter inch, but never deeper than the stud cavity: a 2×4 wall is 3½ inches deep and a 2×6 wall 5½.
Board feet and kits. Board feet are square feet times inches of thickness. Closed-cell DIY kits are sold by the board feet they spray: a small kit makes about 200 board feet and a large one about 630. The calculator uses one small kit if it covers the job, otherwise large kits, topped up with a small one when 200 board feet or fewer are left. Open-cell foam is shown in board feet.
Worked example. 150 square feet of rim joists with closed-cell foam: R-13 ÷ 6.1 = 2.13 inches, so spray 2¼ inches (about R-14). That's 150 × 2.25 = about 340 board feet, one 630 board-foot kit.
📐How it's calculated▼
Thickness = code R-value ÷ R per inch, rounded up to the quarter inch, no deeper than the stud cavity (3½ in for 2×4, 5½ in for 2×6).
Code R-values: rim joists R-13 (the wall level), 2×4 walls R-13, 2×6 walls R-20, basement and crawl space walls R-10. R per inch: closed cell 6.1, open cell 3.7.
Board feet = area (sq ft) × thickness (in). Closed-cell kits: 200 or 630 board feet each.
150 sq ft of rim joists, closed cell: 2¼ in, about 340 board feet, one 630 kit. 300 sq ft of 2×6 wall, closed cell: 3½ in (R-21), 1,050 board feet, two 630 kits. 300 sq ft of 2×4 wall, open cell: 3½ in filling the cavity (R-13), 1,050 board feet.
📎Sources:ICC — 2021 International Energy Conservation Code, residential insulation levels (Table R402.1.3),DuPont — Froth-Pak spray foam kits (R-value and yields),U.S. DOE Building America — Spray foam insulation for existing foundation walls
🔍Finding your inputs▼
Where you're spraying: Rim joists are the band of framing where the floor meets the foundation, usually seen from the basement. 2×4 and 2×6 walls are open stud cavities, 3½ and 5½ inches deep. Basement or crawl walls are the concrete foundation walls themselves.
Area to spray: For rim joists, multiply the house's perimeter by the height of the joists, about ¾ of a foot. For walls, multiply the length by the height and subtract windows and doors.
Type of foam: Closed cell is denser, insulates about R-6 an inch and is the usual choice for rim joists and foundation walls. Open cell is lighter, about R-3.7 an inch, and needs nearly twice the thickness for the same R-value.
⚠️Special situations▼
I want to spray under the roof deck
That takes far more foam: the energy code asks for R-49 at the ceiling in most mixed climates, about 8 inches of closed-cell foam or 13 inches of open cell, deeper than most rafters. It is usually a job for an installer; our attic insulation cost calculator prices it per square foot.
I live in a cold climate
The code asks for more there: walls at R-20 plus continuous insulation, and R-15 on basement walls in zones 5 to 8. For basement walls, set the thickness for R-15: about 2½ inches of closed-cell foam, half again what the calculator shows for zone 4.
Do I need to cover the foam?
Usually, yes. Building codes generally require exposed spray foam in living spaces and basements to be covered with an ignition barrier such as drywall. Check your local code before leaving foam exposed.
❓Common questions▼
How many board feet of spray foam do I need?
Multiply the area by the thickness in inches. Rim joists in a typical basement, about 150 square feet at 2¼ inches of closed-cell foam, take about 340 board feet. A 300-square-foot 2×6 wall at 3½ inches takes 1,050.
How thick should closed-cell spray foam be?
About 2¼ inches for R-13, the usual level for rim joists and 2×4 walls, and 3½ inches for R-20 in 2×6 walls. On basement and crawl space walls, about 1¾ inches reaches R-10.
How much does a spray foam kit cover?
By board feet: a small closed-cell kit sprays about 200 board feet and a large one about 630. At 2¼ inches thick, a 630 kit covers about 280 square feet; at 1 inch, 630 square feet.
Is closed-cell or open-cell spray foam better?
Closed cell insulates about R-6 an inch versus about R-3.7 for open cell, so it reaches the same level in roughly half the thickness, and federal building guidance calls for it on foundation walls. Open cell is lighter and fills a wall cavity at lower cost per board foot.
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