Calculate spray foam insulation board feet, R-value, kits, and estimated cost from area and thickness for open or closed cell foam, or the area a kit covers.
Spray Foam Insulation Formula
Spray foam is measured in board feet. One board foot is one square foot of surface covered at one inch thick. The core calculation multiplies the area you are covering by the thickness you plan to spray:
BF = A * T
Add a chosen overage percentage to the geometric volume for quantity planning. This is an additive allowance, not a measured yield-loss correction or a guarantee of sufficient usable product. For example, 10% overage multiplies by 1.10; compensating for a measured 10% loss would instead divide by 0.90:
BF_total = BF * (1 + W / 100)
Variables:
- BF is the board feet of spray foam for the job
- A is the area you are covering, in square feet
- T is the installed thickness, in inches
- BF_total is the planned board feet including the chosen additive overage
- W is the chosen additive overage percentage; the calculator defaults to 10%, not a universal loss rate
The calculator uses a linear nominal R-per-inch model for the following equations. Actual product ratings may depend on thickness, aging and test conditions; the model does not verify a finished assembly’s R-value, vapor control or code compliance. R here uses US customary units (ft²·h·°F/Btu), not SI RSI:
R = T * Rpi T = R_target / Rpi
You can also run the math backward. If you already own a kit or a set with a known board foot yield, divide that yield by the thickness to see how much surface it covers, and divide the total board feet by a kit size to see how many kits to buy:
A_cov = BF / T Kits = roundup(BF_total / K)
Variables:
- R is the nominal R-value calculated by the linear model
- Rpi is the assumed R-value per inch; 3.7 open cell and 6.5 closed cell are editable illustrative defaults, not universal product ratings
- R_target is the R-value you want to reach
- A_cov is the area a fixed amount of foam will cover, in square feet
- K is the board feet per kit
- Kits is the number of whole kits to buy, rounded up
The calculator offers project quantities and cost, coverage from available board feet, thickness from target R-value, and R-value from thickness. Project quantities accept total area or length × width and either depth or target R-value. Enter product-specific assumptions where available. Exact decimal calculations determine whole kits without rounding intermediate board feet; displayed quantities may be approximate. An explicitly entered zero cost is valid; negative assumptions are rejected.
Foam Type and Thickness Reference
This table preserves archived source examples for comparison, not current prices or product specifications. Product-specific data determine R-value, vapor permeance and usable yield. The listed applications do not establish suitability or a compliant assembly.
| Property | Open cell | Closed cell |
|---|---|---|
| Archived illustrative R/in ranges; not universal | 3.5 to 3.8 | 6.0 to 7.0 |
| Archived example cost per board foot; not current quotes | $0.44 to $0.65 | $1.00 to $1.60 |
| Vapor control | Product- and assembly-specific | Product-, thickness- and assembly-specific |
| Structural contribution | Not assessed by this calculator | Not assessed by this calculator |
| Archived set-yield examples; verify the actual product | 16,000 board feet | 4,000 board feet |
| Source example applications; suitability unverified | Interior walls, vented attics, sound control | Crawl spaces, basements, exterior walls, roof decks |
The following table retains illustrative source calculations per 100 square feet, using nominal R-3.7/in open cell or R-6.5/in closed cell and archived example point rates of $0.55 or $1.30 per board foot. It includes no overage or unentered labor, equipment, shipping, tax or other costs. These depths and applications are arithmetic examples, not design requirements or installation recommendations; the rates are not current quotes.
| Application | Foam | Thickness | R-value | Board feet / 100 sq ft | Material / 100 sq ft |
|---|---|---|---|---|---|
| Roof deck (unvented) | Closed | 5 in | R-32.5 | 500 | $650 |
| Attic floor (vented) | Open | 10 in | R-37 | 1,000 | $550 |
| Exterior wall | Closed | 3 in | R-19.5 | 300 | $390 |
| Exterior wall | Open | 5 in | R-18.5 | 500 | $275 |
| Basement wall | Closed | 2.5 in | R-16.3 | 250 | $325 |
| Crawl space wall | Closed | 2 in | R-13 | 200 | $260 |
| Rim joist | Closed | 3 in | R-19.5 | 300 | $390 |
Use the selected product’s tested data and project requirements. Manufacturer-rated yield is theoretical or test-condition yield; installation, temperature, technique and cavity packing can change usable output, with no universal loss percentage. See manufacturer technical-data examples for product-specific thickness, aging, vapor-permeance and yield conditions. Follow product installation instructions and safety data for PPE, ventilation and re-entry; EPA’s archived SPF safety guidance explains why these precautions matter.
Example Problems
Example 1: Foam and cost for an attic roof deck.
You are spraying an unvented attic roof deck that measures 1,000 square feet with open cell foam at 8 inches. Multiply area by thickness for board feet, then add a 10 percent overage:
BF = 1,000 * 8 = 8,000 board feet. With 10 percent overage, BF_total = 8,800 board feet. The nominal model gives R = 8 * 3.7 = R-29.6. At the archived example rate of $0.55 per board foot, the volume-based estimate is $4,840, and at an assumed usable 600 board feet per kit the exact whole-kit ceiling is 15 kits.
Example 2: Foam for a basement wall.
For a quantity example using 500 square feet at 3 inches of closed cell foam, BF = 500 * 3 = 1,500 board feet, or 1,650 board feet with a 10 percent overage. The nominal model gives R = 3 * 6.5 = R-19.5. At the archived example rate of $1.30 per board foot the volume-based estimate is $2,145, and an assumed usable 600 board feet per kit gives an exact ceiling of 3 kits. This example does not verify a basement assembly.
Frequently Asked Questions
How much spray foam do I need per square foot?
One square foot at one inch thick is one board foot, so 100 square feet at 3 inches represents 300 geometric board feet before overage. Choose a suitable thickness using the product’s tested performance and project requirements, then multiply by area. A nominal R-value calculation alone does not establish an installation depth or compliant assembly.
Is open cell or closed cell cheaper for the same R-value?
Under the source’s illustrative assumptions, nominal R-30 requires about 8.1 inches at R-3.7/in or 4.6 inches at R-6.5/in. The fixed archived $0.55/$1.30-per-board-foot comparison gives different estimated costs; actual quotes and products can change the result. The comparison rows retain those fixed source assumptions rather than custom selected-product overrides. Cost, vapor control, structural contribution and available cavity depth require product- and project-specific assessment; the calculator does not select a suitable foam.
How many board feet are in a spray foam kit or set?
The 200, 300 and 600 board-foot choices and the archived 16,000-open/4,000-closed set choices are planning examples, not standard capacities for every kit or drum set. Enter the actual product’s usable or rated board feet as appropriate. Theoretical yield excludes some application losses, and actual yield depends on product and conditions; there is no universal 10–20% reduction. Coverage divides the selected board feet by thickness without silently applying a loss factor.
