How Much Spray Foam Insulation Do I Need?
Spray foam coverage depends on thickness and product type. Here's how to calculate board feet and convert to kit or drum quantities.
Open Cell vs. Closed Cell Spray Foam
Open cell spray foam is soft and spongy, with an R-value of about 3.5 to 3.8 per inch. It's less expensive, great for soundproofing, and ideal for interior walls and attics where moisture vapor barriers are installed separately.
Closed cell spray foam is rigid and dense, with an R-value of 6.0 to 7.0 per inch. It also acts as a vapor barrier. Use closed cell in crawl spaces, exterior walls, basements, and any area exposed to moisture.
For the same R-value, closed cell requires half the thickness of open cell — but costs roughly twice as much per board foot. The choice depends on available space and moisture conditions.
What Is a Board Foot of Spray Foam?
Spray foam coverage is measured in board feet (BF) — one square foot at one inch thick. A kit that covers 200 board feet applied at 2 inches thick covers 100 square feet.
To calculate board feet needed: square footage × desired thickness in inches = board feet. A 500-square-foot crawl space at 2 inches of closed cell: 500 × 2 = 1,000 board feet.
DIY spray foam kits (two-component, 200 to 600 BF) are sold at home centers. For large jobs, renting a proportioner and buying in drums is more economical above 1,000 board feet.
Free tool: Spray Foam Calculator →
R-Value Targets by Application
Attic (spray foam at roof deck): R-38 to R-60. At 6.5 R per inch for closed cell, that's 6 to 9 inches. Open cell at 3.7 R/in needs 10 to 16 inches — use closed cell in tight attics.
Exterior walls: R-13 to R-21. Closed cell at 2 to 3 inches achieves R-13 to R-21 while also acting as a vapor barrier. This is common in new construction and deep energy retrofits.
Crawl space walls and rim joists: R-10 to R-19. Two to three inches of closed cell on crawl space walls achieves R-13 to R-20 and seals against moisture intrusion simultaneously.
How to Estimate Spray Foam Quantity
Measure the total surface area to be sprayed in square feet. Multiply by the target thickness in inches to get board feet. Add 10 to 15% for overspray and hard-to-reach areas.
Example: rim joists in a 50×30-foot house. Perimeter = 160 linear feet. At 8 inches tall, rim joist area = 160 × 0.67 ft = 107 sq ft. At 3 inches closed cell: 107 × 3 = 321 BF + 15% = 370 BF. One 600-BF kit covers this with room to spare.
Our spray foam calculator converts square footage and thickness directly to board feet and kit count — saving you the back-of-envelope math.
Cost of Spray Foam Insulation
DIY spray foam kits cost $100 to $150 per 200-board-foot kit, or $0.50 to $0.75 per board foot. Professional installation costs $1.00 to $2.00 per board foot for open cell, $1.50 to $3.00 per board foot for closed cell.
A typical 2,000-square-foot house with 2 inches of closed cell on all exterior walls (2,000 sq ft × 2 in = 4,000 BF) costs $6,000 to $12,000 professionally installed.
The payback period for spray foam is typically 3 to 7 years through energy savings. Energy audits and utility rebate programs often offset 10 to 30% of installation cost.
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FAQ
- How much does a spray foam kit cover?
- A standard 200-board-foot kit covers 200 square feet at 1 inch thick, or 100 square feet at 2 inches thick. Larger kits (600 BF) cover proportionally more. Always calculate board feet (area × thickness) first.
- What R-value does spray foam give per inch?
- Open cell spray foam: R-3.5 to R-3.8 per inch. Closed cell spray foam: R-6.0 to R-7.0 per inch. Closed cell provides more than double the insulation per inch.
- Should I use open cell or closed cell spray foam?
- Use closed cell in crawl spaces, basements, exterior walls, and anywhere moisture is a concern — it's a vapor barrier. Use open cell in interior walls and attics where you want soundproofing and cost savings.
- Can I spray foam myself?
- Yes, with DIY two-component kits. Wear a respirator, eye protection, and disposable coveralls — cured foam is inert but the components are hazardous during application. Ventilate well and keep the area temperature above 60°F.