Spray Foam Insulation Calculator
Spray polyurethane foam (SPF) insulation is measured in board feet — one board foot is one square foot at one inch thick. Closed-cell spray foam achieves R-6.5 per inch and is a vapor barrier; open-cell foam achieves R-3.7 per inch and is vapor-permeable. Choose closed-cell for exterior walls, rim joists, roof deck (unvented attic), and any location with moisture risk. Choose open-cell for interior walls (soundproofing), interior ceilings, and where vapor barriers are not desired. Enter area to insulate, foam type, and target R-value to get board feet required, foam thickness, optimal mix of DIY kit sizes (200 BF, 600 BF, 1500 BF), and estimated material cost. Always wear full PPE (respirator, gloves, protective suit) when applying — uncured spray foam is a strong respiratory irritant.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How many board feet of spray foam do I need for a 200 sq ft crawl space?
For closed-cell foam at R-20 in a crawl space: R-20 / 6.5 R/inch = 3.1 inches thick. Board feet = 200 sq ft × 3.1 in = 620 BF. Use one 600-BF kit plus one 200-BF kit, or one 1500-BF kit if you have more areas to do.
What is the difference between open-cell and closed-cell spray foam?
Closed-cell foam (R-6.5/in) is dense, rigid, and acts as a vapor barrier — best for exterior walls, rim joists, and below-grade applications. Open-cell foam (R-3.7/in) is soft and spongy — better for interior walls and soundproofing where vapor barriers are not needed. Closed-cell costs about 2× more per board foot.
Can I spray foam over existing insulation?
Yes, in some applications. You can spray foam over existing fiberglass batts in rim joists — the foam air-seals where the batts can't. Spraying over existing blown-in attic insulation is generally not done, as it encapsulates the existing insulation and the cost rarely justifies the marginal R-value gain. For walls, spray foam is applied to open cavities before drywall is hung.
Is spray foam insulation worth the cost?
Spray foam costs $1.50–$3.50 per board foot installed by a contractor, vs. $0.50–$0.75/BF for fiberglass batts. The premium pays off primarily for air sealing — spray foam both insulates and air-seals in one step, eliminating the separate air sealing step required with batts. For rim joists, crawl spaces, and vaulted ceilings where air sealing is critical and access is difficult, spray foam is often the right choice despite the higher cost.