Spray Foam Insulation Calculator: Open-Cell vs Closed-Cell, Board Feet, R-Value by Thickness
Enter your insulation area and target thickness or target R-value. Compare open-cell (R-3.7 per inch) and closed-cell (R-6.5 per inch) side by side. Get board feet needed, R-value achieved, DIY two-component kit count by size, and a vapor barrier note for closed-cell at 2 inches. Application presets for walls, attic roof deck, rim joists, and crawlspace walls.
Select open-cell (R-3.7 per inch), closed-cell (R-6.5 per inch), or compare both. Use an application preset or enter custom thickness. Switch to R-value mode to enter a target R-value and get the required thickness. Results show board feet, R-value achieved, DIY two-component kit count in four standard sizes, and a vapor barrier note for closed-cell at 2 inches.
Open-Cell vs Closed-Cell Spray Foam: R-Value per Inch, Board Feet, Vapor Barrier, and US
Spray polyurethane foam (SPF) insulation is the highest-performance insulation product available for US residential and commercial construction. Unlike fiberglass batts or blown-in cellulose, spray foam expands on contact to fill cavities, gaps, and cracks completely, simultaneously providing both air sealing and thermal insulation in a single application. The result is a significantly more airtight building envelope than any other insulation product can achieve. This combination of air barrier and thermal barrier makes properly installed spray foam one of the highest-return energy investments available for US homeowners.
Two distinct chemistries are used in US spray foam applications, with dramatically different performance profiles. Open-cell foam (0.5 lb per cubic foot density) expands aggressively, fills cavities completely, is vapor permeable, and achieves R-3.7 per inch. Closed-cell foam (2.0 lb per cubic foot density) is denser, more rigid, vapor-resistant, and achieves R-6.5 per inch — nearly double the R-value per inch of open-cell. The right choice depends on the application, the climate zone, the available cavity depth, and whether a vapor retarder is needed.
Open-Cell vs Closed-Cell Spray Foam: Key Properties for US Applications
| Property | Open-Cell (0.5 lb) | Closed-Cell (2.0 lb) |
|---|---|---|
| R-value per inch | R-3.7 | R-6.5 |
| Typical cost (installed, per BF) | $0.44 to $0.65 | $1.00 to $1.50 |
| Vapor permeability | Permeable (not a vapor barrier) | Less than 1 perm at 2+ inches (Class II vapor retarder) |
| Air barrier | Yes (at 3.5 inches or more) | Yes (at 1 inch or more) |
| Structural rigidity added | Minimal | Significant (adds racking resistance) |
| Moisture resistance | Absorbs water if exposed | Resists water absorption |
| Expansion ratio | Up to 100x original volume | Up to 40x original volume |
| Best US applications | Interior wall cavities, attic floors, interior sound | Rim joists, crawlspace walls, roof deck, exterior walls |
How the Spray Foam Calculator Works
Board feet = area (sq ft) x thickness (inches). This is the standard US unit for spray foam quantity: 1 board foot = 1 square foot of coverage at 1 inch thick. In thickness mode: board feet = area x thickness x waste factor. R-value = thickness x R-per-inch (open-cell: 3.7, closed-cell: 6.5). In R-value mode: required thickness = target_R / R_per_inch, then board feet = area x thickness x waste factor. DIY kit count = ceil(board_feet / kit_size) for each kit size. Vapor barrier note triggers when closed-cell thickness is 2 inches or more. The board foot is also used by professional spray foam contractors for bidding — knowing your board foot requirement is essential for getting accurate quotes.
3 Real Spray Foam Estimates
Application: Sealing and insulating rim joists in a 1,500 sq ft ranch house. Total rim joist area (perimeter of 130 lin ft x typical 10-inch joist depth): approximately 108 sq ft. Closed-cell foam at 2 inches.
Board feet: 108 sq ft x 2 in = 216 BF raw. With 15% waste: 248 BF.
R-value: 2 in x 6.5 = R-13.
DIY kits: Two 200-BF two-component kits cover 400 BF total. 2 kits of 200 BF.
Vapor barrier note: At 2 inches of closed-cell, this assembly becomes a Class II vapor retarder (less than 1 perm). This is appropriate for a rim joist application where moisture control is important.
Application: Unvented attic created by spraying open-cell foam against the underside of the roof deck (between and over the rafters). Roof deck area: 2,400 sq ft. Open-cell at 5.5 inches to meet Georgia R-19 attic requirement for an unvented assembly.
Board feet: 2,400 sq ft x 5.5 in = 13,200 BF raw. With 15% waste (overhead application): 15,180 BF.
R-value: 5.5 in x 3.7 = R-20.35 (meets R-20 code minimum for unvented attic in Georgia).
Professional application: At 15,180 BF, this requires a professional spray foam contractor with a proportioner rig. DIY kits in 600 BF size: 26 kits at approximately $400 to $500 each = $10,400 to $13,000 in material alone. Professional installed cost (materials + labor) in the Atlanta market: typically $1.75 to $2.25 per BF for open-cell applied overhead = $26,565 to $34,155. The professional cost includes equipment, labor, and warranty.
Application: Exterior wall cavities in a 1,200 sq ft addition. Gross wall area: 800 sq ft. 2×4 framing (3.5-inch cavity). Question: which foam type fills the cavity with adequate R-value?
Open-cell at 3.5 inches: 800 x 3.5 = 2,800 BF x 1.10 waste = 3,080 BF. R-value: 3.5 x 3.7 = R-12.95 (just under R-13 code minimum — marginally short for Zone 5).
Closed-cell at 3.5 inches: 800 x 3.5 = 2,800 BF x 1.10 = 3,080 BF. R-value: 3.5 x 6.5 = R-22.75 (exceeds R-20 Zone 5 wall requirement significantly). Acts as vapor retarder.
Recommendation: For Denver Zone 5, closed-cell at 3.5 inches in 2×4 cavities achieves R-22.75, significantly exceeding code and eliminating the need for a separate vapor retarder. Open-cell at 3.5 inches only achieves R-13 and is vapor permeable, which may create condensation risk in Denver’s cold winters without an additional interior vapor control layer.
3 Expert Tips for Spray Foam Insulation
When US contractors give you a spray foam bid, they price the work in board feet — not square feet, not cubic feet, and not per bag. A board foot of spray foam is exactly 1 square foot of surface covered to 1 inch of thickness. If a contractor quotes you 500 board feet of closed-cell foam at $1.25 per board foot, you know the material plus labor cost will be approximately $625. This allows direct comparison between multiple contractors and between professional installation versus DIY two-component kits. For DIY projects using two-component kits (products like Foam it Green, Touch ‘n Foam Professional, or Froth-Pak), the kit sizes are labeled in board feet: a 15-board-foot kit covers 15 square feet at 1 inch, 7.5 square feet at 2 inches, or 5 square feet at 3 inches. Use this calculator to determine your total board feet requirement, then check it against the coverage chart printed on the specific kit you plan to purchase — DIY kit outputs vary by product line. The board foot volume gives you the information you need to compare kits from multiple brands and select the most cost-effective combination of kit sizes for your specific project.
A critical and often misunderstood fact about closed-cell spray foam: at approximately 2 inches of thickness, closed-cell foam achieves a vapor permeance of less than 1 perm, meeting the definition of a Class II vapor retarder under the 2021 IRC. This has significant implications for the building assemblies where it is used. For rim joists and crawlspace walls: the 2-inch closed-cell layer is specifically the standard specification because the vapor retarder function is desired in these locations to prevent moisture migration from the soil or exterior. For exterior wall cavities: applying 2 or more inches of closed-cell foam on the interior face of the sheathing (between the studs) means the foam itself becomes the vapor control layer, eliminating the need for a separate poly vapor barrier or kraft-faced batts. For roof decks: closed-cell foam on the underside of the roof decking in an unvented attic assembly creates a vapor control layer immediately below the roof deck, which is the desired location to prevent wintertime condensation on the cold sheathing. Understanding the vapor behavior of your chosen foam type and thickness is essential for designing a building assembly that manages moisture correctly in your US climate zone. In warm-humid climates (Gulf Coast, Florida), vapor drive reverses in summer (moisture pushes from the hot exterior into the cool interior), and the vapor retarder placement for spray foam assemblies differs from cold-climate design. When in doubt about vapor control for your specific application and climate zone, consult buildingscience.com for US climate-specific assembly guidance.
Two-component spray foam DIY kits are powerful products that can be used successfully for projects under 600 board feet by US homeowners who follow the manufacturer’s instructions carefully. However, they require precautions that are non-negotiable and that many DIYers discover only after a failed or dangerous application. Temperature: the A and B component cylinders must be at 70 to 80 degrees Fahrenheit at the time of application. Cold cylinders (below 60 degrees F) produce under-expanded, crumbly foam that does not cure properly and provides poor R-value. Warm the cylinders to room temperature in a heated space for 24 hours before use if they have been stored in a cold garage or vehicle. Temperature of the substrate (the surface being sprayed) also matters — do not spray onto surfaces below 40 degrees F. Ventilation: the isocyanate component of two-component foam is a respiratory sensitizer and potential carcinogen during the application period. Work in a space with fresh air circulation (open windows, fans) and wear a full-face respirator rated for isocyanates (not a dust mask) throughout the application and for 24 hours after spraying while the foam cures. PPE: chemical-resistant gloves, a Tyvek suit (foam is extremely difficult to remove from skin and clothing), and safety glasses are all required. Foam that contacts skin must be removed immediately with acetone or isopropyl alcohol before it cures — cured foam on skin requires mechanical removal, which is painful. For projects requiring more than 600 to 800 board feet, the complexity, equipment cost, PPE requirements, and health hazards of DIY application make professional installation more practical and often more cost-effective when total project cost including PPE and equipment is considered.
Frequently Asked Questions About Spray Foam Insulation
Related Insulation Calculators
Board feet = area x thickness x waste_factor. In R-value mode: thickness = target_R / R_per_inch (open-cell 3.7, closed-cell 6.5). R-value = thickness x R_per_inch. Kit counts = ceil(board_feet / kit_size). Vapor barrier note triggers when closed-cell thickness is 2 inches or more (Class II vapor retarder per IRC). R-value and yield data based on published US manufacturer specifications for two-component open-cell (0.5 lb/cu ft) and closed-cell (2.0 lb/cu ft) spray polyurethane foam. Actual yields vary by temperature, substrate condition, and applicator technique. Always verify board feet yield on the specific product label. Two-component spray foam is a hazardous material — use full PPE as directed. For building science guidance visit buildingscience.com. For EPA spray foam guidance see epa.gov. Last reviewed: August 2026.