Spray foam insulation cost

Quick numbers

Board feet, and why foam is quoted that way

A board foot is one square foot at one inch thick. Foam is priced this way because thickness varies across a job in a way it doesn't with batts.

Board feet = area in sq ft × thickness in inches

A 500 sq ft crawl space wall at 2 inches of closed-cell is 1,000 board feet — roughly $1,000–$1,500 installed.

When comparing quotes, check they specify the same thickness. Under-applying thickness is the most common way foam jobs go wrong, and it's invisible once the crew leaves.

Which one you need

Open-cell

Open-cell foam

R per inch

3.6

Installed

$0.45–$0.75 / bd ft

Per sq ft at R-20

$2.50–$4.20

Vapor retarder

No

• Expands further, filling irregular cavities
• Vapor-open, so assemblies can dry
• Better sound absorption than closed-cell
• Roughly half the cost per board foot

Choose this when: Interior walls, sound control, and cavities that need to dry inward.

Closed-cell

Closed-cell foam

R per inch

6.5

Installed

$1.00–$1.50 / bd ft

Per sq ft at R-20

$3.10–$4.60

Vapor retarder

Yes, at 2 inches

• Nearly double the R-value per inch
• Acts as its own vapor retarder
• Adds structural rigidity to the assembly
• Handles below-grade and rim joists

Choose this when: Crawl-space walls, rim joists, and anywhere depth is limited.

Open-cell is cheaper, expands more, and is vapor-permeable — so assemblies can dry. Good for interior walls, sound control, and irregular cavities. Wrong for below-grade, and risky on unvented roof decks in cold climates where moisture reaching the sheathing causes rot.

Closed-cell costs more but delivers nearly double the R-value per inch, blocks vapor, and adds rigidity. It's the correct choice for crawl-space walls, rim joists, and anywhere depth is limited.

This choice has real building-science consequences, and it's the part most worth getting professional advice on rather than deciding from a price list.More on the differences.

Why DIY kits rarely pay

Two-part kits are sold at roughly $1.00–$2.00 per board foot — often more than a professional charges installed, because contractors buy chemicals in bulk.

The failure modes are also expensive:

• Chemicals are temperature-sensitive; foam applied outside the window may never cure properly
• Off-ratio mixing produces foam that stays tacky and off-gasses persistently
• Uncured or shrunken foam has to be physically cut out
• Proper respiratory protection is required, not optional

Kits do make sense for one thing: rim joists. Small area, high leakage, closed-cell is the right material, and a single kit covers a typical basement perimeter.

When foam is worth the premium

Foam earns its cost in specific situations, not as a default:

• Depth-limited assemblies — cathedral ceilings, 2x4 walls where you need more R than will fit
• Rim and band joists — small area, disproportionate air leakage
• Encapsulated crawl spaces — closed-cell handles moisture that would ruin fibrous insulation
• Insulating at the roofline — bringing ductwork inside the thermal envelope

For a standard vented attic floor, foam is the wrong tool. Air sealing plus deeper blown insulation achieves the same result for a fraction of the cost.

Related

• Spray foam insulation
• Insulation cost overview
• How getting quotes works