Spray Foam Insulation Cost Per Square Foot

Per-square-foot price depends entirely on thickness. Here's the real math, by foam type — for projects across Hamilton County.

Quick Answer

Spray foam runs about $0.35 to $0.45 per board foot for open-cell and $1.25 to $1.40 for closed-cell — and a board foot is one square foot at one inch thick. So per square foot, cost scales with thickness: an open-cell attic at 6 inches runs about $2.10 to $2.70 per square foot, while closed-cell walls at 2 inches run about $2.50 to $2.80.

Square Foot vs Board Foot

The biggest source of confusion in spray foam pricing is that a "per square foot" number means nothing without a thickness. Spray foam is applied in custom thicknesses — 1 inch for an air seal, 3 inches for walls, 6 inches or more for an attic — and the cost scales directly with how thick it goes. That's why contractors quote by the board foot (area times thickness). To compare quotes fairly, always ask for the price per board foot or the per-square-foot price at a stated thickness.

Cost Per Square Foot by Thickness

Feature	Open-Cell ($/sq ft)	Closed-Cell ($/sq ft)
1 inch (air seal)	$0.35 - $0.45	$1.25 - $1.40
2 inches (walls)	$0.70 - $0.90	$2.50 - $2.80
3 inches (roof deck)	$1.05 - $1.35	$3.75 - $4.20
6 inches (attic, open-cell typical)	$2.10 - $2.70	—

1 inch (air seal)

Open-Cell ($/sq ft)

$0.35 - $0.45

Closed-Cell ($/sq ft)

$1.25 - $1.40

2 inches (walls)

Open-Cell ($/sq ft)

$0.70 - $0.90

Closed-Cell ($/sq ft)

$2.50 - $2.80

3 inches (roof deck)

Open-Cell ($/sq ft)

$1.05 - $1.35

Closed-Cell ($/sq ft)

$3.75 - $4.20

6 inches (attic, open-cell typical)

Open-Cell ($/sq ft)

$2.10 - $2.70

Closed-Cell ($/sq ft)

—

Cheaper Per Inch vs More Per Inch

Open-Cell (lower cost per inch)

Pros

About a third the price of closed-cell per board foot
Full air seal at the thicknesses attics and walls use
Best value for large areas like attic rooflines

Cons

Lower R-value per inch (R-3.6), so it needs more thickness
No moisture barrier — not for crawl spaces or below grade

Closed-Cell (more cost per inch)

Pros

Highest R-value per inch (R-6.5), so thin applications hit high R-values
Built-in moisture barrier for crawl spaces, rim joists, and metal
Adds structural rigidity to the assembly

Cons

Roughly three times the per-square-foot cost at the same thickness
Overkill for dry interior spaces where open-cell performs fine

How to Estimate Your Square Footage

To estimate a job, start with the area you're covering. For walls, multiply the perimeter by the wall height. For an attic or ceiling, use the footprint of the space. Multiply that area by your target thickness in inches to get board feet, then multiply by the per-board-foot rate. The cost calculator does this math for you in seconds — enter your dimensions and it returns a board-foot estimate and a price range.

Our Verdict

Ignore any "per square foot" number that doesn't state a thickness — it isn't a real quote. Decide the foam type and target R-value first, and the per-square-foot cost follows from there. Use the cost calculator to turn your dimensions into a price range, or read How Much Does It Cost to Spray Foam a House for project-level budgets.

Frequently Asked Questions

It depends entirely on thickness and foam type. Open-cell runs about $0.35 to $0.45 per board foot (so $2.10 to $2.70 per square foot at a typical 6-inch attic application), while closed-cell runs $1.25 to $1.40 per board foot ($2.50 to $2.80 per square foot at 2 inches). Always tie the price to a thickness.

At the same thickness, closed-cell costs about three times open-cell because it's denser, delivers nearly double the R-value per inch, and acts as a moisture barrier. You use it where those properties matter — crawl spaces, rim joists, metal buildings — not for general interior insulation.

Yes. The rates here are all-in installed prices covering foam material, labor, equipment, and standard overspray. They are not material-only figures, so the number you see is close to what you'd actually pay for a straightforward, accessible application.

It depends on the application: 1 to 2 inches of closed-cell for an air seal or rim joist, 2 to 3.5 inches for walls, 6 to 10 inches of open-cell for an attic roofline, and 2 to 3 inches of closed-cell for a crawl space. Indiana code calls for R-49 in the attic, which is about 13.5 inches of open-cell.