Spray Foam Insulation Calculator

One square foot at one inch thick. The Spray Polyurethane Foam Alliance defines it as a volume of 12 by 12 by 1 inches, which makes the arithmetic exact: board feet equals area in square feet times thickness in inches. Spray foam is quoted this way rather than by the bag, because thickness varies by application.

Thickness, and this calculator asks for it that way deliberately. Working backwards from an R-value produces impossible answers: R-49 of open-cell foam needs 13 inches, which does not fit in any rafter bay. Thickness is the physically real input, so we preset what installers actually apply for each application and show you the R-value that results. You can override the thickness, and we warn you if it will not fit or will not develop its rated R-value.

Closed-cell gives nearly twice the R-value per inch, acts as its own vapor retarder from about 2 inches, and adds structural stiffness — the usual choice for crawl space walls and rim joists. Open-cell costs roughly a third as much per board foot and stays vapor-permeable, so the assembly can dry, which is right in some places and wrong in others. For a wall cavity or an attic roofline either can be correct depending on your climate zone.

Almost never, once you account for real-world yield. The board-foot figure on the box is a theoretical laboratory number — DuPont says so about its own product. Cold substrate alone costs 10 to 25% of yield, and a first-timer wastes considerably more through ratio errors and technique. Large closed-cell kits land at or above professional installed cost per board foot before counting your own labor, and small kits run several times higher. The genuine case for a kit is a job too small for any contractor to mobilise for.

Yes, a little. The blowing agent diffuses out over the first five to ten years, costing roughly 5 to 10%. Initial values of R-7 per inch are commonly advertised; the aged figure is nearer R-6.5, and that is what this calculator uses, because the aged number is what your house actually gets for most of its life. Open-cell foam is water-blown and does not drift.

Usually, and it is fire code rather than a finish preference. IRC R316.4 requires a 15-minute thermal barrier — normally ½ inch gypsum board — over foam in an occupied space. Attics and crawl spaces entered only for repairs need only an ignition barrier under R316.5.3, and in practice most installers use an approved intumescent coating instead, at roughly $0.75 to $1.50 per square foot installed. Rim joists are the exception: the IRC permits exposed foam below 3¼ inches, which is a good reason to cap the thickness there.

Because mobilisation dominates a small foam job. The rig has to arrive, the chemical has to come up to temperature, and the hoses have to be purged whether you are spraying 200 board feet or 4,000. Expect $500 to $1,500 as a floor. Labor is 50 to 60% of a spray foam job — a higher share than any other insulation — which is also why regional rates move these estimates more than they would elsewhere.

No. Spray foam expands as it cures and needs an open cavity to expand into; injecting it behind drywall can bow or crack the wall. For a finished wall the options are dense-pack cellulose, blown fiberglass, or a purpose-made injection foam, all installed through drilled holes.