About the Spray Foam Insulation Calculator
Spray polyurethane foam is sprayed as a liquid that expands into a foam, filling gaps, cracks and irregular cavities that batts and boards cannot reach. As well as insulating, it seals air leaks, which is often where most of a building's heat loss happens. It comes in two main types, open-cell and closed-cell, which differ greatly in insulating value per inch, cost and behaviour.
This spray foam insulation calculator works out how much foam you need. Enter the area to insulate, the foam type and the R-value you want to reach. It gives the thickness required, the board feet of foam, the number of kits to buy with a waste allowance, and the installed cost at your price per board foot.
How to Use the Spray Foam Insulation Calculator
Enter the area to insulate in square feet.
Choose the foam type and enter the target R-value.
Enter the kit yield in board feet, a waste allowance and the installed price per board foot.
The Formulas
thickness = target R-value ÷ R-value per inch
board feet = area × thickness
kits = ⌈board feet × (1 + waste) ÷ kit yield⌉
cost = board feet × priceR-values used: closed-cell R-6.5 per inch, open-cell R-3.7 per inch.
Step-by-Step Example
1,000 ft² of wall to R-21 with closed-cell foam, 600 board foot kits, 10% waste, $1.50 per board foot.
thickness 21 ÷ 6.5 = 3.23 in
board feet 1,000 × 3.23 = 3,231
kits 3,231 × 1.1 ÷ 600 = 5.9 → 6 kits
cost 3,231 × $1.50 ≈ $4,846With open-cell foam to R-13, the thickness is 3.51 inches, filling a 2×4 wall cavity.
Board Feet Explained
Spray foam is measured in board feet, a unit borrowed from lumber. One board foot is a piece one foot square and one inch thick, 144 cubic inches. Covering 100 square feet with 3 inches of foam takes 300 board feet. Kits for do-it-yourself use are labelled by their theoretical yield, commonly 200, 600 or 1,000 board feet, though real yield is often lower because of temperature, overspray and uneven surfaces, which is why a waste allowance is added.
Open-Cell or Closed-Cell?
Closed-cell foam has tiny sealed cells filled with a gas that insulates well, giving about R-6 to R-7 per inch. It is dense and rigid, adds some strength to walls and roofs, and at about 2 inches or more acts as a vapour retarder. It suits basements, rim joists, crawl spaces, metal buildings and places where space is limited. It costs more per board foot.
Open-cell foam has cells that are broken open and filled with air, giving about R-3.5 to R-3.8 per inch. It is soft, expands a lot, and absorbs sound well. It lets water vapour pass, so in cold climates it usually needs a separate vapour retarder. It is cheaper per board foot and often used to fill wall cavities and the underside of roof decks in conditioned attics.
Choosing the R-Value
Energy codes set minimum R-values by climate zone and by part of the building: walls, attics, floors and basement walls. Typical wall requirements range from R-13 to R-21 or more, attic requirements from R-30 to R-60. Because foam also stops air leaks, it often performs better than its R-value suggests compared with leaky batts, but codes still require the stated R-values. Check your local code or energy program.
Safety and Installation
Spray foam is made by mixing two chemicals that react as they are sprayed. During spraying and for some time afterwards, the fumes can irritate the lungs and eyes, so installers wear respirators and protective suits, and occupants stay out of the building until it is ventilated. The chemicals must be at the right temperature for the foam to cure properly. Most codes require a thermal barrier, such as half-inch drywall, over foam in living spaces because it burns. For these reasons, large jobs are usually done by trained installers.
Where Spray Foam Pays Off Most
Spray foam gives the biggest benefit where air leaks are worst and other insulation is hard to fit: rim joists, where the floor framing sits on the foundation; the underside of roof decks in conditioned attics; crawl space walls; and around complex framing. In ordinary wall cavities, it competes with cheaper options such as dense-packed cellulose and batts combined with careful air sealing.
Comparing Quotes
Contractors may quote by the board foot, by the square foot at a stated thickness, or as a lump sum. Convert each quote to board feet at the R-value you need, so you can compare them fairly, and confirm which foam type and brand they will use.
Understanding Your Result
The headline gives the board feet of foam.
The thickness line shows the R-value calculation.
The kits line gives the number of kits.
The cost line gives the installed cost.
When Should You Use This Calculator?
Use it to estimate the cost of spray foam before getting quotes.
Use it to compare open-cell and closed-cell foam for a project.
Use it to work out how many DIY kits a small job needs.
Common Mistakes
Confusing square feet with board feet. Multiply by the thickness.
Assuming open and closed cell are interchangeable. Their R-values differ greatly.
Ignoring the thermal barrier. Most foam must be covered.
Spraying in the cold. Foam cures poorly outside its temperature range.