Tools Web

Calculators

Insulation Calculator

Use this insulation calculator to find how thick insulation must be to reach a target R-value and how much to buy for walls, an attic or a floor. Enter the area as length × width, wall length × height or a known area, subtract openings, pick a material and set its R-value per inch. It shows the thickness in inches and millimetres, whether that fits a 2×4 or 2×6 cavity, 4 × 8 ft rigid foam boards, spray foam board feet and, with the coverage from your package label, batts or bags. R-per-inch values are typical and adjustable, so check your product label. For the wall covering see the drywall calculator, and for heating and cooling load see the BTU calculator.

Math runs in your browser. Nothing is uploaded.

Net area to insulate
—

Thickness needed
—

Thickness = target R-value ÷ R-value per inch. The R-per-inch values are typical figures you can edit: the U.S. Department of Energy's Guide to Home Insulation lists about 3.1–3.5 for batts, 2.9–3.7 for loose-fill, 3.5–6.2 for rigid board and 3.5–6.0 for spray foam, so check your product label. Net area is the area minus openings, then waste is added. A 4 × 8 ft board covers 32 sq ft and layers are counted from the board thickness. Spray foam is shown in board feet (1 sq ft at 1 in). Batt and bag coverage depends on R-value and thickness, so enter the sq ft per package from your label. Recommended R-values depend on climate zone and local code, so enter your own target.

Drywall Calculator · Roofing Calculator · BTU Calculator · Square Footage Calculator

More in this group

How it works

How do I calculate the insulation thickness for an R-value?
Divide the target R-value by the R-value per inch of the material. R-19 with a material at 3.2 R per inch needs 19 ÷ 3.2 = 5.9 in, about 150 mm. A material with more R per inch, such as closed-cell spray foam at about 6, needs less thickness for the same R-value.
What is the R-value per inch of common insulation?
The defaults here are typical, not exact: fiberglass batt 3.2, loose-fill 3.3, open-cell spray foam 3.6, closed-cell spray foam 6.0, EPS 4.0, XPS 5.0 and polyiso 6.5. The U.S. Department of Energy's Guide to Home Insulation gives ranges such as 3.1–3.5 for batts and 2.9–3.7 for loose-fill. Edit the value to match your product label.
Does R-19 fit in a 2×4 or 2×6 wall?
A 2×4 stud cavity is 3.5 in deep and a 2×6 cavity is 5.5 in deep. At 3.2 R per inch that holds about R-11 and R-17.6, so R-19 does not fit either with a typical batt. Use a material with more R per inch, such as closed-cell spray foam, or add continuous rigid board. Pick the cavity depth to see the check.
How many batts or bags of insulation do I need?
Divide the net area plus waste by the coverage printed on the package. Coverage depends on the R-value and thickness of that product, so enter the sq ft per package from your label. Rigid foam is counted in 4 × 8 ft boards, 32 sq ft each, and spray foam in board feet.
What R-value do I need for my home?
It depends on your climate zone, which part of the house you are insulating and your local building code. The U.S. Department of Energy and ENERGY STAR publish recommended R-values by zone, but this calculator does not hard-code a table, so enter the target from your code or contractor.