Free tool · Takeoff
Insulation Calculator
Enter each area you are insulating, such as an attic floor, a wall or a crawlspace, then choose what you are buying. The calculator gives the packages of batts or bagged fill, the sheets of foam board or the board feet of spray foam, with a waste allowance. A second part works out the R-value of a thickness, or the thickness you need for a target R-value.
The numbers stay in your browser: we do not store or receive them.
How much to buy
Enter each area to insulate, such as an attic floor, a wall or a crawlspace, then what you are buying.
Area
1,200 sq ft
1,260 sq ft with 5.0% waste
Packages
13
1,300 sq ft covered at 100.0 each
| Area | Gross (sq ft) | Openings | Net (sq ft) |
|---|---|---|---|
| Attic floor | 1,200.0 | — | 1,200.0 |
R-value and thickness
R per inch used
3.40
DOE range R-2.9 to R-3.8
R-value of 3.5 in
R-11.9
For R-38
11.2 in
R per inch is approximate and starts at the middle of the U.S. Department of Energy range. The real value depends on the product, its density and how it is installed. Check the package.
How the quantity is figured
Area = length × width, or the area you enter, less the openings. The net areas are added and the waste is added on top: need = area × (1 + waste).
Batts, rolls and bagged blown-in: packages = need ÷ the square feet one package covers, rounded up. The coverage is printed on the package, and for loose-fill it depends on the thickness you want, so read the bag’s coverage chart for the R-value you are installing.
Rigid foam board: sheets = need × layers ÷ the area of one sheet, rounded up. A 4 ft by 8 ft sheet is 32 sq ft.
Spray foam is sold by the board foot, which is 1 sq ft one inch thick, so board feet = need × the thickness in inches. If you know the board feet in a kit, it gives the kits.
R-value and thickness
R-value is resistance to heat flow, and a bigger number insulates better. For a material, R = R per inch × the thickness in inches, and the thickness for a target R = the target ÷ R per inch.
The calculator starts each material at the middle of the range in the U.S. Department of Energy’s Building America insulation guide (2012): standard fiberglass batts R-2.9 to R-3.8 per inch, blown-in cellulose R-2.9 to R-3.4, blown-in fiberglass R-2.2 to R-2.7, mineral wool R-3.7, EPS R-3 to R-4, XPS R-5.0, polyisocyanurate board R-5.6 to R-8, open-cell spray foam R-3.5 to R-3.8 and closed-cell spray foam R-7 to R-9. The guide says these are approximate and for general comparison only.
The real figure depends on the product, its density and how it is installed. Batts lose R-value when they are compressed, loose-fill when it is over-fluffed or settles, and polyisocyanurate as its gas escapes over time. Enter the value on your product in the Other box, and use the R-value your building code requires for your climate zone.
A worked example
An attic floor 30 ft by 40 ft is 1,200 sq ft, or 1,260 with 5 percent waste. With bags that cover 100 sq ft each that is 12.6, so 13 packages. In 4 ft by 8 ft foam board it is 1,260 ÷ 32 = 39.4, so 40 sheets. Sprayed 4 in thick it is 1,260 × 4 = 5,040 board feet.
At the middle of the DOE range for fiberglass batts, 3.4 per inch, 3.5 in is about R-12, and reaching R-38 takes about 11.2 in.
What this does not do
It does not subtract the framing, so a wall or floor filled between studs or joists has less insulated area than its gross size, and the batt width has to match the framing spacing. It does not account for thermal bridging, air sealing, vapor retarders, ventilation baffles or the code R-value for your climate zone. Check the coverage and the R-value on the product you buy.
FAQ
How do I figure how many bags or rolls of insulation I need?
Find the area to insulate, add a few percent for waste, and divide by the square feet one package covers. The package states its coverage, and for loose-fill the coverage depends on the R-value or depth you want.
What is R-value?
It is a measure of resistance to heat flow. A higher R-value means better insulation. The R-value of a layer is its R per inch times its thickness in inches.
How thick does insulation need to be for R-38?
It depends on the material. At 3.4 per inch it is about 11 in, and at about 5 per inch, as with XPS, it is about 7.6 in. Enter the material and the target R and the calculator gives the thickness.
What is a board foot of spray foam?
It is one square foot of foam one inch thick. A 1,000 sq ft area sprayed 2 in thick is 2,000 board feet.
Why is my real R-value lower than the table?
R-value per inch depends on the product and its density, and falls if batts are compressed or loose-fill is not installed to the right density. The DOE table lists approximate ranges for general comparison.
Is anything I type saved?
No. The calculation runs in your browser and nothing is stored or sent to us.
Pricing the whole building envelope? PoPs Estimating counts insulation, framing and finishes from your plans and keeps the quantities tied to the bid.
Questions?
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