Insulation Calculator — R-Value & Quantity
Bring your attic, walls or floors up to the recommended R-value. Choose your climate zone and insulation type to get the thickness to add and the batts, bags, boards or spray foam you need.
Insulation to add
How to use the insulation calculator
- Enter the area. For an attic floor, that is roughly the footprint of the house below it.
- Choose your climate zone to set a target R-value, or type your own target, and estimate the R-value already there.
- Choose the insulation type and enter the coverage from the bag or package chart at your target depth.
How much insulation do you need?
R-value measures resistance to heat flow; higher is better. ENERGY STAR recommends R-30 to R-49 for uninsulated attics in the warmest zone, R-30 to R-60 in zones 2 and 3, and R-49 to R-60 in zones 4 through 8, which covers most of the northern half of the U.S. When adding to existing insulation, subtract what is already there.
Different materials provide different R-value per inch, so the thickness needed varies: R-38 is about 15 inches of blown fiberglass but about 11 inches of cellulose. Blown-in bags list how many square feet they cover at each R-value; use that number in the coverage box for an accurate bag count.
Seal air leaks around wiring, plumbing and light fixtures before adding insulation, keep insulation away from recessed lights not rated for contact, and maintain attic ventilation at the eaves with baffles. Spray foam quantities are given in board feet (one square foot, one inch thick).
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Frequently asked questions
What R-value should my attic have?
Generally R-38 to R-60 depending on climate; R-49 or more in cold climates.
How thick is R-38 insulation?
About 12 inches of fiberglass batts, 11 inches of cellulose or 15 inches of blown fiberglass.
Can I add new insulation on top of old?
Yes, as long as the old insulation is dry and not moldy. Unfaced batts or blown-in work best on top.
What is a board foot of spray foam?
One square foot at one inch thick; 1,000 sq ft at 3 inches is 3,000 board feet.