Insulation Calculator
Find the thickness and amount of insulation needed to reach your target R-value for an attic, wall or floor.
Free · No sign-up · Runs in your browser · Last reviewed November 2026
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About the Insulation Calculator
R-value measures how well insulation resists heat flow: the higher the number, the better it insulates. This insulation calculator divides your target R-value by the R-value per inch of the material to find the required thickness, and estimates packs from the area. Typical values are R-3.2 to R-3.7 per inch for fiberglass and cellulose and about R-6.5 per inch for closed-cell spray foam.
How to use it
- Enter the length and width of the area to insulate.
- Enter your target R-value and choose the material.
- Press Calculate insulation to see the thickness and quantity.
R-value formula
Thickness (inches) = target R-value ÷ R-value per inch. To reach R-38 with blown cellulose at R-3.7 per inch, you need about 10.3 inches.
Typical recommended R-values (check the DOE map for your climate zone)
| Area | Common range |
|---|---|
| Attic | R-30 to R-60 |
| Walls (2x4) | R-13 to R-15 |
| Walls (2x6) | R-19 to R-21 |
| Floors over unheated space | R-25 to R-30 |
| Crawl space walls | R-10 to R-13 |
Tips
- Air seal gaps before adding insulation.
- Do not compress batts, which lowers their R-value.
- Keep insulation away from recessed lights not rated for contact.
Insulation planning: a second example and common pitfalls
Consider a 30 × 25 ft attic (750 sq ft) with a target of R-49. With blown cellulose at R-3.7 per inch you need about 13.2 inches. With blown fiberglass at R-3.5 per inch it is about 14.0 inches. Settling is why installers often aim a little above the target depth, since loose-fill insulation compresses over time.
Mistakes that reduce performance
- Skipping air sealing. Leaks around ducts, wiring and the attic hatch let heat bypass the insulation altogether.
- Blocking soffit vents. Use baffles so air can still flow into the attic.
- Mixing up R-value and thickness. Different materials have different R-per-inch, so a 10 inch layer is not always R-38.
- Covering recessed lights. Fixtures not rated for insulation contact need clearance.
Recommended levels depend on your climate zone, so check the Department of Energy guidance for your area. If you are finishing walls afterward, estimate sheets with the drywall calculator.
Frequently asked questions
How thick should attic insulation be?
Most U.S. climates call for R-38 to R-60, which is roughly 10 to 16 inches of fiberglass or cellulose.
What is R-value?
R-value is a measure of thermal resistance. Higher values mean better insulation.
How do I calculate how many batts I need?
Divide the area by the coverage printed on each pack and round up.
Is spray foam better than fiberglass?
Closed-cell foam gives more R-value per inch and an air seal, but costs more.
Can I add new insulation over old?
Often yes, if the old insulation is dry and in good condition.
Is more insulation always better?
There are diminishing returns. Going from R-11 to R-38 saves a lot, while going from R-60 to R-80 saves little. Air sealing and the right level for your climate usually matter more.
Can I add new insulation over old insulation?
Often yes, if the existing material is dry and free of mold or pest damage. Do not use a vapor-barrier-faced layer on top of existing insulation, since it can trap moisture.
Last reviewed: November 2026.