Insulation Calculator
Batts, bags, or blown depth for walls and attics. Works from framed area and stud spacing.
Common batt sizes and R values
| Cavity | Typical batt | R value |
|---|---|---|
| 2x4 wall | 3.5 in | R-13 to R-15 |
| 2x6 wall | 5.5 in | R-19 to R-21 |
| 2x8 floor | 7.25 in | R-25 |
| 2x10 floor | 9.25 in | R-30 |
Bag coverage varies a lot by manufacturer and R value. Check the bag and override the default above.
Insulation estimating FAQ
How many bags of insulation do I need?
Divide the square footage of framed wall by the coverage printed on the bag, then round up. Coverage is commonly 40 to 110 square feet per bag depending on R value and batt thickness.
What R value do I need in an attic?
Most of the US is R-38 to R-60 for attics under current energy code. Check your climate zone because the requirement changes by region and by whether the attic is vented.
How deep is R-49 blown insulation?
About 19 inches of loose fill fiberglass at R-2.6 per inch, or about 14 inches of cellulose at R-3.5 per inch. Settled depth matters, so blow to the depth markers on the rafters.
Do I subtract windows and doors from insulation?
Yes for anything over a few square feet. You still need batts around openings, so small windows tend to wash out.
What size batt for 16 inch on center studs?
A 15 inch wide batt friction fits between 16 inch on center studs. Twenty four inch on center takes a 23 inch batt. Using the wrong width leaves gaps and kills performance.
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