Attic Insulation Calculator
Pick your climate zone and measure your current insulation. You'll see the R-value to aim for and how many inches to add.
Recommended attic R-values
These are the ENERGY STAR recommendations for existing homes, based on the 2021 International Energy Conservation Code:
| Climate zone | Uninsulated attic | Attic with 3–4" already |
|---|---|---|
| 1 | R30 | R25 |
| 2 and 3 | R49 | R38 |
| 4 to 8 | R60 | R49 |
This calculator aims for the "uninsulated attic" target, the full recommended level, and subtracts what you already have.
How the calculation works
R you have = existing depth × R per inch of that material
inches to add = (target R − R you have) ÷ R per inch of new material
The R-per-inch figures are typical values: loose fiberglass about 2.2 to 2.7, cellulose about 3.2 to 3.8, and fiberglass batts about 2.9 to 3.8. Check the coverage chart on the bag you buy. It lists the bags per 1,000 sq ft and the settled depth for each R-value.
Worked example
In zone 5 the target is R60. An attic with 4" of old loose fiberglass has about R10, so it needs R50 more. That's 20 inches of blown fiberglass or about 14 inches of cellulose.
Tips
- Air seal first. Caulk and foam gaps around wires, pipes, light fixtures and the attic hatch before adding insulation. Leaks undo much of the benefit.
- Keep soffit vents open with baffles, so the attic still breathes.
- Many hardware stores lend the blowing machine for free when you buy enough bags.
Frequently asked questions
How much attic insulation do I need?
Anywhere from R30 in the warmest zone to R60 in most of the US. In inches, R60 is about 24" of blown fiberglass or 17" of cellulose.
Can I put new insulation on top of old?
Yes, as long as the old insulation is dry and free of mold or pests. Don't put faced batts (with a paper or foil layer) on top of existing insulation, because the facing can trap moisture.