How much attic insulation is enough?
Insulation is rated by its R-value, its resistance to heat flow: the higher the number, the better it insulates. ENERGY STAR publishes recommended attic levels for each US climate zone, based on the 2021 International Energy Conservation Code (IECC). For an uninsulated attic the targets are R-30 in zone 1, R-49 in zones 2 and 3 and R-60 in zones 4 to 8. This calculator subtracts what is already there and turns the difference into a depth and a volume of insulation.
R-value to add = target R − existing R
Depth (in) = R-value to add ÷ R-value per inch
Volume (cu ft) = attic area (sq ft) × depth (in) ÷ 12
ENERGY STAR attic recommendations
| Climate zone | If the attic is uninsulated | If it already has 3–4 in |
|---|---|---|
| 1 | R-30 | Add R-25 |
| 2 | R-49 | Add R-38 |
| 3 | R-49 | Add R-38 |
| 4A and 4B | R-60 | Add R-49 |
| 4C, 5 and 6 | R-60 | Add R-49 |
| 7 and 8 | R-60 | Add R-49 |
Climate zones run from 1 (the warmest, including Hawaii, Guam, Puerto Rico and the Virgin Islands) to 8 (the coldest parts of Alaska). They follow county lines, so look up your county on the ENERGY STAR or DOE climate zone map if you are not sure.
R-value per inch of common attic insulation
- Blown (loose-fill) cellulose: about R-3.2 to R-3.8 per inch after settling, according to the US Department of Energy. This calculator uses R-3.5 by default.
- Blown (loose-fill) fiberglass: about R-2.2 to R-2.7 per inch after settling; default R-2.5.
- Batts and rolls are sold by R-value (R-13, R-19, R-30, R-38), so read the label rather than measuring.
Products vary, and US rules require manufacturers to print R-value and coverage charts on the bag. If you know your product, type its R-value per inch, or better, use its bag chart directly.
Worked example 1: topping up a zone 5 attic
- A 1,200 sq ft attic in zone 5 has 3–4 inches of old insulation, roughly R-11. Target R-60, so add R-49.
- Blown cellulose at R-3.5 per inch: 49 ÷ 3.5 = 14 in settled depth, or 1,200 × 14 ÷ 12 = 1,400 cu ft (about 52 cu yd).
- Blown fiberglass at R-2.5 per inch would need 19.6 in for the same R-value: a deeper layer, but lighter.
Worked example 2: an empty zone 3 attic
A 1,000 sq ft attic with no insulation in zone 3 needs R-49. With cellulose that is 14 in deep. If the bag chart says one bag covers 40 sq ft at R-49, you need 1,000 ÷ 40 = 25 bags. Always count bags from the chart, because coverage per bag falls as the target R-value rises.
Settling and installed depth
Loose-fill insulation settles after it is blown in, cellulose more than fiberglass. Bag charts therefore list an initial installed thickness and a minimum settled thickness for each R-value. The depths here are settled depths; blow to the chart's initial thickness so you still have the full R-value after settling. Attic rulers stapled to the joists make it easy to check an even depth across the whole attic.
Before you add insulation
- Air-seal first. Seal gaps around plumbing, wiring, chimneys and top plates; insulation does not stop air leaks.
- Keep soffit vents open with baffles so the attic stays ventilated.
- Keep clearances around chimneys, flues and recessed lights that are not rated for insulation contact (IC-rated).
- Fix roof leaks and moisture problems before covering them.
- Old vermiculite insulation (grey-brown, pebble-like) may contain asbestos; do not disturb it and get professional advice.
Common mistakes
- Using the settled depth as the blowing depth, which leaves the attic short once it settles.
- Compressing batts under boards or storage, which lowers their R-value.
- Counting bags by volume alone. Densities differ between products; the bag chart is what counts.
- Measuring the roof instead of the attic floor. Insulation goes on the attic floor, so use the area of the ceiling below it.
Frequently asked questions
How many inches of insulation is R-60?
About 17 in of blown cellulose at R-3.5 per inch, or 24 in of blown fiberglass at R-2.5 per inch, settled depth. Check the bag chart for your product.
How many inches of insulation is R-49?
About 14 in of blown cellulose (R-3.5 per inch) or about 19.6 in of blown fiberglass (R-2.5 per inch).
What R-value should my attic be?
ENERGY STAR recommends R-30 in climate zone 1, R-49 in zones 2–3 and R-60 in zones 4–8 for an uninsulated attic.
How many bags of blown-in insulation do I need?
Divide the attic area by the coverage per bag that the bag chart lists for your target R-value. For example, 1,000 sq ft ÷ 40 sq ft per bag = 25 bags.
Can I put new insulation over old insulation?
Usually yes, if the old insulation is dry and not vermiculite. Add the new layer on top, and lay batts unfaced (without a vapour retarder) over existing insulation.
Sources
- ENERGY STAR (US EPA / DOE), Recommended home insulation R-values (based on 2021 IECC Table R402.1.3).
- US Department of Energy, Loose-Fill Insulations (DOE/GO-10095-060): settled R-values per inch for cellulose and fiberglass.
- US Federal Trade Commission, R-Value Rule (16 CFR Part 460): labeling of R-value and coverage on insulation packages.
Last reviewed 31 October 2026. Results are estimates; see our disclaimer.