How to calculate attic insulation
R-value to add = ENERGY STAR's level for your zone − what's there now
Blown-in bags = area ÷ 1,000 × bags per 1,000 sq ft (from the bag's chart)
Batts = area ÷ area of one batt, for each layer
Spray foam thickness = R-value ÷ R per inch
Example: a 1,000 sq ft attic in zone 5 that already has 3 to 4 in of insulation.
- ENERGY STAR: add R-49 (R-60 in total)
- Blown fiberglass, Owens Corning AttiCat: 25 bags per 1,000 sq ft at R-49, so 25 bags, 17 in deep
- Or blown cellulose, GreenFiber: 70.8 bags per 1,000 sq ft, so 71 bags, 13.3 in settled (blown in at 14.3 in)
- Or batts: R-30 between the joists plus R-19 across them, 1,000 sq ft of each
- Installed blown-in at a typical $2 per sq ft: about $2,000
ENERGY STAR’s recommended levels
| Zone | Attic with no insulation | Attic with 3 to 4 in, add | Floor |
|---|---|---|---|
| 1 | R-30 | R-25 | R-13 |
| 2 | R-49 | R-38 | R-13 |
| 3 | R-49 | R-38 | R-19 |
| 4A and 4B | R-60 | R-49 | R-19 |
| 4C, 5 and 6 | R-60 | R-49 | R-30 |
| 7 and 8 | R-60 | R-49 | R-38 |
These are ENERGY STAR’s recommended levels for upgrading existing wood-framed homes, based on the 2021 IECC. Local code may require different values for new construction or permitted work. For walls, ENERGY STAR suggests blowing insulation into empty cavities and adding insulated sheathing when the siding comes off; the calculator covers batts for open walls only.
Coverage charts
Blown-in insulation is sold by the bag, and its maker publishes a chart of R-values with the minimum bags per 1,000 sq ft and the minimum thickness. GreenFiber’s chart says both have to be met to reach the R-value. The calculator uses two of them:
| R-value | AttiCat bags | Depth | Cellulose bags | Depth |
|---|---|---|---|---|
| R-30 | 14.6 | 10.75 in | 38.6 | 8.3 in |
| R-38 | 19 | 13.5 in | 52.6 | 10.4 in |
| R-49 | 25 | 17 in | 70.8 | 13.3 in |
| R-60 | 31.5 | 20.5 in | 86 | 16.1 in |
Bags per 1,000 sq ft: Owens Corning AttiCat blown fiberglass and GreenFiber SANCTUARY cellulose. The cellulose depth is after settling.
If the R-value you need falls between two rows, the calculator takes the next one up. Other brands have their own charts, so check the bag you buy.
Batts and spray foam
R-values of stacked layers add up, so batts can reach high attic levels in two layers: R-30 + R-19 makes R-49 and R-30 + R-30 makes R-60. The calculator counts pieces in Owens Corning’s sizes (16 or 24 in wide for joists 16 or 24 in apart) and doesn’t subtract the framing, so you’ll have a few spare.
Spray foam is rated per inch: about R-3.6 for open-cell and R-6 for closed-cell (DOE Building America). Its price depends on the thickness, so the per-square-foot ranges in the cost box are rough. To size a room air conditioner, use the BTU calculator.