How to use the insulation calculator
Pick your climate zone and what's in the attic now. The calculator looks up ENERGY STAR's recommended attic R-value for retrofits and shows how much to add. Enter the attic floor area and the material, and it turns that into bags or packages using the coverage figure printed on the product, or into a depth for spray foam.
The bag chart matters: loose-fill products differ a lot in how many bags reach a given R-value, so this calculator uses your product's own chart instead of a generic guess.
Recommended R-values (ENERGY STAR)
| Zone | Uninsulated attic | Attic with 3-4 in | Floor |
|---|---|---|---|
| 1 | R30 | add R25 | R13 |
| 2 | R49 | add R38 | R13 |
| 3 | R49 | add R38 | R19 |
| 4A, 4B | R60 | add R49 | R19 |
| 4C, 5, 6 | R60 | add R49 | R30 |
| 7, 8 | R60 | add R49 | R38 |
Worked example: a 40 × 30 ft attic in zone 4A with 3-4 in already there
- ENERGY STAR says add R49.
- Attic area: 40 × 30 = 1,200 ft².
- If your blown-in bag chart says 22 bags per 1,000 ft² for R49, you need 1.2 × 22 = 26.4, so 27 bags.
- Closed-cell foam at about R-6.5 per inch would need about 7.5 in to add R49, which is why attics usually get loose fill instead.
Before you add insulation
- Air seal first. Seal gaps around plumbing, wiring, recessed lights, chimneys and the attic hatch. It's often worth more than the insulation itself.
- Keep soffit vents open with baffles so air can flow from the eaves to the ridge.
- Clearance around heat sources: keep insulation away from chimneys, flues and non-IC-rated recessed lights, per code and the product's instructions.
- Mark the depth. Staple depth rulers to trusses before blowing so you can see when you've reached the chart's minimum settled thickness.
- Wear protection: a respirator, goggles and long sleeves.
Questions people ask
How much attic insulation do I need?
ENERGY STAR recommends R30 for an uninsulated attic in climate zone 1, R49 in zones 2 and 3, and R60 in zones 4 to 8. If you already have 3 to 4 inches, add R25 in zone 1, R38 in zones 2-3, and R49 in zones 4-8.
How many bags of blown-in insulation do I need?
Every bag has a coverage chart listing bags per 1,000 square feet for each R-value. Find the row for your target R, multiply by your attic area in thousands of square feet, and round up. The calculator does that once you enter the figure from the chart.
How thick is R-38 insulation?
It depends on the material. Loose fill varies by product, so use the minimum settled thickness on the bag chart. As a reference, the Department of Energy's insulation guide lists closed-cell spray foam at about R-6.5 per inch and open-cell at about R-3.6 per inch.
What climate zone am I in?
The US is split into zones 1 (south Florida, Hawaii) to 8 (northern Alaska) in the International Energy Conservation Code. Most of the Southeast is 2-3, the Mid-Atlantic and central states 4, the North 5-6. Your local building department or ENERGY STAR's map can confirm it.
Should I add insulation on top of old insulation?
Usually yes, as long as the old insulation is dry and free of mold or pests. Air-seal the attic floor first: gaps around lights, pipes and the attic hatch leak more heat than thin insulation does.
Sources and assumptions
- ENERGY STAR: recommended insulation levels for retrofitting existing wood-framed buildings (based on 2021 IECC Table R402.1.3)
- U.S. Department of Energy, Guide to Home Insulation: closed-cell spray foam R-6.5 at 1 in, open-cell R-12.6 at 3.5 in, fiberglass batt R-13 at 3.5 in
- Loose-fill bag counts must come from the product's coverage chart; they vary widely by brand.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.