Attic Insulation Calculator
Choose your climate zone and what's in the attic now to see the R-value to aim for, how deep the new insulation needs to be, and how many bags to buy.
Enter the attic floor area.
On this page
How to use the insulation calculator
- Measure the attic floor — usually the footprint of the heated part of the house below it.
- Choose your climate zone. The labels are a rough guide; the U.S. Department of Energy publishes a county-level climate zone map.
- Tell it what's there now. Measure the depth of the existing insulation. If it's about 3–4 inches, choose that option; otherwise enter its R-value (depth × R per inch for its type).
- Pick the insulation you'll add and, for bags, enter the coverage from the bag's chart at your target R-value.
Recommended attic R-values (ENERGY STAR)
| Climate zone | Uninsulated attic | Attic with 3–4 in existing |
|---|---|---|
| 1 | R-30 | add R-25 |
| 2–3 | R-49 | add R-38 |
| 4–8 | R-60 | add R-49 |
ENERGY STAR recommendations for retrofitting existing wood-framed homes.
R-value per inch
| Material | R per inch (DOE range) | Inches for R-38 | Inches for R-49 |
|---|---|---|---|
| Loose-fill fiberglass | 2.2–2.7 | 14–17 | 18–22 |
| Loose-fill cellulose | 3.2–3.8 | 10–12 | 13–15 |
| Fiberglass batts | 2.9–3.8 | 10–13 | 13–17 |
The calculator uses the middle of each range (2.5, 3.5 and 3.3). Blown-in products list the exact installed thickness and bag count for each R-value on the bag — follow that chart when you install.
Worked example: a 1,200 sq ft attic in zone 5 with 3–4 inches today
- ENERGY STAR recommends adding R-49
- With blown-in fiberglass at R-2.5 per inch: 49 ÷ 2.5 = 19.6 inches on top of what's there
- With cellulose at R-3.5 per inch: 14 inches
Before you add insulation
- Seal air leaks first — around chimneys, plumbing and wiring holes, recessed lights and the attic hatch. Air leaks can undo much of the benefit.
- Keep ventilation open: install baffles at the eaves so insulation doesn't block soffit vents.
- Mind clearances around chimneys, flues and recessed lights that aren't rated for insulation contact.
- Deal with moisture — wet or moldy insulation points to a roof leak or ventilation problem that needs fixing first.
Frequently asked questions
How many inches of insulation is R-49?
Roughly 18 to 22 inches of blown-in fiberglass, 13 to 15 inches of cellulose, or 13 to 17 inches of fiberglass batts.
Can I put new insulation on top of old?
Usually yes, if the old insulation is dry and free of mold or pests. Don't put faced batts (with a vapor retarder) on top of existing insulation — use unfaced batts or loose fill.
How many bags of blown-in insulation do I need?
Divide your attic area by the coverage per bag listed on the bag's chart for your target R-value. Coverage per bag drops as the target R-value rises.
Sources
- ENERGY STAR, Recommended home insulation R-values — attic recommendations by climate zone.
- U.S. Department of Energy, Insulation materials — R-value per inch ranges.