R-Value by Insulation Type
| Type | R-Value per Inch | Depth for R-38 | Depth for R-60 |
|---|---|---|---|
| Blown-In Fiberglass | R-2.2–2.7/in | ~15–17 in | ~22–27 in |
| Blown-In Cellulose | R-3.2–3.8/in | ~10–12 in | ~16–19 in |
| Spray Foam (open-cell) | R-3.5–3.6/in | ~11 in | ~17 in |
| Spray Foam (closed-cell) | R-6.0–7.0/in | ~6 in | ~9 in |
| Fiberglass Batts | R-3.1–3.4/in | ~11–12 in | ~18 in |
| Mineral Wool Batts | R-3.0–3.3/in | ~12 in | ~18–20 in |
DOE Recommended R-Values by Climate Zone
How to Calculate Insulation Quantity
- 1
1. Measure attic square footage
Length × width of the attic floor area. Include all accessible areas.
- 2
2. Determine current R-value
Measure existing insulation depth and multiply by the R-value per inch for that material.
- 3
3. Find your target R-value
Use the DOE climate zone table above. Most US homes should target R-38 to R-60.
- 4
4. Calculate additional depth needed
Subtract current R-value from target, then divide by the R-value per inch of your new material.
- 5
5. Estimate bags or rolls needed
Blown-in bags typically cover 40–45 sq ft at recommended depth. Check the bag's coverage chart.
About this calculator
How do I calculate how much attic insulation I need?
Measure your attic square footage, determine your current R-value, and identify your target R-value based on your climate zone. Divide the required R-value increase by the R-value per inch of your chosen insulation type to find the depth needed. Multiply depth (in inches) by square footage, then divide by the coverage per bag to get bag count. For example, blown-in cellulose covers ~40 sq ft at 3.5 inches per bag (R-13).
What R-value do I need for my attic?
The DOE recommends: Zone 1–2 (South/Gulf Coast): R-30 to R-38. Zone 3–4 (Southeast, Mid-Atlantic): R-38 to R-49. Zone 5–6 (Midwest, Mountain): R-49 to R-60. Zone 7 (Northern Alaska): R-49 to R-60. Most homes have R-11 to R-19, well below current recommendations. Adding insulation from R-11 to R-49 typically saves 15–25% on heating/cooling costs.
What is the difference between blown-in and batt insulation for attics?
Blown-in insulation (cellulose or loose-fill fiberglass) is sprayed into the attic and fills gaps and irregular spaces better than batts. It is typically more effective for existing attics. Batt insulation (fiberglass or mineral wool rolls) is easier for DIY installation between joists in accessible attics. Blown-in achieves a higher R-value per inch for cellulose (R-3.7/inch) compared to fiberglass batts (R-2.9–3.8/inch).
How much does attic insulation cost to install?
Attic insulation costs $1,500–$4,500 for a typical 1,000–1,500 sq ft attic. Blown-in cellulose to R-49 costs $1.80–$3.00 per sq ft installed. Blown-in fiberglass costs $2.00–$3.50 per sq ft. Spray foam costs $4.00–$7.00 per sq ft. The federal 25C insulation tax credit ended after December 31, 2025, but many state and utility programs still offer insulation rebates — check DSIRE (dsireusa.org).