Insulation Calculator
Square footage, batt bundles, blown-in bags and the depth needed to hit your R-value.
Bags needed
33
- Gross area
- 1,200 sq ft
- Net insulated area
- 1,080 sq ft
- Area with waste
- 1,188 sq ft
- Required depth
- 13.2 in
- Material R per inch
- R-3.7
- Cavities across span
- 24
Always confirm coverage against the printed chart on the bag or bundle you actually buy.
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How this is calculated
net area = gross area × (1 − framing %) order area = net area × (1 + waste %) depth (in) = target R-value / material R per inch bundles = ceil(order area / bundle coverage) bags = ceil(order area / bag coverage at depth)
R per inch constants used: fiberglass batt 3.2, mineral wool 4.2, blown fiberglass 2.5, blown cellulose 3.7. These are mid-range published values; specific products vary by density and manufacturer.
Depth needed by R-value and material
| Target R | Fiberglass batt | Mineral wool | Blown cellulose | Blown fiberglass |
|---|---|---|---|---|
| R-13 | 4.1 in | 3.1 in | 3.5 in | 5.2 in |
| R-19 | 5.9 in | 4.5 in | 5.1 in | 7.6 in |
| R-30 | 9.4 in | 7.1 in | 8.1 in | 12.0 in |
| R-38 | 11.9 in | 9.0 in | 10.3 in | 15.2 in |
| R-49 | 15.3 in | 11.7 in | 13.2 in | 19.6 in |
| R-60 | 18.8 in | 14.3 in | 16.2 in | 24.0 in |
Frequently asked questions
What R-value do I need in my attic?
Most of the United States now targets R-49 to R-60 in attics, with R-38 acceptable in the warmest southern zones. Walls are limited by cavity depth, so R-13 to R-21 is typical. Check your local energy code, since it sets the legal minimum for permitted work.
How many square feet does a batt bundle cover?
Coverage is printed on the bag and depends on width and thickness — a common R-13 3.5-inch bundle covers roughly 88 square feet, while thick R-30 bundles may cover only 40 to 60. This calculator lets you enter the actual bundle coverage from the label rather than guessing.
How deep does blown-in insulation need to be?
Divide the target R-value by the material's R per inch: R-49 needs about 13 inches of cellulose at R-3.7 or 20 inches of blown fiberglass at R-2.5. Blown material settles, so install to the manufacturer's settled-depth marker, not just to the number you calculated.
Does insulation lose R-value over time?
Fiberglass and mineral wool hold their rating almost indefinitely if kept dry and uncompressed. Loose-fill cellulose settles a few percent in the first year, which manufacturers already account for in coverage charts. Wet insulation of any type loses most of its value until fully dried.
Can I put new insulation over old?
Yes, as long as the existing layer is dry and not moldy, and the new layer is unfaced. Adding faced batts on top traps moisture between the vapor retarders. Blown material over old batts is a common and effective attic upgrade.
What is the difference between R-value per inch and total R-value?
R per inch is the material property; total R is that number multiplied by installed thickness. A high R-per-inch product in a shallow cavity can still underperform a cheap product installed thick, which is why attic depth usually matters more than material choice.
Quantities are planning estimates. Verify local energy code requirements and manufacturer coverage charts before ordering — this is not professional design advice.