Insulation Calculator — Blown-In & Batts
Enter your area and target R-value to find out exactly how many bags of blown-in insulation or how many batt packages to buy — including the depth you'd actually need to blow in.
R-3.7/inch at settled density.
Both numbers are printed on the bag's coverage chart, usually next to the R-value table.
How this is calculated
For blown-in (loose-fill) insulation: required depth is your target R-value divided by the material's R-value per inch at settled density. That part is simple — but bag coverage isn't a fixed number per bag. Manufacturers print coverage at a specific reference depth (for example '32.6 sq ft per bag at 13.5 inches for R-38'), and coverage scales inversely with the depth you actually install: blow the same bag in thicker and it covers less floor area, blow it in thinner and it covers more. This calculator scales the bag's printed reference coverage to your actual required depth before dividing into your area — skipping that step is the most common reason online insulation calculators overestimate or underestimate bag counts.
The installed depth is rounded up to the nearest quarter inch, since that's the practical resolution of a depth-rake marker on site, and the achieved R-value shown reflects that rounded depth rather than the theoretical exact figure.
For batts and rolls, the math is simpler since batts are a fixed-thickness, fixed-R product: packages needed is just your area divided by the coverage printed on the packaging, rounded up. If your chosen batt's R-value is below your target, the calculator flags it — a higher-R batt or a second layer is the usual fix, not stretching the same batt.
The DOE climate-zone quick-picks are a simplified, approximate starting point based on general U.S. Department of Energy attic insulation guidance — always confirm against your local energy code, which can specify different minimums than the general federal guidance.
Frequently asked questions
Why does the number of bags change if I pick a different target R-value?
Because bag coverage is printed at a specific reference depth, and coverage scales inversely with the depth you actually install. Aiming for a higher R-value means a deeper install, which means each bag covers less area — so you need more bags per square foot, not just because the material itself changed.
How do I find my bag's coverage numbers?
They're on a coverage chart printed on the bag, usually showing several rows of (R-value, minimum depth, bags-per-1000-sq-ft or sq-ft-per-bag). Pick the row matching your target R-value and enter its coverage and depth directly, or enter any row's figures — the calculator scales correctly either way since it works from the material's R-value-per-inch rate.
Should I use blown-in or batts?
Blown-in is generally faster for open attic floors and fills irregular spaces (around joists, wiring) more completely. Batts are more common for wall cavities and smaller, regularly-shaped areas where precise cutting is practical. Many homes use both in different areas.
What if my area has obstructions (vents, joists, recessed lights)?
This calculator assumes a clear, flat area. For attics with a lot of obstructions, it's standard practice to buy slightly more material than the raw calculation — rounding your area up before entering it, or simply buying one extra bag or package, avoids a mid-job shortage.
Sources
- U.S. Department of Energy, Energy Saver — insulation R-value recommendations by climate zone
- Manufacturer coverage-chart methodology for blown-in/loose-fill insulation (depth-scaled bag coverage)
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Disclaimer
This tool provides an estimate for informational purposes. Check your specific product's packaging for its exact coverage figures, and confirm required R-values against your local building/energy code rather than relying solely on the climate-zone quick-picks here.
Last updated 2026-11-02.