How Much Attic Insulation Do I Need?
Wondering how much attic insulation you need? This calculator turns your target R-value into inches of insulation, an estimated number of bags, and a material cost estimate — for blown cellulose, blown fiberglass, batts, mineral wool, and spray foam.
Enter your numbers
Measure the attic floor (length × width). Do not subtract for joists.
Your results
| Added R-value needed | – |
|---|---|
| Bags needed (estimate) | – |
| Loose-fill volume | – |
| Material cost (estimate) | – |
How to use this calculator
Measure your attic's floor area in square feet — just length times width for a rectangular attic. If your attic is an odd shape, break it into rectangles and add the pieces together. You can skip subtracting the area taken up by joists; the small overcount acts as a built-in waste allowance.
Estimate your current R-value by measuring the existing insulation depth in inches and multiplying by the material's R-value per inch (see the table below). Enter 0 if the attic is bare. Then pick your target R-value: R-30 for warm southern climates, R-38 for most of the US, or R-49 for cold northern zones.
Choose your insulation type and optionally enter a price per bag to get a material cost estimate. Results update instantly as you change any input — the bag count rounds up to whole bags, and we recommend buying 10–15% extra to cover settling and uneven spots.
How it works
R-value measures resistance to heat flow: the higher the number, the better the insulation. Each material delivers a certain R-value per inch of thickness, so the depth you need is simply the R-value you still have to add, divided by the material's R-per-inch. This tool uses typical published R-per-inch values:
| Insulation type | R-value per inch (typical) |
|---|---|
| Blown cellulose | 3.5 |
| Blown fiberglass | 2.5 |
| Fiberglass batts | 3.2 |
| Mineral wool batts | 4.0 |
| Closed-cell spray foam | 6.5 |
The math is: added R = target R − current R (never below zero — if you already meet the target, you need no new insulation), then inches needed = added R ÷ R-per-inch. For example, going from R-0 to R-30 with blown cellulose needs 30 ÷ 3.5 ≈ 8.6 inches.
For loose-fill bag counts, the tool estimates the volume as area × inches ÷ 12 (cubic feet) and divides by an assumed 25 cubic feet of loose-fill per bag — a typical figure, though it varies by brand and blower settings — then rounds up to whole bags. That is why bag counts and costs are labeled estimates: always check the coverage chart printed on the bags you actually buy, and treat spray-foam results as a volume equivalent, since foam is sold by board-foot kits rather than bags.
Frequently asked questions
What R-value should my attic have?
The U.S. Department of Energy recommends attic insulation of R-38 to R-60 for most of the country, with colder climates at the higher end. R-30 is common in warmer southern zones. When in doubt, R-38 is a safe middle-ground target for most homes.
How many inches of insulation is R-30?
It depends on the material: blown cellulose needs about 8.6 inches, blown fiberglass about 12 inches, fiberglass batts about 9.4 inches, mineral wool batts about 7.5 inches, and closed-cell spray foam about 4.6 inches. Use the calculator above to get the exact depth for your insulation type.
How do I find the R-value of my existing attic insulation?
Measure the depth in inches with a ruler or yardstick in several spots, then multiply by the material's R-value per inch: about 3.5 for blown cellulose, 2.5 for blown fiberglass, 3.2 for fiberglass batts, and 4.0 for mineral wool batts. If the attic is uneven or compressed, use the thinnest measurement.
Are the bag counts and cost estimates exact?
No — they are planning estimates. Bag coverage varies by brand, blowing-machine settings, and settling over time. Always buy 10–15% more than calculated and check the coverage chart printed on the bags you buy.
Can I add new insulation on top of the old insulation?
Yes, in most cases. Adding blown insulation over existing insulation is the most common way to reach a higher R-value. Just make sure there are no moisture problems first, and never add faced batts with a second vapor barrier — trapped moisture can damage the structure.