Blown-in insulation calculator: how many bags do I need for my attic?

Blown-in insulation is sold by the bag, but a bag doesn't cover a fixed area: the deeper you blow it, the fewer square feet each bag covers. That's why "how many bags of blown-in insulation do I need?" has no single answer until you know two things: the R-value you're adding and which material you're blowing. This guide gives the formula, a bag table for a 1,000 sq ft attic, and a free calculator that does it with the coverage chart printed on real bags.

Short answer: bags = R-value you're adding × attic area ÷ the bag's rating (R·sq ft per bag), plus 10% for settling. A 19 lb cellulose bag is rated about 600 R·sq ft and a large fiberglass bag (AttiCat-style) about 1,950. Taking a bare 1,000 sq ft attic to R-49 is about 90 bags of cellulose or 28 bags of fiberglass.

Step 1: how much R-value do you need?

The U.S. Department of Energy's recommendations for an uninsulated attic, by climate zone:

• Zone 1 (south Florida, Hawaii): R-30 to R-49.
• Zones 2 and 3 (the Gulf Coast, the South, southern California): R-30 to R-60.
• Zone 4 (the mid-Atlantic, the middle of the country, the Pacific Northwest coast): R-38 to R-60.
• Zones 5 to 8 (the northern states and Alaska): R-49 to R-60.

Already have some insulation? Measure its depth and subtract. Old fiberglass is about R-2.5 per inch, so 4 inches is roughly R-10: to reach R-49 you add about R-39. Blown cellulose is about R-3.5 per inch.

Step 2: the bag formula

1. Attic area in square feet: length × width of the attic floor (the ceiling below, not the roof).
2. R to add = target R-value − existing R-value.
3. Bags = R to add × area ÷ bag rating.
4. Multiply by 1.10 for settling and uneven coverage, then round up.

The bag rating is printed on the bag's coverage chart as square feet per bag at each R-value; multiply any line of the chart (R-value × sq ft per bag) to get it. A 19 lb cellulose bag that covers 12 sq ft at R-49 is 49 × 12 ≈ 600. Ratings differ between brands, so use the chart on the bag you're buying.

Bags for a 1,000 sq ft attic

Starting from a bare attic floor, with 10% added:

For other sizes, scale in proportion: a 1,500 sq ft attic needs one and a half times as many bags. Fiberglass needs fewer bags because each large bag is much bigger, not because it insulates better per inch: blown fiberglass has to be blown deeper than cellulose for the same R-value.

Cellulose or fiberglass?

• Cellulose is denser, blocks more air movement and settles more. It's usually the cheaper choice per R-value at retail.
• Fiberglass is lighter (worth knowing over older drywall ceilings), doesn't absorb water and settles less.
• Either one needs air sealing first (top plates, wiring holes, the attic hatch) and baffles at the eaves so the soffit vents stay open.

Renting the blower

Home centers often lend the blowing machine free with a minimum bag purchase, typically 10 to 20 bags; check your store's current offer. Have one person feeding bags at the machine and one in the attic with the hose, mark target depths on a few rulers stapled to joists, and work from the far end back to the hatch.

Use the calculator

The blown-in insulation calculator at smartinsulationcalculator.com uses each material's bag chart, subtracts what's already in the attic, and gives bags, depth and cost for DIY and for a contractor. The same calculators are in the free Smart Insulation Calculator app for Android, which works offline in the attic. On iPhone, the insulation calculator is part of Roofing Calculator & Estimator from version 1.7.

Not sure of your target? Read what R-value insulation do I need. Insulating walls and floors too? See how much insulation do I need.