Attic and bag coverage
Attic area
Floor of the attic, not wall area. 1,000 sq ft at 40 sq ft/bag → 25 bags.
Read the bag chart. Prefill 40 sq ft/bag at R-30 is a typical blown-cellulose example, not a spec. R-30 vs R-49 are different rows — type the row you want. Same honesty as paint: coverage from the bag, not a brand table.
Waste
10% on the 1,000 sq ft fixture is 27.5 → 28 bags (ceil).
Bag count
- Bags ceil
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- Exact
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- Coverage
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- Waste
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Enter attic square feet. Coverage comes from the bag at the R you want. 1000 sq ft at 40 sq ft/bag → 25 bags.
Coverage comes from the bag. The 40 sq ft/bag at R-30 prefill is a typical blown-cellulose example, not a spec. Not a Manual J, not a rebate form, not an energy audit. Thickness is a result of R — not mulch thickness. Confirm the bag chart. Nothing on this page leaves the browser.
How bags are counted
Paint gallons come from the can. Insulation bags come from the bag chart at the R you want — not a hidden brand table, and not depth like mulch.
bags = attic_sq_ft / coverage_sq_ft_per_bag with_waste = bags × (1 + waste/100) buy = ceil(with_waste) 1,000 sq ft, 40 sq ft/bag (R-30 example): 1000 / 40 = 25 bags + 10% waste = 27.5 → 28 bags (ceil) R-30 and R-49 are different rows on the same bag. Thickness is a result of R, not the input.
A 1,000 sq ft attic at the example 40 sq ft/bag is 25 bags, not a thickness you type.
FAQ
How do you count bags of blown insulation?
Bags = attic sq ft / (coverage sq ft per bag at the R you want). Coverage is a row on the bag chart, not a thickness you pick like mulch. 1,000 sq ft at 40 sq ft/bag is 25 bags. Add waste, then round up: 10% waste is 27.5 → 28 bags.
Why is coverage from the bag, not a built-in brand table?
Same honesty as paint: read coverage from the bag, don’t invent a brand’s secret table. The default 40 sq ft per bag at R-30 is a typical blown-cellulose example, not a spec. Type the number printed on the bag you are buying.
Why are R-30 and R-49 different rows on the same bag?
One bag chart has a coverage column for each R. R-30 and R-49 are different rows — more R means fewer square feet per bag. Thickness is what that R lands at after you blow it, not an input you type here. That is the opposite of mulch, where depth is the starting number.
Why isn't thickness the input (like mulch)?
Mulch is volume: area × depth. Blown insulation is coverage-at-an-R from the bag chart. You pick the R you want, read that row’s sq ft per bag, and divide attic area by it. Thickness is a result of R, not the input.
How many bags for a 1,000 sq ft attic?
1,000 sq ft → 25 bags at the 40 sq ft/bag R-30 example, 0% waste. Same attic with 10% waste is 27.5 → 28 bags (ceil). Change coverage to the R-49 row on your bag and the count changes. Empty attic or coverage is invalid, not a bag count.
Is this a Manual J, rebate form, or energy audit?
No. Not a Manual J, not a rebate form, not an energy audit. Bag-count math only: attic square feet, coverage from the bag at the R you want, optional waste, round up. Related tools: paint, drywall, window AC, and the Acorn hub.