How many bags of blown insulation you need for 1000 square feet depends on the insulation material, the desired final R-value, and the bag coverage chart provided by the manufacturer, because different products have different densities. For fiberglass blown insulation like AtticCat or GreenFiber, which is very fluffy and low density, to reach R-38 which is about 12 to 14 inches deep you typically need about 1 bag per 60 to 80 square feet, so for 1000 square feet you would need roughly 13 to 17 bags. To reach R-49 which is about 16 to 18 inches you would need about 18 to 22 bags per 1000 square feet. For cellulose blown insulation, which is denser and heavier, you need more bags, about 1 bag per 35 to 45 square feet for R-38, so roughly 22 to 28 bags per 1000 square feet, and about 30 to 36 bags for R-49. Always check the coverage chart on the bag that lists square feet per bag at a given R-value and depth, add 10 percent extra for waste and settling, and measure your existing insulation depth to calculate only the additional bags needed to top up.
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