Blown-In Insulation Calculator
Work out how many bags of blown-in attic insulation you need based on area, target R-value, and existing insulation depth.
Bags needed
30bags
Depth to add
10.5in
Volume needed
878ft³
This covers blown-in cellulose or fiberglass loose-fill only. Spray foam uses different coverage math (based on kit yield, not bag coverage) and isn't calculated here.
How the blown-in insulation calculator works
Blown-in insulation is estimated differently from batts: instead of covering a wall area with a fixed-size roll, loose-fill material is blown to a target depth that achieves your desired R-value, and bags are counted by volume (cubic feet) rather than square footage.
Formula: effective R-value = target R − existing R. Depth (in) = effective R ÷ R-value per inch (≈3.7 for cellulose, ≈2.5 for blown fiberglass). Volume (ft³) = area × (depth ÷ 12). Bags = volume ÷ bag yield, rounded up.
Frequently asked questions
The U.S. Department of Energy recommends R-38 to R-49 for warmer climate zones (1-3) and R-49 to R-60 for colder zones (6-8). Check your specific climate zone recommendation before setting your target.
Yes — if your attic already has some insulation, subtract its R-value from your target so you're only calculating the depth you need to add. For example, going from R-10 to R-49 only requires adding R-39 worth of new material.
Bag yield depends on the specific product's density and settled R-value per inch, which differs by brand and material. Always use the coverage chart printed on your actual bag rather than a generic figure — that's why bag yield is a field you set yourself here.
No — spray foam expands from a liquid and is estimated by kit yield (board feet per kit at a target thickness), which is a different calculation from loose-fill bag coverage. This calculator is for blown-in cellulose or fiberglass only.