To calculate blown insulation needed for your attic, measure the square footage of the space, determine your target R-value, and account for the insulation's settling factor.
Step 1: Measure attic square footage. Measure the length and width of your attic space and multiply them together. This gives you the total coverage area.
Step 2: Determine your target R-value. R-value requirements vary by climate zone and local building codes, typically ranging from R-30 to R-60 for attics. Check your local building codes or the Department of Energy's recommendations for your region.
Step 3: Check the insulation product's coverage. Blown insulation bags specify how many square feet one bag covers at a given R-value. For example, a bag might cover 100 square feet at R-38 but only 80 square feet at R-60.
Step 4: Account for settling. Blown insulation compresses over time. Add 10-20% extra material to compensate for this settling, or follow the manufacturer's settling recommendations.
Step 5: Calculate bags needed. Divide your attic square footage by the coverage rate per bag at your target R-value, then multiply by 1.1 to 1.2 for settling factor.
Example: 1,000 sq ft attic at R-38 target. If each bag covers 100 sq ft at R-38, you need 10 bags base amount. With 15% settling factor: 10 × 1.15 = 11.5 bags (round up to 12).
Consider hiring a professional if your attic has unusual dimensions, obstructions, or existing insulation to work around.