How to Calculate Insulation Requirements
Coverage area and R-value, not just square footage alone
Start with the area to insulate
Measure the wall, floor, or attic (ceiling) area you're insulating, the same way you would for drywall or paint — length × height for walls, length × width for a floor or attic plane.
R-value determines the product, not just the area
Required R-value (a measure of thermal resistance) varies by climate zone, application (wall vs. attic), and local energy code — a higher R-value target typically means thicker batts or more layers of loose-fill, which changes the coverage-per-package figure. Always check the product's own coverage chart for the R-value you're targeting rather than assuming coverage is constant across R-values.
Batts vs. loose-fill/blown-in
Batt insulation is quantified by area (square feet) at a given R-value and thickness — divide your area by the coverage per package. Loose-fill (blown-in) insulation is quantified by bags, where each bag's coverage chart states square feet covered per bag at a target R-value and depth — always follow the manufacturer's chart rather than estimating a bag's coverage independently.
Worked Example
An 800 ft² attic floor, targeting R-38 with a product covering 40 ft² per bag at that R-value.
Bags needed: 800 ÷ 40 = 20 bags