How to calculate insulation
Multiply surface length by width or height, subtract openings, and add waste. Divide the adjusted area by the exact square-foot coverage printed on a batt, roll, or bag package.
Insulation formula
Adjusted area = (gross area − openings) × (1 + waste). Packages equal adjusted area ÷ package coverage, rounded up.
Worked example
A 20 × 8 foot wall has 160 square feet. Subtracting a 21-square-foot door and adding 10% waste gives 152.9 square feet, requiring two packages that each cover 88 square feet.
Practical tips
- Match insulation width and thickness to the framing cavity.
- Choose R-value based on climate, assembly, local code, and existing insulation.
- Do not compress batts; seal air leaks and follow vapor-retarder guidance for the assembly.
Assumptions and limitations
This estimates coverage only. It does not select an R-value, assess moisture, model framing area, or determine code compliance. Blown-in yields vary with installed depth and density.
Frequently asked questions
Should I subtract studs?
Usually not for a purchase estimate: framing reduces cavity area, but trimming and small losses consume material. Use product-specific guidance.
Can this estimate blown-in insulation?
Yes, if you enter the bag coverage for the required R-value and installed depth from the manufacturer's chart.