How to Measure for Insulation
Start by measuring each surface that will receive insulation. For walls, multiply the wall length by its height. For ceilings, attic floors, and other flat areas, multiply the length by the width.
If you're insulating several walls or separate areas, measure each one individually and add the areas together. Doors, large windows, and other spaces that will not receive insulation can be subtracted from the total.
Batts, Rolls, Rigid Foam, or Loose-Fill?
The amount of insulation you need depends partly on the type of product you're installing.
Batts and rolls are commonly sized to fit between standard framing members and are sold in packages with a stated coverage area. Rigid foam insulation is sold in sheets or boards, so both the board dimensions and number of boards needed matter.
Loose-fill or blown-in insulation is different because coverage depends on the installed depth and the particular product being used. Use the coverage information printed on the manufacturer's bag for the depth or R-value you intend to install rather than assuming every bag covers the same area.
What Does R-Value Mean?
R-value is a measure of an insulation material's resistance to heat flow. In general, a higher R-value provides greater resistance to heat transfer, but the appropriate amount of insulation depends on where it is being installed, the building construction, climate, and applicable building or energy codes.
Don't choose an insulation product based only on the amount of material calculated. Verify the recommended or required R-value for your particular project before purchasing.
Why Insulation Coverage Can Vary
Two packages of insulation that appear similar may not cover the same amount of area. Product thickness, density, dimensions, intended R-value, and packaging can all affect the manufacturer's stated coverage.
For this reason, use the coverage printed on the actual product you're considering whenever possible. This is especially important with loose-fill insulation, where the number of bags required can change substantially depending on the intended installed depth.
Before Buying Insulation
Double-check your measurements and the product specifications before purchasing. Pay particular attention to framing dimensions, insulation width and thickness, package coverage, and the R-value of the product.
Also check the manufacturer's installation instructions and any applicable local requirements. Insulation performance can be affected by improper installation, compression, gaps, moisture, and other conditions that a simple material calculator cannot account for.