How to Figure for Bundles of Blown Insulation

Measure the length and width of your attic and multiply the two numbers together to get the total square footage. Subtract only for significant areas that won't be covered by the blown insulation (more than 16 square feet). Round up to the nearest whole number.

Measure the depth of any existing insulation to determine your current R-value. According to DIYHomeInsulation.com, fiberglass batts and recycled denim batts have a typical R-value of 3 per inch; blown cellulose insulation has an R-value of 3.5 per inch; blown fiberglass and rock wool have an R-value of 2.5 per inch. Multiply the depth of your type of insulation by the approximate R-value and round this number down to the nearest whole number.

Consult the U.S. Department of Energy's R-value recommendation calculator to determine the recommended amount of insulation you should have for your area of the United States. (The address is ornl.gov/sci/roofs+walls/insulation/ins_16.html.)

Subtract the current R-value of your attic insulation from the recommended R-value. This is how much new blown-in insulation you will need.

Consult the manufacturer's directions for the R-value of the type of insulation you will be adding. This is typically printed on the side of the bundle. Or you can check the manufacturer's website for more information.

Purchase enough bundles to meet your new insulation goal. Always purchase an extra bundle or two because you can return unopened bundles to the store. Or keep them on hand in case you need more insulation in the future.