How Much Blown-In Insulation Is Needed?

R-Values

• Insulation is rated by a universal standard called an R-value that measures a material's ability to resist the transfer of heat. Materials tested in laboratories are assigned R-values. The values vary by the type of insulation, meaning the higher the R-value, the greater a material's heat resistance. These R-values also vary by climate zones. Therefore, houses in Minnesota need higher R-values than those in Texas.

Cellulose is Highest

• Most blown-in insulation is cellulose that's made from old newspapers. Cellulose has the highest R-values, which are from 3.2 to 3.8 per inch, but it's most typical R-value is 3.7. Rock wool has R-values between 3 and 3.3 per inch. Fiberglass beads have the lowest R-values, which are between 2.2 and 2.7 per inch.

Walls and Attics Differ

• Attics generally require about double the insulation of walls. The North America Insulation Manufacturers Association recommends R-values between 30 to 60 for attics. It recommends R-values between 13 and 21 for walls. Minimum recommendations are slightly higher in the Midwest than in the Deep South and West Coast.

Typical Thicknesses

• A typical 2-by-4-inch stud wall filled with cellulose will have an R-value of about 13. That same wall filled with rock wool has an R-value of about 12. Fiberglass beads in a stud wall have R-values between 8 1/2 and 9. An attic with an R-value of 30 requires between 8 and 9 inches of cellulose. For rock wool, it's between 9 and 10 inches. For fiberglass beads, it's about 12 inches. Most attics have an R-value of 38. They are typically filled with between 10 and 12 inches of cellulose, about 15 inches of rock wool or slightly more inches for fiberglass beads.

Other Materials

• Recycled cotton, vermiculate and perlite are also blown-in materials. Blown-in cotton is similar to rock wool, but it has a lower R-value, and it's subject to moisture damage. Vermiculite and perlite have R-values between 2 and 3 per inch. They are naturally fire-resistant but are not used much except for concrete block walls and around fireplaces and chimneys.

Spray Foam

• Polyurethane foam is usually used to insulate walls in older houses that were built without any wall insulation. It has R-values between 6.5 and 7. It must be professionally installed. An installer cuts a hole at the top of every stud cavity and flows the liquid foam into the opening. The foam expands to fill the space and solidifies. It is rarely used in attics.