Cellulose

The R-factor of cellulose insulation is approximately 3.8 per inch and it does not vary significantly over a wide range of densities. In an attic, 10 inches of cellulose insulation will have an

R-value of about R-38, regardless of the

density of the material. "Fluffing" cellulose by adding excessive air during installation does not change the R-factor of the insulation, so it is easy for an inspector or home owner to check the R-value of the installation. As long as the insulation maintains the required thickness it will have the

specified R-value. (Refer to the section on Settlement for more information.) In addition to maintaining R-value over a wide density range, cellulose insulation also maintains R-value under cold conditions. At an attic temperature of 20o F below zero the R-value of cellulose insulation is higher than at 70o above zero. It's important to emphasize that while R-value is important, many other factors are nearly as significant in determining the real world thermal performance of buildings. Focusing on R-value to the exclusion of these can lead to poorly-performing buildings.