Insulation performance is measured by R-Value. R-value is a measure of the insulation’s ability to resist heat flow. The higher the R-Value, the better the thermal performance. In Southern U.S. climates, the recommended level for most attics is to insulate to a minimum of R-30 (at least 10 inches, depending on the insulation type). For fiberglass and cellulose insulation, you can estimate about R-3 per inch. In Central and Northern U.S. climates, a minimum of R-49 is recommended (or about 16 -18 inches, depending on the insulation type). Visit Seal and Insulate with ENERGY STAR for more information, including recommendations for how much insulation you should have.