The ability of a material to resist heat -- its "thermal resistance" -- is measured in units called R-values. Even wallboard and wallpaper insulate to a degree, but the best insulators trap air, much as multiple-pane windows do. New buildings have levels of insulation required by local building codes, and building materials can add values of R-5 or R-6 to roofs. Most roofs, however, need additional insulation to reach levels recommended by the Department of Energy (DOE).
The DOE divides the U.S. into eight zones and recommends insulation levels for each. Zone 1 includes only the southern tip of Florida; a total value of R-30 to R-49 keeps heat out of homes. In zone 8 portions of Alaska, where the recommended R-value in roof insulation is as high as R-60, the objective is to keep heat inside buildings. Building specifications only list R-values, not materials, says the Oregon Department of Energy. As a result, materials must be labeled as to the thickness of batt or the weight of loose fill material required to accomplish specific R-values.
Batts and loose fill are laid between joists in attic spaces; "cathedralized" ceilings in attics or rooms can be insulated with spray or batts. A pitched roof requires more material to insulate because of its greater area and because gravity pulls insulation down toward the eaves, leaving less at the peak. In zones 2 and 3, covering most of the Deep South, the average depth of insulation should have an R-value of 30 to 60. In zone 4, which includes the mid-South and Northwest Pacific coastal areas, it should average 38 to 60.
Much batt insulation consists of wool or fiberglass; loose fill may be cellulose, fiberglass or polyethylene. Polyisocyanurate, polyurethane and extruded polystyrene foams work well for filling small openings or limited access spaces. R-value of existing insulation is figured by a simple calculation, depending on the material; 18.75 inches of fiberglass batt multiplied by 3.2 yields an R-value of 60. The same R-rating would require 24 inches of loose fill fiberglass. If 3 to 4 inches of roof insulation already exists, buildings would need materials adding up to the recommended levels. Zone 5, 6, 7 and 8, for example, could add a combination of batts or batts and loose fill that would total ratings of R-38 to R-49 to reach the recommended levels of R-39 to R-60.