Expanded and extruded polystyrene are two types of roof insulation. Other types are blown-in fiberglass or cellulose. Proper use depends on budget, ease of installation, available space and resistance to moisture.
Sloped roofs are properly insulated with polystyrene. The expanded type does not contain ozone depleting hydrochlorofluorocarbons. High recycled content, low density and high vapor permeability are characteristics of EPS. The extruded type is water-resistant rigid foam made with HCFC-142b. XPS is a high-density material with low vapor permeability. Installation is optimal above roof sheeting or between rafters for sloped roofs, according to Green Building Advisor.
For flat roofs, blow-in insulation works well. The fiberglass type contains a minimum of 50 percent recycled content, whereas cellulose contains 85 percent recycled paper fiber. A 6-inch gap between the top of the insulation and roof sheeting allows for venting and prevents rot and mold. Approximately 12 to 16 inches of either type of insulation is suitable.
For energy efficiency, the Department of Energy recommends using R-value insulation levels based on regional climate zones. For example, Zone 4 includes New Jersey, where R38 to R60 levels are appropriate. One bag of Owens Corning AttiCat fiberglass blow-in insulation covers 38.8 square feet of space to the R49 level.