Foam insulation can be made through closed-cell or open-cell foam. Open-cell foam contains broken surfaces that air fills, making it softer and lighter. Open-cell foam insulation should not be used where it can absorb moisture. Closed-cell foam is much more stiff and dense. The cells are often filled with gas to increase the efficiency of the insulation, referred to as the R-value. This rating is a measurement of an insulation's resistance to heat flow. The higher the R-value, the less insulation and weight you will need to install.
Polyisocyanurate Foam Board
Polyisocyanurate foam board is often referred to under the name of Polyiso, a major brand that markets this form of insulation. Made from foam plastic, this insulation does not need a thermal barrier and is used for more than 70 percent of all roof insulation projects. Polyisocyanurate is a closed-cell foam and generally has a 2-pound-per-cubic-foot density. It has a general R-value of 5.56, but can go as high as 8. Due to its increased R-value, consumers can add a thinner layer of the insulation while still achieving a higher efficiency.
Polystyrene Foam Board
Polystyrene foam board is a closed-cell foam insulation that has a strong resistance to moisture absorption. Typically, foam densities for polystyrene foam board are 1.5 pounds per cubic foot. Although the R-value often depends on the material's density, the R-value for polystyrene foam board is usually about R-5 per inch.
Polyurethane Foam Board
Polyurethane is another closed-cell foam insulation board. This material has a high R-value around R-7 to R-8 per inch. As time goes by, however, air can replace the gas inside the insulation, resulting in a decreased R-value of 1 to 2. It typically weighs 2 to 3 pounds per square foot. It is sold in 12-pound, 15-pound, 20-pound and 30-pound densities.