Rigid insulation is made of foam panels and used to insulate exterior and interior walls, roofs and crawl spaces of homes and buildings. It is made of synthetic materials and blended with fibrous materials such as wood, along with binders. The mixture is molded into panels, which are often coated with asphalt to provide a moisture barrier. There are four main types of rigid insulation panels: molded expanded polystyrene called MEPS, extruded expanded polystyrene called XEPS, polysisocyanurate called polyiso and polyurethane.
The R-value is a number used to describe a material’s ability to insulate a home or building from heat loss and heat gain. The former is a lay description for thermal conductivity; the latter, for thermal resistance. The higher the R-value number, the greater the thermal properties. Rigid insulation materials have some of the highest R-values of all types of insulation, ranging from 3.6 per inch to 6.8 per inch, in comparison to common “pink blanket” fiberglass insulation, which ranges from 3.1 per inch to 4.1 per inch. MEPS have an R-value range of 3.7 per inch to 4.0 per inch. XEPS has an R-value of 4.7 to 5.0. Polyiso has an R-value of 5.6, and polyurethane has the highest R-value of all, ranging from 7.1 to 8.0. However, polyurethane is commonly used for industrial applications and polyisco is used for residential applications.
Local building code agencies and the U.S. Department of Energy set required R-values for new home construction. In most areas of the United States, the minimum R-value set for building exterior walls is R-19. Therefore, many builders use materials that will achieve an R-value of 20 or higher to make sure they comply with building codes.
Installing insulation to achieve an R-value of 25 or higher is often used for exterior walls in cold climates. Contractors comply with a zone map authored by the U.S. Department of Energy, which identifies six primary climate zones. For example, states in the upper Northeast, including Maine and Vermont, are in zone 1, where an exterior wall with an R-value of 25 an even higher might be used. Likewise, a contractor might use multiple layers of rigid insulation to achieve an R-value of 25 or more for the roofing.
The difference between an R-20 and an R-25 value is 5 points. You can achieve this amount using thicker layers of the rigid form insulation material. For example, you would need between 5 inches and 6 inches of MEPS to achieve an R-20 value, since it has an R-value range of 3.7 per inch to 4.0 per inch. While with polyiso, you only need 2 ½ inches to 2 ¾ inches to achieve an R-20 value, since it has an R-value of 7.1 per inch to 8.0 per inch. Likewise, it would take 6 ¼ inches to 6 ¾ inches to achieve an R-25 value with MEPS, and around 3 ¼ inches to 3 ½ inches of polyiso rigid foam insulation.
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