Rigid insulation is a building material used to provide thermal resistance to walls, foundations, and roofs. It is available in formed or extruded sheets of varying sizes, which are made from polystyrene or polyurethane. This material helps to improve the energy efficiency of a home by keeping cold or hot air from penetrating the walls. The thermal resistance of rigid insulation is measured by the material's R-value. Higher R-values mean that the material is more resistant to temperature transfer, while low R-values mean that air can easily pass into or out of the home.
The use of rigid insulation in the home can dramatically improve the comfort of occupants, while helping to lower energy bills. The resulting reduction in energy use also benefits the environment, leading to fewer greenhouse gas emissions. While many types of insulation may perform this function, rigid insulation generally provides a higher thermal resistance than other materials of a similar size. Rigid panels are denser and thinner than other insulation materials, and can be installed with fewer gaps or joints, resulting in less air leakage.
Sheets of rigid insulation are typically installed between a home's framing members and the exterior sheathing or cladding. The panels are placed tightly together, and joints may be caulked or sealed to provide maximum thermal resistance. In situations where moisture infiltration is a possibility, the insulation may be coated with a water-resistant coating.
Rigid insulation may also be installed on roofs or foundations. Roof panels must be dense enough to walk on, and are usually installed above the roof framing, but below the roof tiles or shingles. Water-resistant rigid panels can be placed on the exterior of foundations, but it is more common to install them on the interior of basement walls, or below a ground-level floor. When installed on the exterior of the foundation, the panels can be treated with insecticides to prevent termite damage.
There are three main types of rigid insulation, and each offers its own benefits and drawbacks. Molded expanded polystyrene (MEPS) is made sheets formed from liquid polymers. This is the most affordable and most common type, of rigid panel and is often referred to as beadboard. Extruded expanded polystyrene (XEPS) is more expensive, but is also stronger and denser, making it well suited for roof installations. Polyurethane foam board is the thinnest of the three, and works best in small spaces or in homes subject to extreme temperatures.