Insulating materials are given an R-value rating, which measures the material’s resistance to conductive heat flow. A higher R-value indicates greater insulating effectiveness. Keep in mind that the R-value of foam board insulation is only one part of construction; for example, when measuring the insulation of a wall, add together the R-value of the insulation, structural and sheeting materials and other components. Hot air flows to cooler areas, so a well-insulated roof will more effectively maintain desired temperatures than insulating only the walls.
Foam board insulation can be used to insulate unfinished foundation walls and finished exterior and interior walls, floors, ceilings and roofs. For interior application, the foam board is usually covered with ½ inch thick drywall. For exterior applications, the foam board must be covered with weatherproof material. The most common size for standard construction is 4-by-8-foot sheets, but foam board is also available in 2-by-8-foot and 4-by-9-foot sheets. Standard thicknesses are 1-inch, 1½-inch, 2-inch, 3-inch and 4-inch. Per square foot, foam board insulation is more expensive and has a lower R-value than fiberglass batts or blown-in cellulose insulation, so it isn't commonly used to insulate walls and ceilings in homes and other heated spaces.
Foam board insulation is easy to use as it is lightweight, manufactured in sizes that fit standard construction framing and does not require special tools. It is easy to cut with a knife or saw, and some products are scored so it is easy to snap them into smaller sizes. It does not cause skin irritation, and when properly installed, it does not break down or exude toxic fumes. Foam boards are often covered with a plastic membrane for weather protection or foil, which provides a reflective surface to further stop heat penetration.
If not sealed with tape or spray foam, air will leak between the joints of foam board insulation sections, and between the insulation and wall studs or other adjoining material. Foam board can only be installed in existing walls by removing the wall sheathing and any existing installation. Because it is easy to snap, it can break if not stored or installed on a smooth surface. Extruded polystyrene foam board is manufactured with hydrochlorofluorocarbons, which deplete the ozone layer.