Foam board insulation is a sheet, usually four by eight feet (1.2 by 2.4 m) in size, of rigid foam used in nearly all aspects of building construction to provide thermal resistance in floors, ceilings and walls. Basically, three materials are used to make foam board: polyurethane, polyisocyanurate and polystyrene.
Polyurethane and polyisocyanurate foam board are very similar in content. The closed-cell foam board is made by first liquefying the pellets, and then introducing a blowing agent. In the case of these two products, the blowing agent is usually one of the hydro fluoro compounds (HFC) or chloro fluoro carbons (CFC). The blowing agent forms gas bubbles in the mixture when it is heated. The hot, liquefied polyurethane or polyisocyanurate is then forced into a board-shaping dye. After the mixture cools, it is cut into any desired length. The resulting foam board insulation is of higher R (resistance to heat flow) value than a polystyrene foam board, usually R7 or R8 per inch (2.54 cm) of thickness.
Polystyrene foam board insulation is made in much the same way as Polyurethane and polyisocyanurate foam board, except for the differences in expansion. There are two basic types of this type of insulation: molded expanded polystyrene (MEPS), and extruded expanded polystyrene (XEPS). The difference in these two types of foam board is the strength of the finished product, though the density, about 1.5 pounds per cubic foot (0.21 kilograms per cubic meter), is usually the same.
MEPS foam board insulation, often called beadboard, is made by mixing unexpanded polystyrene pellets or beads containing pentane with a blowing agent. The resulting mixture is poured into a closed container and heated. The heat causes the beads to expand to many times their normal size. The mixture is then poured or injected into a mold, more heat is applied, and the resulting foam blocks are shaped into insulation.
XEPS is made by mixing polystyrene pellets with chemicals to liquefy them. Once a blowing agent is injected into the mixture and it is heated, gas bubbles form, and the hot material is poured into forms. When cool, it is cut into whatever shapes desired. XEPS is stronger than MEPS, having higher compression strength. Because of its greater strength, XEPS foam board insulation is often used for roofs and walls in new construction. The extruded polystyrene also has a higher rating for resistance to moisture.
Both MEPS and XEPS have lower R-values than Polyurethane and polyisocyanurate foam board insulation, rating about R5 per inch (2.54 cm) of thickness.