By: James Meehan

Rigid foam insulation has become the preferred way for homebuilders to insulate foundations and basement slabs. Traditionally, builders have used the bright pink insulation that is placed in wall cavities between the studs in the house structure. Rigid foam insulation, however, is applied over the entire wall to give continuous insulation for the frame assembly rather than the interrupted insulation provided by traditional insulation.

Rigid foam insulation helps to increase the R-value of a structure – or it helps to increase thermal resistance and acts as an air and moisture barrier for the cavities of walls. The larger the number, the more effectively a house is insulated. Large numbers also help guarantee better heating or cooling for the house. Cities and inspectors required certain R-values in order for a building to meet code.

Rigid foam insulation is usually made of three kinds of materials. Expanded polystyrene (EPS) is semi-permeable and has the lowest average R-value (R-4) of the three, but it is the most affordable. After treatment, it can resist insects, but ground contact should be avoided, since it can absorb water. Extruded polystyrene (XPS) is middle ground for cost and R-value (R-5), and comes in three general colors: pink, blue or green. XPS retards moisture, but does not completely block it. Polyisocyanurate (ISO) is high-end with the highest cost and highest R-value (R-8), though the R-value over time will degrade. ISOs are made with facing (sometimes made of fiberglass) on them and are considered impermeable.

Depending on the kind of rigid foam insulation you choose, you can eliminate the need for house wrap, which prevents outside water and air from entering the walls. House wrap usually guards against high-energy bills by making your house easier to cool and heat. Be careful when installing rigid foam in an attic. Although it can protect the wood from damage, it has to meet certain building codes if it is placed in an attic.