Rigid foam board insulation (sometimes called foam board or rigid boardstock) is used primarily in new construction and major renovations. While other types of insulation are installed in wall or ceiling cavities between the studs, rafters or joists, rigid foam board insulation is used on exterior walls beneath the siding and on exposed interior basement walls where studs are not used. Rigid board insulation is made primarily from extruded or expanded polystyrene (also known as XPS and EPS, respectively) and polyisocyanurate or polyiso. XPS resists moisture best and is often used below grade to insulate the exterior of foundation walls and under basement slabs.
It's worth noting that both polystyrene and polyiso are made from petroleum-derived materials that contain toxic chemicals. In particular, the flame retardant used in polystyrene releases toxic gases if it burns (although polystyrene does not ignite easily). Building codes may require a covering like gypsum wall board as a fire barrier for foam board insulation products. Green building experts say that rigid foam board insulation offers some significant benefits that can offset its environmental drawbacks, including higher R-values (insulating power) per inch than other types of insulation; rigid foam board insulation typically has an R-value of between 5 and 8.
Extruded polystyrene (XPS) is the most rigid as well as the most moisture resistant. Expanded polystyrene (EPS) has a slightly lower R-value and costs a little less; it is also less moisture-resistant. A higher-density EPS is available with a higher R-value and greater moisture resistance, but it is more expensive. Polyiso has the highest R-value of the three materials, but it is not as moisture resistant and is not recommended for use below grade. Polyiso boards are often made with a foil facing that acts as a vapor barrier. ESP foam board costs about $0.07 per square foot per R-value, while XPS and polyiso cost around $0.10.
Need help figuring out how much insulation and R-value you'll need? See our What to Look For page for details on those calculations, plus installation considerations and other factors to consider.