UPDATED on January 25, 2018 with information on phenolic foam.
Maybe you’ve decided that your floor, wall, or roof assembly needs one or more layers of rigid foam. Which type of foam should you choose: polyisocyanurate, expanded polystyrene (EPS), or extruded polystyrene (XPS)?
The answer depends on several factors, including your R-value target, your local climate, whether the insulation will be in contact with soil, and your level of environmental concern.
R-value per inch
Manufacturers of insulation products are required to provide consumers with R-value information. If you’ve purchased rigid foam insulation that isn’t clearly labeled, contact the manufacturer to learn the product’s R-value.
Over a period of decades, the R-value of polyisocyanurate and XPS gradually declines. For more information on this phenomenon, called “thermal drift,” see Thermal Drift of Polyiso and XPS.
Cold weather performance of polyiso
Rigid foam manufacturers are required to perform R-value tests using an ASTM method specifying that the test be performed at a mean temperature of 75°F. At lower mean temperatures, EPS and XPS perform better than their R-value label indicates. In other words, as the temperature drops, the ability of EPS and XPS to resist heat flow improves.
Polyiso behaves differently: as the mean temperature drops, it does a worse job of resisting heat flow. For that reason, some cold-climate builders assign a lower R-value for polyiso — perhaps R-4.5 or R-5 per inch — than the R-value on the product label.
For more information on this issue, see Cold-Weather Performance of Polyisocyanurate.
Soil contact limitations
XPS and most types of EPS are rated for ground contact. These products can be buried without worrying that the materials will lose their insulating value or absorb significant quantities of moisture. If you are in doubt about the suitability…