UPDATED on January 25, 2018 with information on phenolic foam.

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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.

• The R-value of EPS ranges from about R-3.6 to R-4.2 per inch. In most cases, the higher the density of the EPS, the higher the R-value per inch.
• The R-value of XPS is R-5 per inch.
• The R-value of polyisocyanurate is R-5.7 to R-6 per inch (although cold-climate builders sometimes use a lower R-value because polyisocyanurate performs poorly at cold temperatures).

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…

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