The thermal resistance of each of the products will vary. In general, EPS foam has the lowest R-value per inch, with XPS being slightly more efficient, and with Polyisocyanurate having the best R-value per inch. The R-value of EPS foams can be increased by increasing the density of the product, however, the more dense expanded foams are less common in the market. Typically EPS foam has a rated value of approximately R-4 per inch. XPS foams are pretty consistent with an R-value of approximately R-5 per inch.

While the thermal resistance of these thermoplastic foams is generally stable over the long term and therefore the initial R-value at the time of manufacturing will not change over time, polyisocyanurate foams are rated with a Long Term Thermal Resistance (LTTR) R-value representing a 15 year weighted R-value. This is in response to issues of thermal drift of the polyisocyanurate products. Thermal drift occurs due to the gasses produced during the forming of the foam. These gasses slowly diffuse out of the product over time and are replaced by air. Since these gasses also have more thermal resistance than air, the R-value of polyisocyanurate diminishes over time as the gasses diffuse out of the product. Facings on the insulation board, such as aluminum foil, will slow this process down as the diffusion can only occur out the edges of the product and not through the front and back faces. Most polyisocyanurate products have an LTTR R-value of R-6.5 per inch.