The differences in performance of expanded polystyrene rigid foam insulation (EPS) and extruded polystyrene rigid foam insulation (XPS) has been debated since the 1980’s. Esch’s 1986 study showed that the R-value of EPS degraded more than XPS when installed in roadway embankments. Pouliot and Savard (2003) noted similar results. This study adds 15 additional samples from three additional installations to the dataset. Using the combined data from these sites, ratios of R-values of EPS/XPS were developed which can be used to estimate equivalent thicknesses of the two products. R-value multipliers were also developed which allow thicknesses of the products to be computed based on the long-term performance of the insulation used in roadway and airport embankments. The data appear to be consistent between Esch, Pouliot and Savard and this study. There is no consensus as to how the data or ratios are to be applied if at all. However, the study does provide two approaches. A multiplier can be applied to each product which provides a long-term equivalent thickness for each product. Alternatively, a multiplier can be applied to the R-value of each product which yields a thickness that yields a long-term R-value of in-service applications.
