15-year test offers variances between EPS and XPS

Geosynthetics | February 2009

ACH Foam Technologies announced in December the completion of a 15-year test to determine water absorption characteristics of EPS (expanded polystyrene) and XPS (extruded polystyrene). The test revealed variances between the two materials in their resistance to water absorption.

The test of below-grade insulation, conducted by AFM Corp. (ACH Foam Technologies’ marketing and technical support company) in cooperation with the Expanded Polystyrene Molders Association, determined the water absorption of foam-control EPS and an extruded polystyrene (XPS) product. Samples of EPS and XPS were excavated from the exterior foundation of a building in St. Paul, Minn. The insulation was placed into service in 1993 and had 15 years of use as vertical wall insulation separating the heated building foundation from soil.

“The results of the independent testing are dramatic,” said Todd Bergstrom, VP of Technology for AFM Corp. “The EPS insulation maintained 94% of its stated R-value of 3.6 after the 15-year time period and had a moisture content of only 4.8%.

“However, the XPS retained only 52% of its stated R-value of 5.0. The loss in R-value for the XPS is quite dramatic and can be explained very simply by the 18.9% of moisture absorption during the 15 years of use. These results suggest very clearly that short-term laboratory tests of water absorption for XPS do not necessarily reflect the long-term, below-grade performance of these materials.”

Even though the ASTM C578 water absorption requirement for XPS is less than 0.5%, the XPS absorbed 18.9% of moisture during the 15-year period. According to Bergstrom, it is apparent that moisture that migrates through the soil, insulation, and foundation system is trapped in the cell structure of XPS. In contrast to the XPS, EPS is maintaining an equilibrium condition with the adjacent soil and is not accumulating water during the life of the building.

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