Polystyrene Moisture Resistance Facts

Superior Moisture Resistance and Stable R-Value of Foam-Control® 

Architects, Engineers, Contractors, and Building owners are all concerned with the performance of their insulation. The long term performance of insulation is critical to ensuring the energy savings the insulation was specified to provide. The most common issue surrounding the long term performance of below grade insulation is loss of R-value over time.

Recent Independent testing of below grade insulation has determined the water absorption of Foam-Control and an extruded polystyrene product. Samples of molded polystyrene and extruded polystyrene were excavated from the exterior foundation of a building in St. Paul, MN. 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. The molded polystyrene insulation maintained 94% of its stated R-value of 3.6 after the 15 year time period and had a moisture content of 4.8%. However, the extruded polystyrene retained only 52% of its stated R-value of 5.0. The loss in R-value for the extruded  is quite dramatic and can be explained very simply by the 18.9% of moisture absorption over the 15 years of use.

It is apparent that moisture that migrates through the soil, insulation, and foundation system is trapped in the cell structure of extruded polystyrene. In contrast molded polystyrene maintains an equilibrium with the adjacent soil and does not accumulate water over the life of the building.

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