The purpose of building insulation is to reduce heating and cooling energy consumption, contribute to durability, and provide comfort for occupants. However, there are numerous locations where significant exposure to moisture—which severely affects a material’s thermal performance—occurs, such as in protected membrane roofs, vegetative assemblies, below grade, and frost-protected shallow foundations (FPSFs).
Neither expanded nor extruded polystyrene (EPS nor XPS) are intended to provide the primary waterproofing or dampproofing on below-grade foundation walls or under slabs. However, rigid foam insulation can offer an additional barrier to ground water, especially those products designed with that goal in mind.
Balancing performance and cost for insulation installed on below-grade foundation walls and under floor slabs can require specifiers to rethink common assumptions about rigid foam insulation. Misperceptions about the moisture absorption of expanded polystyrene (EPS) compared to extruded polystyrene (XPS) may be limiting more cost-effective choices. Test results demonstrate the long-term durability and moisture resistance of EPS insulation and the stability of its thermal performance. In many applications, substituting a strong and adequate, yet lower compressive resistance insulation, can result in significant cost savings.