by Ram Mayilvahanan
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.
Two classes of products to consider for enhanced moisture protection are faced insulation panels and panels with pre-cut drainage grooves.
Rigid foam insulation is available with polymeric laminate facers virtually impervious to moisture. The thin factory-applied facer keeps water from entering the panel, and thereby away from concrete foundations and slabs.
In instances where a building sits on a high water table or the soil is otherwise regularly saturated, rigid foam insulation drainage boards can help reduce the hydrostatic pressure of the backfill on the foundation wall. Such boards have narrow, regularly spaced channels cut into the face of the foam. A factory-applied filtration facer installed over the grooved face keeps soil out of the channels so water continues to flow. One such widely available product can drain up to 62 l/min/meter (5 gal/min/ft).
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