Designed for exterior foundation walls, Perimate™ XPS Insulation provides a lightweight, durable system that protects the waterproofing membrane while it fends water drainage away from the foundation. Perimate™ extruded polystyrene (XPS) foam insulation is a moisture resistant, durable and lightweight foam board designed specifically for use on exterior foundation walls. Perimate™ Insulation features patented dovetail grooves on one face and shiplap edges on the long edges. When installed on the exterior of basement walls, it provides insulation, protects the waterproofing membrane and assists water drainage away from the foundation.