Over the 60 years I have been writing specifications, I have not read a single satisfactory suggestion as to how to deal with the thorny subject of “or equal” when it applies to below-grade waterproofing.
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 occurs, such as in protected membrane roofs, vegetative assemblies, etc.
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.