GreenGuard? extruded polystyrene (XPS) does not absorb water, is not a food source for mold, and is not subject to moisture-related damage and deterioration.? An R-Value of 5.0 per inch of thickness makes it an excellent thermal insulator that increases the energy efficiency of structures and retains its insulating properties over time.? GreenGuard? XPS insulation provides a barrier to moisture infiltration.? GreenGuard? XPS Insulation Board is ideal for a variety of applications, including below slab, below grade along the foundation, inside basement walls, exterior insulation over wood sheathing, and residing.