Styrofoam Brand DeckMate 18 PSI is an extruded polystyrene foam insulation with a planed surface. It has excellent insulating characteristics, high resistance to water or water vapor, exceptional compressive strength, excellent freeze-thaw resistance and long-term durability. STYROFOAM™ Brand DECKMATE™ Insulation is designed for use in fully adhered conventional low-slope roof applications. Available in flat and tapered board stock, the rigid foam insulation boards can be installed directly on structural steel decks beneath a non-bituminous sheet membrane, eliminating the need for a thermal barrier.