GreenGuard PB4 Protection Board comprises closed-cell extruded polystyrene (XPS) with tough plastic capsheets on both sides. Its tough plastic capsheets resist puncturing and its high compressive strength and impact resistance withstand even the toughest load pressures. Because it is made of extruded polystyrene (XPS), GreenGuard PB4 Protection Board has a dense, closed-cell construction that resists both moisture absorption and decomposition from long-term exposure to chemicals in the soil.