Using the wrong foam board on a foundation, below-slab, or exterior application is a callback waiting to happen — expanded polystyrene (EPS) drinks water and loses R-value, styrene foam delaminates under UV, and the wrong compressive strength cracks under load. Extruded polystyrene handles all three scenarios correctly because it's a closed-cell structure that resists water intrusion in ways EPS doesn't. 1-1/2-inch thickness gives approximately R-7.5 to R-9 depending on the manufacturer — enough for below-slab and frost wall applications where you need consistent thermal performance in consistently damp conditions. The 4x8 sheet covers 32 sq-ft per board, which works cleanly on standard wall and foundation layouts. Compressive strength handles the loads you see in below-grade applications Stocked at Crofton Lumber and Akron Lumber.