Using the wrong R-value foam board for a below-grade or exterior application means callbacks — EPS soaks up water and loses R-value, ordinary foam board isn't rated for permanent moisture exposure, and the wrong compressive strength crushes under backfill or vehicle loads. Extruded polystyrene handles below-grade conditions in ways that expanded foam products don't. 1-inch thickness gives approximately R-5 to R-6 of thermal resistance in a closed-cell foam that resists water absorption over long-term below-grade exposure. The 4x8 sheet covers 32 sq-ft per board — standard for foundation wall and below-slab layouts. Compressive strength handles typical backfill loads and temporary construction traffic without permanent compression. The smooth skin surface on both sides provides better bondi Stocked at Crofton Lumber and Akron Lumber.