Your interior basement wall is finishing-ready but the insulation behind it has to perform for years under drywall — and in a below-grade application, moisture is the enemy that fiberglass cannot fight. By the time you notice the degradation from the living space side, the R-value is already gone. 1-1/2-inch-thick extruded polystyrene bead-board. 4-foot by 8-foot sheet. R-7 per inch closed-cell XPS construction — the foam that keeps its R-value in below-grade applications season after season because it was designed for exactly this environment. The tongue-and-groove edge detail helps manage moisture at the seams. Standard on below-grade foundation insulation across northeast Nebraska and northwest Iowa — the bead-board that goes on the foundation wall before the finish wall goes up. In sto Stocked at Crofton Lumber and Akron Lumber.