Rim joist insulation that compresses, absorbs moisture, or sags in the cavity is insulation that doesn't do its job — and you've got cold floors above and higher heating bills you can't explain. Expanded polystyrene bead board handles rim joist and foundation wall insulation without the moisture issues that plague fiberglass in below-grade and ground-adjacent applications. 1-inch thickness gives R-4 per inch — the board covers gaps and air leaks in one pass where batt insulation requires stacking and sealing around wires and pipes. The 4x8 sheet covers 32 sq-ft per pass, with a board format that cuts cleaner than faced batts and doesn't require the same level of fitting around obstructions. Standard on framing crews and insulation contractors across northeast Nebraska and northwest Iowa — Stocked at Crofton Lumber and Akron Lumber.