Your exterior sheathing needs a thermal break before the siding goes on — and in northwest Iowa winters, a 2x4 wall with nothing behind the sheathing loses heat at a rate that shows up on your heating bill from November through March. Zip sheathing is great but it's not cheap, and it still needs a thermal break. 2-inch extruded polystyrene bead-board rated for use with cementitious finishes. 4-foot by 8-foot sheet. R-10 total — the full R-value of a 2x4 wall in a single board that also provides the thermal break your exterior assembly needs. The cement-compatible surface accepts most adhesive-based cladding systems. Standard on exterior foam and cement-board assemblies across northeast Nebraska and northwest Iowa — the 2-inch bead-board that goes behind the cladding when the spec calls for Stocked at Crofton Lumber and Akron Lumber.