It costs less to build a high-R wall using polyisocyanurate sheathing than using thicker stud cavities filled with fiberglass. That was the conclusion of a study performed by Energy Conservation Management, Inc. (EMC) for the Polyisocyanurate Insulation Manufacturer's Association (PIMA).

Three standard wall systems were defined that used various thicknesses of wood framing to achieve R-13, R-19 and R-30 walls. Three wall systems achieving the same insulation levels were defined using polyiso sheathing. Because foam sheathing cannot provide racking resistance, let-in 1x6 diagonal braces were specified.

Costs for six U.S. cities were calculated using Means Construction Cost Data, including labor, materials and a 30 percent contractor markup.

The walls with foam sheathing were all less expensive than the standard counterparts. Nationwide average savings ranged from 2 percent to 15 percent. For example, a nominal R-19 wall in Boise, Idaho was $1,024 (10 percent) less expensive with foam sheathing. In Minneapolis, a nominal R-30 wall cost $2,611 (15 percent) less using foam sheathing than the wood-framed option, which used 2x6 studs with 2-in. interior furring strips to create a cavity for fiberglass.

One cost factor was not included in the report. With foam sheathing, many builders feel that additional nailing strips are needed at corners so that siding has a solid backing. Materials and labor costs for these additional nailers were not specifically mentioned.

More information on the study can be obtained from ECM at 410-332-0644.