Which exterior wall assembly meets the minimum required R-values for a nonresidential building with insulation above the roof deck and steel-framed walls above grade?

• A. Standing seam metal roof with 2 inches of polyiso insulation, 1 inch of polystyrene rigid insulation in the brick veneer cavity, and 6 inches of fiberglass in the metal studs.
• B. Standing seam metal roof with 4 inches of polyiso insulation, 1.5 inches of polystyrene rigid insulation in the brick veneer cavity, and 4.5 inches of rock-wool blankets in the metal studs.
• C. Standing seam metal roof with 3 inches of polyiso insulation, 2 inches of polystyrene rigid insulation in the brick veneer cavity, and 4 inches of fiberglass in the studs.
• D. Standing seam metal roof with 5 inches of polyiso insulation, 2 inches of polystyrene rigid insulation in the brick veneer cavity, and 3 inches of fiberglass in the studs.

Answer: (B)

The key idea is to maximize the total R-value of the entire exterior-wall assembly by accounting for all layers and the effect of thermal bridging through steel studs. When a wall uses steel framing above grade, bridging can significantly reduce the effective insulation, so the design must emphasize higher insulation values in the studs and in the cavity, plus substantial continuous insulation in the wall assembly.

The best option combines substantial roof insulation (which contributes to the overall envelope performance), a modest yet effective cavity insulation in the brick veneer, and a thick, high-R-stud insulation. Using rock-wool blankets in the metal studs provides a higher R-value per inch than fiberglass, helping offset the steel framing’s bridging effect. With four inches of polyiso on the roof (high continuous insulation), 1.5 inches of polystyrene in the brick veneer cavity, and 4.5 inches of rock-wool in the metal studs, the total R-value is the largest among the choices and meets the minimum required for a nonresidential building with insulation above the roof deck and steel-framed walls above grade.

The other configurations fall short because they reduce the stud insulation or rely on thinner cavity/roof insulation, resulting in a lower overall R-value that does not meet the minimum.