Well I just stumbled across "Rstud". These are a sandwich of polyIso and LVL that performs structurally as a 2x4 & 2x6 but the 2x4 is r 11 and the 2x6 is r21. My plan is to use the 2x6 for the outer, non structural wall with 16" OC, so I can use fiberboard sheathing, and an inner wall out of ordinary 2x4 but 24" OC to align with my roof and floor. With the inner wall as the load bearing one and it is sheathed on it's outer face with 1/2" plywood as the primary air barrier and structural bracing. The outer wall will be sheathed before erection and cavities filled with Roxul before the inner wall is erected. The inner wall will be sheathed before erection and 1" of Polyso installed before erection. The inner wall's sheathing is the primary air barrier and will be sealed accordingly. The inner wall's cavities will be filled with more Roxul then drywall. My exterior is "thin brick" on a metal substrate over DC14 (a really neat product that addresses the solar driven moisture concerns of adhered masonry). From the outside I'll have brick, metal mounting plates, DC14 (r1) 3/4" fiberboard (r2), 2x6 wall with r21 studs and r23 cavities for about r22, 1" Polyiso (r6), 1/2" plywood (r0.6), 2x4 24" OC wall with normal studs and r15 cavities with thermal bridges in headers and such certainly better than the r8.4 needed to get to r 40 overall. The r studs are not all that cheap at $10.65 for 12 footers but their cost is made up for by the need for much less cavity insulation. In effect using the Rstuds replaces about 2" of Roxul between the walls. I'm resorting to poly iso in the middle because it's easy to add to the inner wall before erection, will serve as an added air & vapor barrier and replaces 1 1/2" of Roxul for very little added cost. This r40 wall is almost exactly as thick as the walls in the house I live in now (2x6 16" OC with fiberglass bats and conventional brick veneer) that are probably under r 15 actually.
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