Air barrier in middle of double wall in mixed climate?
I’m in central Arizona, at the cold end of the Climate Zone 2B range, where I have around four months of moderate heating and five of moderate to heavy cooling. In the midst of the complexity of planning for building a 20×48 single-story house entirely by myself, simplicity tempted me briefly away from my plan to use a double wall, but after thinking in more detail about insulating around all of the wires and electrical boxes in a single exterior wall and creating airtight drywall, I’ve concluded that, in addition to the obvious benefit of greater energy efficiency, a double wall will be much easier to build–unless my plan to put an air barrier in the middle of the wall has some major flaw I’m not seeing.
From the exterior in, here’s my wall:
SmartSide panel siding/sheathing
2×4 exterior, load-bearing wall insulated with Roxul batts
Vapor-permeable air barrier
Plywood gussets to keep the 1×3 studs of the interior wall straight and 1-1/2″ wide strips of 1″ polyiso to fill the 1″ gaps between the parallel 2×4 and 2×3 studs
2×3 interior, non-load-bearing wall insulated with 3-1/2″ Roxul batts (extending into the 1″ gap)
Vapor-permeable latex paint
My wires will all run through the gap between the two parallel walls, and I can fit standard electrical boxes in the interior wall plus gap.
I understand that dense-packed cellulose would have made insulating around wires and boxes much easier, but it’s not compatible with SmartSide, I can’t do it myself, and it’s more expensive.
Putting the air barrier on the inner plane of the exterior wall would seem to offer all of the advantages of simplifying and protecting it that Martin notes in his “Service Cavities for Wiring and Plumbing,” https://www.greenbuildingadvisor.com/blogs/dept/musings/service-cavities-wiring-and-plumbing?page=1, the double wall would leave the insulation in the exterior wall almost entirely uninterrupted, and the inner wall would be well insulated, albeit less perfectly. The nominal R-value of the wall with two layers of R-15 Roxul would be R-30, ample for this climate, and thermal bridging would be minimized by the gap. Martin’s article and the discussion that follows are oriented mostly toward a cold climate. Is there anything about a mixed climate that would make my plan unsuitable? If not, what would be a good, economical choice for the air barrier in the middle of the wall?
Also, if I use 3/4″ plywood for the gussets, what’s the narrowest the strips of gusset could be?
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