Construction sequence for mineral wool insulated double walls?
This concept is base on the assumption of a PWF with it’s top flush with the bottom of the sub-floor but may be adapted to other situations. My current plan is 2 ea 2×4 walls, both sheathed on their outer faces. Fiberboard on the outer and OSB on the inner. I I hope to use 3 layers of 3 1/2″ mineral wool bats. I’m thinking that it may be possible to build all 3 layers on the sub floor then erect all at once as an assembly. Here’s how: Key to the process is that the sub-floor has a joint under the inner face of the inner wall and is trimmed to that location before wall construction commences. The inner wall is framed first, then sheathed, taped (the OSB is my primary air barrier) then a strip of sub-floor material the width of the complete assembly before outer wall sheathing is nailed to the bottom plate of the inner wall, the sub-flolor material is taped to the OSB, this strip of sub-floor is the length of the outer wall minus outer sheathing, corner segments, without sheathing, of outer wall are built on the ends of the sub floor if needed. The inter wall insulation is placed then the bottom plate of the outer wall is nailed to the strip of sub-floor, temporary blocks connect the top of the inner wall to the outer wall studs and the outer wall is framed above the inter-wall insulation. the outer wall insulation is placed, last the outer wall is sheathed missing some corner bracing segments which are installed after all the exterior walls have been erected. For a little less thermal bridging and more cost the strip of sub-floor might be replaced by a 10 15/16: wide piece of 1/4″ plywood for the full width and an added strip 4 15/6″ wide layer just under the inner wall plate. The downside is added “joints” in the outer wall probably requiring more studs. FWIW I’m planning on the outer wall being 16″ OC as required for structural fiberboard with the inner wall @ 24″OC with every other inner wall stud aligning with an outer wall stud. Also the outer wall’s top will be higher to make a floored “attic” filled with cellulose under flat bottomed roof trusses.
Does it sound workable? Has anyone tried something similar?
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