Thermal breaks for window bucks in double stud walls
We’ll be buliding double stud walls for a new house, with the inner wall being structural (the outer wall will consist of modified Larsen trusses). Total wall thickness will be around 16″ and windows will be “in-betweenies”. This will be my first double-stud wall house and I’m working in the wall details trying to minimze thermal bridging.
All the penetration details for window bucks on double stud walls I’m seeing on GBA and elsewhere are built with plywood (typically 3/4″) joining both walls. This creates a thin thermal bridge that adds-up. (For example, for a large 6’x6′ window, this corresponds to 1.5 square feet thermal bridge, which I consider significant.)
Has anyone used a plywood buck that would extend from the inner wall just far enough to support the window, and then finish the full depth of the buck with foam? Structurally (which may be a concern for larger windows), I see this as equivalent to bucks that extend outwards for walls that would have a few inches of foam on the outside and “outies” windows.
Is there any flaw with my system that I’m overlooking?
We’re owner-builders, doing the work almost exclusively ourselves, so are not worried about extra labor and we’ll be attentive to details. We’ll be built in a small valley and as such will not be subjected to strong winds.
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