Below grade wall to above grade wall transition – gap?
Climate zone 6a, new construction. I’m struggling to imagine what to do with the transition between below grade wall and above grade wall. I had posted here about this in the conceptual stage but we started framing this week and the conceptual plan has gone out the window.
I have a 2×6 exterior framed cavity with BIBS blown into the cavity. To the exterior of the sheathing I’ll have 2.5″ of foil faced polyiso wrapping the house.
On the below grade walls of the finished walk out basement I have 1.5″ of polyiso wrapped around the interior side of the concrete. However the concrete foundation wall doesn’t go all the way up to the basement ceiling.
The conceptual plan was to have a shelf around the transition on the interior, like I show in the first attachment. This shelf was originally only going to be along the wall shown in the photographs. However due to a screw up in the height of the concrete foundation wall and to achieve the desired 10ft ceiling height we have to frame up by about 18″ around most of the basement perimeter.
Instead of creating the shelf like in the concept the framers want to build the interior 2×4 wall all the way up to the ceiling. Now I have an awkward gap between the 2×4 and 2×6 wall that I don’t know what to do with.
The quick answer would be to fill it with insulation. However I went to some lengths to design the above grade wall to ensure I had a nice 36% ratio R value of outside foam to inside vapor permeable insulation. Filling the cavity will insulation will screw up that ratio.
I’m worried about leaving the gap between the two walls because of air circulation and little thermal eddies being set up in the gap.
Does anybody have any suggestions?
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