Building envelope question
art_kiwacz
| Posted in General Questions on
Would you pay extra to make your foundations 12″ thick instead of standard 10″ in order to install an additional layer or rockwool over the WRB? My envolope is as follows brick, rainscreen, (possible) rockwool comfortboard 80, wrb (tyvek) plywood sheathing taped at seams with siga tape, 2×6 with rockwool, drywall. I’m in Zone 5 (chicago) and even though I know rockwool is vapor permeable, wouldn’t that add another “barrier” to vapor being slowed to dry at the tyvek point? Been in a house where mold grew inside exterior walls due to vapor being stuck at vapor impermeable sheathing and I’m super cautious about that. Thank you
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Replies
Art,
In CZ5, you need a minimum of R7.5 on the exterior (rigid foam or rockwool). See here for details: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing. So about 2 inches of insulation.
But I'm not understanding why your foundation needs to be thicker. Why is that?
Thank you Steve.
With brick exterior i could do a standard 10" foundation (brick 3.5", 1"space, 1\2" sheathing, 2x6 (5.5"). With a bit of brick overhang it fits tight but fine. So now to add rockwool i would need to go to 12" foundation. Is it worth it?
Walls generally sit on the floor assembly in most construction. This means you can set the walls where you want (within reason) and doesn't need to line up with the foundation bellow, the bottom plate can overhang towards the interior.
You do need to provide the sufficient amount of bearing for your floor joists but there should still be plenty of room for a bit of rigid without making the foundation thicker.