Would this insulation method work?
I notice all the pictures of exterior rigid foam is overhanging the concrete foundation by the thickness of the foam. Is there any reason why it can not be installed flush with the concrete foundation,(the area where the bricks would normally go) and to just have the 1×4 strapping and siding overhang the foundation wall?
This way if you used metal flashing and insect screen it could be caulked right to the concrete foundation.
Also I read many posts you can only have foam insulation on 1 side of the wall .What if you have 4″ of exterior insulation, omit the osb sheathing (brace the walls with other approved methods) then flash & batt the 2×6 wall.
This way would go a long way for air sealing the walls also.
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If you want your rigid foam wall sheathing to be co-planar with the exterior of your concrete foundation, you can certainly install it that way. However, you still need to be sure that there is a good air seal at this transition, and there is more of a risk that you won't have good continuity of your thermal barrier.
Ideally, the wall foam hangs lower than the sill-seal. That way the foam helps reduce air leakage at the bottom of your rim joist, and keeps the top of your concrete warm.
If you do it your way, make sure you do a good job insulation your concrete wall on the interior. You need to include insulation at the top of your concrete wall.
Q. "What if you have 4 inches of exterior insulation, omit the OSB sheathing (brace the walls with other approved methods), and then flash & batt the 2x6 wall?"
A. You could do that. There are no building science reasons why that approach wouldn't work.