I'm gutting the interior and redoing the siding of my old house. Unfortunately the exterior walls are only 2x4 which makes it challenging to reach R20 code let alone go above it to make a greener more energy efficient home.
So if you fill the 2x4 cavity with rock wool, then mount 4 inches of rigid foam to the sheathing, would that surpass R20? What would others do in my shoes? Double studding and losing square footage could be a possibility but would seem a lot more expensive.
Also where does the house wrap and vapor barrier go?
Climate Zone 5 (cold winters/hot summers)