Exposed RO in 2x4 gable wall; convert to equiv. 2x6 wall.
Vinyl siding, insulation and original 50 year old wood siding removed, down to the sheathing. It's in good shape. Saving the removed 25 year old Alcoa vinyl siding for a proposed add-on project. (Aside. Alcoa is the toughest siding I ever dis-assembled. No currently available unZip tool can be fitted to unhook the panels from each other unless you first sacrifice a panel to get at its nailing. Designed not to separate, even in the toughest winds.) House is a cape, with attached garage.
I'm in North country and would like to take this opportunity to insulate my bedroom wall to current 2x6 standards. Since there's a 2" difference in wall thickness, I assume I buy a window for 2x6 application, increase the 2x4 RO depth by 2" (rip a 2x4 to 1 1/2" x2" actual, and install 1 7/8" thick closed cell foam, with its own membrane (skin) on both sides.
If I use 3" long Aluminum siding nails I shouldn't need to furr, since the foam insulation is light. Vinyl siding is the final layer.
I'm puzzled about adding TyVek as a water barrier. Do I install, as is normally done, directly to the sheathing or tape to the insulation's outer skin.?
Am I on the right track? Please advise and/or comment.