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.