The current "best wall we know how to make" is sufficiently different from the existing walls that it basically becomes new construction. Siding or brick, four

The current "best wall we know how to make" is sufficiently different from the existing walls that it basically becomes new construction. Siding or brick, four to six inches of rigid foam overlapping insulation, a fully sealed waterproof, windproof membrane, structural sheathing, 2x4 or 2x6 framing, internal wallboard, latex paint. Consider the existing state of most of those houses: Painted sheathing or siding, 2x4 framing, fiberglass batt insulation with kraft paper vapor barrier inside the framing, wall board, latex paint. Essentially the only piece you get to keep is the 2x4s, assuming you come at it from the inside. If you come at it from the outside, you can do marginally better by keeping the interior walls, but you also have to brace the structure while you replace the exterior sheathing since it's almost always structurally loaded.