Our 1000 sq foot home has 17 old 12 pane windows, and NO insulation in the walls. We keep meaning to at least have the attic insulation increased, but just haven’t gotten around to doing it. And our exterior is old hard asbestos siding (built in early 1950’s), so we can’t have holes drilled in it to have foam sprayed in. Thank goodness utilities are cheap, here in Oklahoma.
I live in Houston. Can I run an extension cord from your house to mine? Our utility companies love to take from the producers and pay for the usurpers. It makes them feel good about themselves.
I've noticed a lot of old homes in the east coast mid-states region are insulating from the outside, using Tyvek wrap on the existing exterior walls, (with or without an additional layer of R16 insulation, which would require some wood lath framing), and then cladding the exterior with wood, pvc or vinyl siding as the final finish. It may be easier than trying to redo your interior walls, and would probably be much tighter than an insertion from inside.
Of course this requires rebuilding deeper window openings; but you would want to refit your windows with pre-built double-glazed tilt-to-cleans, or at least add storm/screen fittings to supplement your sash windows.