I've gone through the exercise of retrofitting insulation. Unfortunately there are a few things that are extremely hard to get past. 2x4 walls can only be insulated to R-13 or so unless you're replacing the siding. Subslab insulation can pretty much only be done when it's poured. Joist insulation & air sealing can be very hard to get to.
I can only hope 2x6 framing becomes more common, the subslab insulation has started to become code.
I can only hope 2x6 framing becomes more common, the subslab insulation has started to become code.