[constGard]

Insulation is a no brainer until you pump it into an older house with poorly sealed walls and end up with a sodden mess next time it rains. Some of the best advice from "green" building professionals is focus on air sealing, efficient electric heat (heat pumps), and eventually rooftop solar. These have complications as well (slave labor solar panels, high gwp refrigerants) but those are often easier to fix than your 1930s wall assembly. The individual cost of a deep energy retrofit to address the systemic problems with a home can be incredibly high (100k+ for people like me in California), so hopefully market innovation and government regulation will help solve some of the issues I mentioned with electrification + rooftop pv.

[ominous_prime]

Yup, old houses can be insulated, but they do not insulate well. Currently in an 1890s house which has been superficially renovated at least 2x in its life, with all the insulation you could possibly pump into the walls. Winter heating is still grossly expensive, and you can never stop all the drafts. During a kitchen reno we tried pulling everything back to the studs and using spray foam as the insulation and vapor barrier -- better than the rest of the house, but still noticeably draftier than a modern house.

There's no real way to fix that without completely stripping the house down to the frame and rebuilding with modern techniques and materials.