We had an insulation upgrade recently when we ripped out our gas furnace and put in a heat pump. The biggest improvement was from spraying foam into the space

We had an insulation upgrade recently when we ripped out our gas furnace and put in a heat pump. The biggest improvement was from spraying foam into the space below the first floor, where it rests on the outer basement walls. There had been too much air leakage there. There was also attic insulation upgrading. No walls had to be penetrated. The house (built just a decade ago) feels much better insulated now.