A lot of stuff in Boston and other places in the East Coast is actually pre-War and pretty terrible. I grew up in the Upper Midwest (Minnesota), and our house, built around 1980, was very well insulated. Double paned windows, etc. Houses built today to even just code are even better, and anyone going a step beyond code has an insanely efficient house.

My current house in Virginia is over 100 years old, and isn’t terribly well insulated and has single pane windows (although we have storm windows which help… also it’s small and is a duplex which helps).

So the places you visited (on East Coast, big old cities) are not necessarily very representative. Any place with newer housing will have better insulation. (And yes, I realize that a lot of people think newer houses are cheap crap, but overall I think building codes have improved average quality significantly, and the reason you don’t see it in some old houses is because the crap old houses were already torn down, ie sample bias.)

It's survivor bias, and a great point to make when people say old construction is better. Really bad construction simply doesn't survive long enough to be considered old

I’m not here to defend US housing quality but if you’d asked me the worst-built houses prior to your post, I’d have said the Eastern US, specifically New England/Boston.