Then again, though, American houses in cold places are built the same way (mostly) as American houses in hot places, and so if they're insufficiently insulated

Then again, though, American houses in cold places are built the same way (mostly) as American houses in hot places, and so if they're insufficiently insulated to keep cool in when it's 100F outside, they're dramatically more inadequately insulated to keep warm when it is -20F outside. (Few people try to use air conditioning to maintain more than a 35F delta between inside and outside (~80F inside on a 110F+ day), but in cold climates, an 80F delta (~60F inside on a -20F night) or more might be required.