I know in my country (NL) modern houses are built to an energy efficiency standard, so that 15 odd year old houses like mine are pretty efficient and well-insulated; how is that in the US?

Over here if you have an older house you can get subsidies or attractive mortgage rates if you spend it on improving your house's energy efficiency, it's worthwhile looking for schemes like that.

Actually I should look into that myself; I don't need to improve insulation, but switching to electric heating and adding solar panels and a heat pump would be interesting, especially given we need air conditioning during the hottest periods nowadays.