Its not all about insulation though. A terrace has much less exterior wall than a row of spread out detached homes. Some still insulate the party wall. but more for sound reasons. But the main advantage of terraces , fossil fuel-wise, is that the medium-density is more likely to give you a walkable grocery + other stuff and a somewhat useful bus service or other public transport.

Though modern suburbs here can be pretty sparse too with more detached homes.