Oh, but I'm not arguing that. That I agree on, that, eventually the inside and outside temperature will reach equilibrium. My only point is that, because of the use of insulation and it's low thermal conductivity, you can slow down this process in such a way that it's not economical anymore to turn the heater down at night. (because in the morning it would have to do a lot more effort to reach the same temperature level as it was the day before and thus consuming more energy).

If you're leaving the house for more than a couple of days, then I agree you'd better turn down the thermostat to save energy and thus cost. But not in the course of 24 hrs, thanks to the insulation. I actually tried this last night now, just because I was curious. I switched off the heater when I went to bed, and when I got up 8 hrs later I looked how much the room temperature had dropped: half a degree. Ok, so this is not yet the coolest period of the year, this night it was 5°C outside. But I don't think the dropping of the inside temperature will increase immensely if the outside temperature would start dropping below zero.