My current apartment in Valenciennes – Northern France – is located in a recent building (2000). And far from me the idea of bragging, but until temperatures dropped below zero (Celcius) I almost didn’t not use my electric heaters.
My flat has double glazing windows and quite well insulated thick walls. The thermometer located in my flat indicates 19°C indoors while we are nearing zero outdoors, and this with little to no heating.
This is empirical proof that good thermal insulation – or weatherproofing – not only works but also saves a LOT of money as you just use little to no heating.