jess, with all due respect, you are the one who said you were banging your head on a wall about your MIL's bathroom window. I was trying, not too successfully it seems, to subtley suggest that you leave her alone! Preaching? Ha! I actually find quite a lot of posts by you rather preachy but I wasn't complaining, just arguing from a different point of view.
With regard to windows open and heating on, again it depends on the house and on personal preferences, as always. Our house is much cooler than nearly every other house I've ever been into, not because the heat goes out of the windows but because we don't overheat it. As it happens, I have a max/min thermometer near the bed which doubles as my alarm clock. Even in winter the variation in room temperature from maximum to minimum is rarely more than (and usually less than) 2°C so I reckon our insulation (thick walls; it's an old house) must be reasonable. In the middle of winter it's usually between 11 and 13°C, not (to reiterate) because we lose the heat but because the heating is rarely on. We only use it to take the chill off the room, not to raise the temperature to sitting-room warmth. At the moment it's a good deal cooler inside my house than it is outside.
I'm not saying that anyone else should do what we do. That would be preaching. I'm just describing what we do and why. I'm not disputing facts, although I may sometimes put a different interpretation on them than someone else might. For instance, yes deaths from a number of causes do increase over the winter months but flu and colds and other winter-associated illnesses (including complicatiions arising from colds and flu) also increase in winter. I suspect that this has always been the case and always will be. It's the nature of winter. Which comes first, chicken or egg?
I was going to finish with some comment about the benefits of global warming for a fuel-poverty stricken country , but I'll refrain.