Yeah I was thinking airtightness might be the difference. My flat seems to be bizarrely hermetic (when you turn on the kitchen extractor fan, it struggles if yo

Yeah I was thinking airtightness might be the difference. My flat seems to be bizarrely hermetic (when you turn on the kitchen extractor fan, it struggles if you don't have a window open somewhere). So maybe a few leaky cracks are enough that when you open a window you get a bit of a through-draft.