I've yet to see an intermittent extractor with a damper to close off the exhaust vent when not in use. Being intermittent, i.e. on a light switched timer, there's a clear 100mm path to the outside world for the vast majority of the time. Being surrounded by centrally heated rooms on all sides, our downstairs loo hasn't got a single outside wall yet it's always the second coldest room in the house. The extractor duct is run up through a boxed-in section in the adjacent utility room and exits near