The extractor from my cooker hood used to go straight through the wall and had a vent on outside wall. Last year I had a garage extension on that side of house meaning that garage came past where the vent was. My buider removed vent cover, joined a 45 bend to exsisting hose through wall and extended up through flat roof. The duct stands vertical approx 8 inch above flat roof and has a "mushroom" on top. The problem I have is that during winter months, because the mushroom is permanantly open we get a terrible draft coming back down ducting making kitchen very cold. I was considering taking off mushroom and fitting a back draft shutter inside duct then refitting mushroom. Does this sound like the right thing to do or is there another way to cure. I really want to sort before the cold weather returns. From the back of cooker to the end of duct on roof is approx 3 and half foot.
