Mick explains why the DraftStoppa is a good idea for your ceiling exhaust fans. It let's the exhaust out, while not letting the cold air get back in. It also doesn't let the warm air out of your house, because the counterbalanced flaps need the push of an exhaust fan to be open.
Check out the DraftStoppa here, and if you have a small ceiling fan or an all-in-one fan then add a DraftStoppa Mounting Plate as well.