You need a flap on the outside to stop the air getting in when not in use is this the piece you are talking about?

Depending on the configuration, there is merit to having a damper at the roof or other exterior point, and one at the exit of the hood.

Yes, thats the piece I'm talking about. There is a flap there but its still pretty drafty on a breezy cold day.

So the options are, stronger spring, two dampers, or electric motorized damper tied to the hood fan switch.