Guys - Looking for a little advice here. In our new to us home we have a professional gas range with a large hood. The hood is plumbed with 7-inch ductwork up into and unheated space and vented outside. On the roof there is a vent cap with a thin metal flapper. During last winter we could feel a lot of cold air coming back in the hood, back draft, when it was not in use.

So far I canít find a good, energy efficient solution to this problem. I have read a lot about inline back draft preventers, but they really are just a thin sheet metal flapper. Will that keep the cold air out? Or is there a better solution. We all spend so much time in trying to keep the cold out and the warm in it seems funny to have a 7-inch opening in the house.