We have a kitchen hood and bath exaust fan the both use 6" ducting.Draft blocker has a 4" louvered insert that keeps out cold as well as heat from the outside that is more (claimed) reliable than the outside vent caps that seem to get rendered useless in time.My question is can i reduce down to the draft blocker each side without affecting the performance of the hood or fan?

Is there another way of dealing with incoming cold or heat such as an insulation?

Thanks,Mark