Backdraft dampers allow air to efficiently flow through exhaust ducts to the outside, yet prevent the unwanted flow of air into a house when the exhaust fans are off. 
They stop cold air from coming in, allowing a more comfortable temperature in your home. These dampers are most often used in bathroom exhaust systems, clothes dryer vents, kitchen range hoods and microwaves.
Fantech sensitive spring-loaded backdraft dampers provide a good seal against backdrafts. They are easy to install and work without noise or vibration.
The Cape Damper from Tamarack is made of a metal duct with a fabric sleeve attached. It is a one way air flow valve, designed to overcome the inefficiencies and limitations of traditional gravity or butterfly dampers. It is quiet and can simply be installed in either a vertical or horizontal position.
Either option will help prevent outside air from leaking into your home and allow you to save on energy costs.