Whenever the level of air in a home is pulled out, more will be pulled in -- it will come through a crack, an open window, or even your kitchen vent. This natural correction of air pressure is called a backdraught, and it can lower the efficiency of your heating and cooling system. It can be dangerous, too, if the backdrafted air contains carbon monoxide. To prevent a backdraught, you'll need to install a backdraught damper.
Remove your kitchen exhaust fan and measure the size of your vent. You will need this measurement when you order your backdraught dampener.
Shop for a backdraught dampener. These devices are traditionally made out of metal, but they can be noisy as they open and close, and are usually reserved for larger commercial hoods. Many people are now switching to a simple fireproof cloth dampener that fits into the vent much like a tapered sleeve. It is laid smooth against the sides of the vent when normal airflow is present, but when a backdraught occurs it is pulled down and folds in on itself, closing the vent.
Install your backdraught dampener by sliding it up into the vent at least 3 feet, if possible. If you cannot manage to do so, try removing a section of vent farther up. Remove the vent and place the dampener inside with the cloth smoothed upward.
Test your dampener by using a hair dryer to blow downward from above the removed vent. It should fold over on itself. It resembles a ventricle closing and should be completely noiseless.
Replace your vent, making sure that the cloth is still facing up properly.