This 4 in. Draft Blocker helps to prevent cold air from entering the home through the vent. The flap opens when the dryer or fan is operating to allow the air and lint to exhaust. The flap remains closed when the dryer or fan is turned off.Helps to keep the cold air out of the home when the dryer or fan is not in useFits all 4 in. ductsMade of polypropyleneFor use in dryer and bathroom fan venting applications