We have a gas fired clothes dryer that is vented to the outside. The venting is aluminum ducting. The vent begins in the wall behind the dryer, goes down into an unheated basement, across the ceiling to the outside wall and vents thru a flappered vent cap. The flapper works fine, the venting is not clogged. On cold days when the dryer door is opened it is like a refrigerator door opening. I have no doubt that the cold air is coming from the outside and the basement. Would indulsting the vent duct in the basment and adding another flapper at the start of the vent duct behind the dryer be of any help, or just a waste of time? Suggestions appreciated.