Kitchen Large Electric Appliances - Electric Dryer Vent

Ok, I need help from all you smart people! We have our washer/dryer upstairs in our new house. We added this feature - wasn't in the original plan. The vent runs out the back of the dryer and down to the soffit. We haven't had any problems, but now that it's getting cooler we notice the cool air coming into the dryer. The flap on the vent doesn't work because it's hanging upside down. So what can we do keep this closed (or have it covered) when the dryer isn't running? We had the vent go thru the roof in our old house and that had its own set of problems - and we couldn't do that here since it would be on the front of the house. Help!!!

You'll always have some cool air coming into the dryer through the vent. If the vent runs vertically rather than horizontally, which is what I understand you are saying, then the flap will always remain open - you cannot defy gravity! It may be possible to run the vent cap to the fascia of the overhang using some flexible duct.