Ultra Seal Dryer Vent

I recently cleaned our dryer vent. The screen to keep pests out was completely clogged. And my wife wondered why the dryer was taking so long to dry. I saw in a book, and on the web, a type of dryer vent sometimes called Ultra Seal Dryer Vent that curves upward, and has a little ball or plug that rises as air is blown out the vent. But when the air stops, the ball or plug drops to plug the vent and keep the cold air out. I decided to go ahead and order one. I think it will help keep cold air out of our house. It all adds up.