Product review (5 stars): Keeps out cold air and bugs
My dryer vents to the outside on the north side of my house, so in the winter with strong icy winds, the old flap-style vent cover just let the cold in.  This thing has a floating cap on the inside--when the dryer is on, the air blows the floating cap up to allow the vented air to escape out the underside.  Kind of hard to explain.  I wasn't sure how it worked until I actually bought one.  Installed quickly with six screws (not included).  The top has a cap that you can pop open to reach in and clean out lint once in a while.  No possible way for cold air, bugs, mice, snakes, birds or anything else to get access to your dryer ducts.  So far I love it.