Louver or flapper style vents are the most commonly sold at local hardware stores.
Check out theHeartland Dryer Vent Closure System.Per the manufacturer literature, "it is designed with an air activated floating shuttle cup, which gives you a positive close when the dryer is not in use.
- It is easy to clean and is far superior to the common "flapper" type that often stick open with lint accumulation or freeze open in the winter."
- I installed one of these on my parents home 17 years ago in Northern Michigan.
- To this day they have not complained of having cold air coming in through the dryer vent.
For more information:
Read "What brand of washer/dryer would you recommend? We want low-maintenance, energy efficient and Energy Star compliant." a Q&A answered by Kristina Hahn.