When we moved into this house, it was the fall. That first winter, if you stood in the kitchen, your hair would move due to the excessive amount of air coming in thru the backdoor casing, window & dryer vent. (Our laundry room is just off the kitchen.)
We fixed the window & door, but didn't know what to do about the vent. Then, we went to one of our favorite internet sites, searched and found this: replace your dryer vent. I ordered the vent, it was about $25 with shipping & handling.
When it arrived, Jerry took the old vent cover off and found quite a bit of lint for something that was so drafty. I had been keeping the door of our dryer shut and really could have used it as a second fridge it was so darn cold inside.
Anyway, it didn't take him too long to install it. Handyman skills don't come naturally to him but he said it was pretty easy, even with our uneven siding.
It works by having a floating part inside. When the dryer is on, heat forces the floating shuttle up, when the dryer is off, the shuttle is down and vent is sealed from air exchange & pests.
It's even paintable. And, cleaning lint out of it is pretty simple. Just twist off the cap.
Photo source: www.GoodCommonSense.net