Product review (5 stars): Effective Vent Seal
I purchased this product 20 years ago and it performed well until recently when it was hit by hail and broken.  I have one on a drier vent and the only problem is I have to clean out lint from time to time. I also have one on a bathroom vent and the problem here is that wasps like to build nests in it which prevents it from opening when the exhaust fan is on. So I have to clean it out every year or so.