I find that if you squeeze one of these check-valve kinds properly, the airflow is just as good as with a free-flow one. And I don't have to put my tongue on a gross piece of plastic that's been in the attic/garage/basement for months.
> if you squeeze one of these check-valve kinds properly, the airflow is just as good as with a free-flow one
That hasn't been my experience, and if you look at one closely you can see that it can't be the case. The size of the opening is substantially smaller even at maximum expansion, and the path for the air is less direct.
