I bought one when I thought I had a cowl leak, but it turned out to be the windshield leaking. I put it on anyway to keep out the leaves from the trees and any water if I got caught in the rain.

When I had the windshield sealed today, the glass guy asked why I had a cowl cover, then told me it was "bad" to have one as it would not allow air flow, so I'd end up with "rot and rust" in there. Another guy told him he was wrong, that they're fine ...

What's the truth, guys?

 

if your ventys are closed, it has no effect, if your cowl vents are open, it can help keep the interrior cooler, so yes, both are correct and wrong