I am having my Central Air installed in my house. I was at work today and I mentioned to this guy who is a tin knocker sheet metal guy that my Installer is using 2" duct board for the feed in the attic and run 7" flex off at the trunk, the guy at work say that ductboard is not a wise choice for a trunk, he says over the years there always be fine fiberglass particles always coming from the ceiling vents and is illegal in some states. any truth to this
Chet