sound attenuation batts

I would guess one is rated in thermal terminology Rx and the other in sound transmission coefficient terminology STCx. Just a guess. Same stuff . No vapor barrier needed for STC.

Also the STC will be proportional to the weight of the fiberglass so panels like Corning 704 being much denser will have better sound ABSORPTION characteristics at low frequencies.

"Deputy Dumbya Dawg" wrote in message news:noxJh.126902$ snipped-for-privacy@newsread2.news.pas.earthlink.net...

Not much really. Batt thickness, and the binder (it's a little more dense). You can get close to the same performance acoustically from the thermal, if that's where you're going with this.