Originally Posted by GSXRJJordan

That fiberglass stuff doesn't harden strong enough to stop vibration, and isn't as effective as dynaliner/Luxury Liner/any mass-loaded closed-cell-foam barriers at stopping sound. From an audiophile standpoint, unlike a room, cars generally don't have enough volume to make stopping ambient noise feasible... once the noise is in, it's most likely going to stay. You can fill that space with anything you want, but the people I've talked to about the same exact problem say don't bother filling it with anything - just mass-load the panels with Dynamat/Damplifier/etc and put a barrier over it.