One thing to know about the acoustic materials is that you need massive amounts of thickness if you want to absorb a lot of the lower bass frequencies. Yes, putting a space behind the fiberglass will help because the sound will bounce off the wall and back into the fiberglass. However, if you want to absorb the lower frequencies, you’ll need a very thick trap (like the 6-7" bass traps from GIK). If you want to put multiple layers of the fiberglass in, you need to keep them at the lower densities (like 2.5 - 3 lbs).
I chose the 2" thick 8 lb Roxul because I wanted to limit how much mid/high frequencies it would absorb, but I still wanted decent bass absorption. At a certain point, the denser material will start reflecting sound at a thickness level. A 4" stack of the 8lb Roxul will probably not work any better than a 2" stack. However, putting a 4-8" stack of 2.5lb Safe-n-sound will absorb a lot more bass (as well as a lot more mids/highs).