We're in the process of turning our large unfinished attic into a bonus room and bathroom. Originally the attic floor contained the insulation that kept the house warm. Now we'll spray foam the underside of the roof and make sure the thermal barrier connects up the the exterior walls. Also, the new attic space has both a heating/cooling air supply and return. So I'm not worried about thermal issues.
I am concerned about noise issues. The bathroom and bonus room (which is where the TV, gaming systems, and rowdy teenagers will be) are directly above some bedrooms (including mine).
Will regular fiberglass insulation provide a good sound barrier? At what thickness? Or is this just a waste of money? Is there a better alternative?
Edit (More detailed info): The finished attic will have a padded, carpeted floor. 3/4" plywood subfloor on 2x8 joists. 1st floor ceilings are 1/2" drywall screwed to 3/4"(?) lathe-and-plaster.