When insulating above and / or around light fixtures, make sure the fixture is rated for insulation. That’s a good point that was brought up earlier. The fixture has light gauge sheet metal shielding that keeps the insulation away from ‘hot spots’.
There is bat fiberglass insulation that is specifically designed for sound reduction. The difference between it and normal bat insulation is fiber density. When insulating for a thermal conductivity barrier, insulation needs to have “loft” where pockets of dead air space is captured between the fibers. A common mistake with installing insulation is to cram it in. You reduce its insulating properties when it is not allowed to expend to its rated dimensions.
There is also sound board that is installed on the joists after you install the bats between the joists. It’s downright obnoxious to install, especially overhead. It breaks apart and you are sure to get the fibers everywhere. It works very well though.
