Should I Insulate Between Floors to Reduce Noise?

Question

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.

Answer 1

I put Roxul Safe n Sound in our entire basement ceiling when I finished the basement, and it made an enormous difference. So much so that it's hard to remember what it used to be like. I would absolutely recommend it.

Answer 2

Yes, insulation will reduce noise, but you might also want to also consider how the ceiling is going to be attached.

Assuming drywall, I would use a second set of ceiling joist that are suspended in between the floor joist above and held up by the walls (you don't need much more than 2x4 or even 2x3 since you are only holding up the sheetrock).

Wood is a very excellent conductor of sound. If you attach the ceiling sheetrock directly to the floor joist, then sound while be much more readily transmitted through the floor above, through the joist and then through the ceiling, all regardless of insulation. This is why car mechanics will often use a piece of wood to listen at various points on a running engine block to isolate the location of a bad connecting rod, for example.

When I finished my basement, I used offset ceiling joist held up by the walls and attached to the floor joist above where necessary (i.e. where there were large expanses of ceiling) using rubber in between the joist and the attaching wood. The noise reduction was astonishing.