Tim writes from Flagstaff, Arizona: "I'm adding a home theatre in the basement, and the bedrooms are directly above. I need to maximize soundproofing, and would like to reach the recommended STC of 50. The ceiling is being framed for drywall (used to be dropdown panels), and I have the opportunity to improve the insulation which is currently R-19. There arrears to be enough room for a equivalent size of R-40 insulation. Though thermal efficiency isn't a concern, soundproofing is. From what I can tell, one layer of drywall on an uninsulated wall gives an STC of 33. QuietZone specs seem to say the batting increases the STC about 6. Would packing multiple layers increase this, ie, could I say packing in three layers may increase the soundproofing 6x3, or an STC of 18? If this theory works, then I could potentially reach the desired STC of 50 by packing in lots of QuietZone insulation. Does this sound feasible? The other option is to use Quietrock drywall, probably in conjunction with QuietZone insulation, but this option appears substantially more expensive. Please advise. I want to do this remodel right while I have the chance."

Answer: While we would love to sell you enough insulation to fill the cavity, it would not be the advisable method to reduce the sound from floor to floor. Insulation provides its greatest sound reduction in the first inch of thickness. It will continue to reduce more sound as it gets thicker but the reduction per inch diminishes as the insulation gets thicker. This means the difference between 6 inches of insulation and 12 inches is minimal (not double). We typically recommend 3-1/2 to around 6 inches for floor/ceiling assemblies.

Adding mass (more drywall) and absorption (more insulation) in reasonable amounts will not get you to an STC of 50. To achieve that number you need to separate the structural sound by using something like resilient channels. Or if you are changing the floor above you could look at Owens Corning QuietZone™ Acoustic Floormat.