From the Wikipedia article:

Sound Transmission Class (or STC) is an integer rating of how well a building partition attenuates airborne sound.

Materials which can improve STCs in walls include mass-loaded vinyl (MLV), standard drywall, “soundproof” drywall (such as QuietRock, Supress, SoundBreak, or ComfortGuard) and damping compounds such as Green Glue.

When researching solutions to soundproof a “paper-thin” wall:

  1. Blow-in insulation / cellulose. Holes are drilled every 6-12 inches in-between each stud on the wall. A hose is inserted into the wall and the material is pumped in. This will not work if there is any existing insulating material in the wall – even if it’s 20+ year old broken down fiberglass.
  2. Additional drywall layer. Simple, just slap up another layer of 5/8″ thick drywall. More material = more soundproofing.
  3. Soundproof drywall. Use acoustically engineered drywall sheets instead of standard drywall. Even more soundproofing at roughly 3x the material cost.
  4. Dampening compound. Add a dampening compound between two layers of drywall to increase the soundproofing.

Depending on who you talk to you will receive a different recommendation. We unfortunately found out about the cellulose limitations during an aborted installation. This led to an evaluation of removing the drywall to change the insulation, or figuring out a plan “C”.

So how do you figure out what to do? First figure out the STC rating that you want to achieve. In my case I’m looking for a rating of at least 50 and anything approaching 60 would be ideal. I found a Wall Assembly Performance guide on GreenGlue’s website with some fantastic comparisons of installations. Additionally on the QuietRock site they list an STC Rating of 53 on their highest performing product.

  • 1 layer of drywall each side: 40
  • 2 layers of drywall + 1 layer of drywall: 42 <– Adding one extra drywall layer to one of the walls
  • 2 layers of drywall on both sides + 1 layer of Green Glue per side: 55 <– Can’t install since there is no access to one of the sides.
  • 1 layer of QuietRock one side: 53 <– This assumes that the original drywall was removed

Unfortunately STC ratings are not additive and increase slowly with each additional layer. In our case we’ve decided to add a layer of GreenGlue and a layer of QuietRock. I’m hoping this achieves an effective rating of 55+.

TLDR;

Install a layer of QuietRock drywall with a layer of GreenGlue to the existing wall.