A.Fernando Pagés Ruiz, a
contractor in Lincoln, Neb.,and the author of Building an
Affordable House,responds: According to Appendix K of
the IRC (which, of course, might not be locally adopted),
dwelling-unit separation walls must have a minimum
sound-transmission class rating of 45 (at which loud speech can
be barely heard through the assembly). In your case, the
double-wall construction you describe should meet this
requirement, but I suspect that construction defects are still
allowing considerable noise to seep through. So the first thing
you need to do is find the sound leaks, which are reducing the
wall's effectiveness.
Start by getting down on your hands and knees and listening.
You may discover that most of the sound is filtering in at
baseboard level. Drywall hangers often leave a gap between the
bottom sheet of drywall and the floor, which allows air and
noise infiltration. After removing the baseboards, fill any
large gaps with drywall and tape the seams, allowing the tape
to mask the joint between the floor and the wall (see
illustrations). Smaller gaps should be sealed with an
acoustical caulk.
Next, check the sidewalls for "flanking paths" that allow sound
to come through the corners. Improperly built corners transmit
noise, and the solution involves removing drywall and adding
framing and blocking as necessary so that the wall goes through
the adjacent corners to the exterior sheathing. Check the attic
or ceiling, too. Sound travels around these joints when they're
built improperly.
If all of these steps fail to reduce sound levels adequately,
you can buy resilient channel at the drywall supply shop and
apply it horizontally to your wall at 16- or 24-inch centers.
Then hang 5/8-inch drywall vertically over the channels, making
sure your screws do not go all the way through the channel into
the fram-ing, since this would defeat the spring action
provided by the channel. Also, be sure to tape the new drywall
to the ceilings, adjacent walls, and floor, which will help
seal the leaks that permit sound transmission. Of course, it
would be even better if you could get the neighbor to take the
same steps.