I’ve renovated professional sound studios, and done a fair number of condos where the party wall needs lots of attention to keep neighbors neighborly. But even in a single-family detached home, there are some relatively easy ways to control sound that will make life much more pleasant for the occupants. In media rooms, home offices, master bedrooms, meditation rooms, home theaters – on almost every home I build or remodel my clients ask me to soundproof at least one room.
In every assembly where you trying to limit sound transfer, you trying to do two things: limit vibration and limit air movement. Sound moves as a wave through air. When it hits a wall it will vibrate the wall materials. Sound waves will also move unimpeded through any cracks and gaps. So the two basic approaches to stopping sound are 1) to isolate materials so vibrations can’t transfer from one to the other, and 2) seal up air gaps to limit air movement.
Here are the usual methods I employ on most builds, for example, in between the master bedroom and an adjacent bedroom, or between a child’s room and the master bath.
This is a pretty common method of building a quiet wall. We use a 2×6 top and bottom plate, then fill-in the studs with 2x4s on a 16-in. o.c. layout, offsetting the layout by 8 inches. You essentially get most of the benefits of two walls, but it is a lot easier to build. When sound hits one side of this wall and starts vibrating the 2x4s, that vibration does not transfer to the other side of the drywall. Sound can only transfer at the plates, which is not very significant, compared to the entire surface area of the wall.
Sealing electrical boxes. With code requirements for outlets (typically required every 6 feet) it’s hard to have a bedroom wall without an outlet. But electrical boxes have a lot of holes in the them for all the wires to poke through. You also end up with a hole in the drywall around the outlet. Which has to be sealed to prevent sound from freely passing through them.
For sealing outlet boxes, we use putty pads. These are made for fire stopping in commercial applications, but they work well for soundproofing electrical outlets. I like the thick red pads from Hilti (CP3617) the best. We get them at Commercial Supply Houses or the Hilti Store; they are much better than the thin ones sold at a big box store. The Hilti pads are 6- x 7-inches; we center them over the back of the boxes, and fold the edges over the sides of the box. The material has a consistency like Silly Putty, and effectively shut down the air flowing through all those holes that might otherwise carry sound.
Once the drywall has been installed, you also have come back and use an acoustical sealant to seal between the drywall and the box to complete the installation. We have had good luck with the Noiseproofing Sealant in St. Gobain’s Green Glue line or Quiet Seal Pro, which is part of the Quiet Rock line. Acoustical sealant stays flexible; it won’t set up and get hard. This flexible seal not only stop airflow, but it also isolates the electrical box from sound vibration coming through the drywall.
Soundproofing batts. Before the wall is enclosed, we insulate the wall cavity. Fiberglass batts will work; wet-spray cellulose works better, but it’s not that common. We’ve had the best results with ROCKWOOL soundproofing batts, which are denser and specifically designed to absorb sound.
Those three steps will do a lot to limit sound transfer through a wall assembly, and can be done with minimal investment.
For that condo party wall, we add one more measure: a double layer of drywall.
The most cost effective way to add this layer is to install the second layer with Green Glue, a compound you squeeze out of a caulk gun on to the backside of the drywall in a zigzag pattern. This material stays flexible over time and helps dissipates sound energy from one sheet of drywall to the next. But with Green Glue, you have to get the details right. You need to use two full tubes per 4×8 sheet of drywall. You can’t skimp on the amount. You also need to do a careful job of sealing the edges of the first layer of drywall with an acoustical sealant. When applying the acoustical sealant, apply lots of pressure as you squeeze it out, so the sealant gets pushed into the crack between adjacent sheets, or between the first sheet and the subfloor. Here again, don’t skimp on the material.
I have also used Quiet Rock effectively. This system essentially uses double sheets that have been pre-bonded together, so you cut down on the installation time. Each sheet gets installed with acoustical sealant around the perimeter, so it’s not as fast as installing one layer of conventional drywall, but it’s a little faster than bonding two layers with Green Glue.
Noisy drains. This last tip is a little different, but it relates to a very common complaint that I hear on my initial meeting with clients on remodels. They can hear upstairs toilets being flushed when they are sitting downstairs, usually in a dining room or family room. This noise is caused by water flowing through a PVC drain running through the downstairs walls. It’s very hard to insulate PVC so you won’t hear water running through it; the only practical way to fix this is to simply replace the drain with cast-iron pipe.
Cast-iron drains can be retrofit easily by cutting out the old PVC stack at the top and bottom plate in the wall, and using a banded rubber coupling (something like Fasco’s ProSeal coupling) to secure the a new cast-iron stack. It’s a relatively affordable fix. Often just replacing the section of drain running through the downstairs wall is enough to fix the problem, depending on how the drain is configured upstairs. You may also have to replace the horizontal sections of drain running overhead in the ceiling.
By: Matt Risinger