Dave asks: We are remodeling a bathroom that connects to our bedroom and shares a wall with another bedroom. What steps can we take to soundproof the bathroom walls and door? It’s rather loud in both bedrooms when the shower is running.
Steve Bliss, of BuildingAdvisor.com, responds: While it’s not possible to make walls 100% “soundproof,” it’s pretty easy to get significant reductions in sound transmission through walls. The four main strategies are listed below, starting with the cheapest. You will get the most for your money by starting with the less expensive options. You can usually achieve the soundproofing level you are looking for by combining a few low-cost techniques.
1) Seal all gaps and joints. Anything that will leak air will also leak sound.
2) Insulate the walls for sound absorption in the cavity.
3) Add mass to the walls.
4) Decouple the wall surfaces.
Wall assemblies are rated by sound-transmission class (STC). The STC rating is the approximate reduction in noise level through the wall. So, for example, a 50 dB noise would sound like a 15 dB noise on the other side of an STC 35 wall system. A typical 2×4 wall has an STC rating of about 35. An STC rating of 40 is pretty good – loud speech will be audible through the wall, but you won’t understand what they are saying. At 50, loud speech is barely audible. Above 55 you won’t hear speech at all. View STC Ratings of Typical Wall Assemblies.
If the walls are opened up for remodeling, it’s easy to caulk and seal (or gasket) the same places you would to block air leakage on an outside wall. In addition to top and bottom plates, seal around electrical outlets, plumbing penetrations and any other openings in the wall.
Use a long-lasting flexible sealant such as silicon or urethane. Traditional non-hardening acoustical sealant can also be used. However this material outgases VOCs for decades, which is not great for indoor air quality. One low-VOC alternative is Green Glue Noiseproofing Sealant. Acoustical putty is a good choice around electrical outlets and other penetrations.
To block sound leaks at doorways, you can install bulb-type weatherstripping to create an airtight seal. Keep the crack at the threshold very small or add a sweep. In recording studios, they often use two facing doors with an airlock in between for extra protection. Also a solid-core door will do a much better job of blocking sound than a hollow-core door.
Standard fibrous insulation (fiberglass, mineral wool, cellulose) all do a good job of absorbing sound within the wall cavity, providing a modest boost to sound control. Foam insulation is less effective as it tends to couple together the two sides of the wall. High-density foam is especially bad for low frequency sounds. And sound-deadening sheathing, also called “sound board” is not very effective in walls (but helps in floors).
Massive materials like masonry are better at blocking sound than wood and low-density materials like wood. For example, lead sheeting is often used in sound studios because it is very heavy, but not rigid, so it tends to deaden the sound that strikes it. The simplest and most cost-effective step is to double the thickness of the drywall on one or both sides of the wall. Stagger the drywall joints and glue, don’t screw, the second layer.
Decouple Wall Surfaces
To reach STC levels near 50, you typically need to use resilient channel to hang the drywall. You can reach even higher levels by staggering the studs on either side of the wall to eliminate the direct transmission of sound through the framing. These methods are very effective, but obviously eat up space, which is often an issue.
You mentioned plumbing noises. These are difficult to control as they are often transmitted directly to the framing by rigid clamps – so called “structure-borne” sound. You can help control plumbing noises by wrapping pipes in foam and stuffing plumbing chases with fiberglass. This can be very effective where PVC drain pipes run through a chase in living areas below. If you are very fussy about plumbing noise, you can also use special sound-deadening pipe clamps to attach supply lines to framing.
Blocking sound transmission is tricky. As with heat loss and air leakage, you block one path and the sound finds another. These roundabout routes are called “flanking paths.” Think under doors, around the wall perimeter, through electrical outlets, through the ceiling or floor, or through ductwork. You can limit these with careful workmanship, and thoughtful layout. For example, don’t put electrical outlets back to back, or forget to seal the wall perimeter. Shared ductwork, however, is not easy to remedy without rerouting the ductwork.
Several new products promise to simplify the task of reaching STC levels close to 50 without adding resilient channel or staggered studs. Two approaches that look promising are Green Glue Noiseproofing Compound and QuietRock sound-dampening drywall. Both of these products install easily and have extensive test data to support their sound ratings.
The noiseproofing compound applies like caulking to the back of the drywall sheet in a two-layer installation. The compound decouples the two sheets, providing performance similar to resilient channels. The cost is similar to high-quality sealant.
Quiet Rock is very pricey drywall embedded with sound-dampening materials. It is a good bet where space is at a premium. However, remember that measured ratings are for wall assemblies, not individual materials. Your results in the field will probably fall short of results in the test lab. Nonetheless, if you follow the manufacturer’s instructions, both these products should provide a substantial boost to sound control.
STC ratings are shown in the table below. Click to enlarge.
A higher STC rating means that less sound passes through a wall. The rating is roughly equal to the reduction in decibel (dB) level through the wall. Source: Best Practices Guide to Residential Construction, by Steven Bliss. © 2006 Wiley