Sound Control: Insulating for Peace and Quiet
One of the big problems with old houses is that noise tends to bounce around in them. These days, homeowners want more sound control.
In fact, sound control has become such an issue that major manufacturers are taking notice.
Five years ago, CertainTeed set out to determine how it could sell builders and homeowners more of its fiberglass insulation. The Valley Forge, Pa.-based company started by looking at research on design trends published by the National Association of Homebuilders and others.
"When we analyzed the research data, a couple of things jumped out," says company spokesman Tom Newton. These included more in-law apartments, in-home offices and media rooms. "The trends," says Newton, "were ideal for sound control."
The nation's two other insulation giants, Owens Corning of Toledo and Johns Manville Corporation (formerly Schuller International) of Denver, have apparently come to the same conclusion. All three companies are aggressively pushing sound insulation as an upgrade from a standard thermal insulation package.
"We know from our research that homeowners want sound control," says Jim Schmiedeskamp, Owens Corning's marketing manager for insulation, "and that they're willing to pay for it."
Sound control is a natural for remodelers, because it can make an existing home more livable. But the key to selling the upgrade is understanding the technical issues behind sound control and being able to explain them to homeowners. With that in mind, here's a short guide to soundproofing walls.
What's an STC?
The ability of a building assembly to block sound is expressed as its sound transmission class, or STC. The higher the STC, the better the assembly's sound-deadening abilities. An STC of 50 is a good rating; anything above 55 is considered excellent.
Most homeowners, and many contractors, equate sound control with more insulation. But while adding insulation is essential, it's just one part of the overall strategy. For example, a 2 x 4 partition wall with fiberglass batt insulation and 1/2-inch drywall on both sides has an STC of only around 38.
To build a really quiet wall, you need to do three more things: decouple the two wall surfaces from one another, increase the wall's mass and block the paths by which sound flows from one side of the wall to the other. The time to plan for all of this is before framing begins.
Framing
There are two ways to decouple the two wall surfaces. If you are building the wall from scratch, use staggered stud framing: 2 x 3 studs 8 inches on center between 2 x 4 top and bottom plates. Alternating studs line up with opposite sides of the plates, so each stud supports the drywall on one face of the wall only.
To decouple the two faces of an existing wall without altering the framing, separate the drywall from the framing with resilient channels. Fastened horizontally to the studs, the channels allow the sheetrock layer to absorb sound vibrations rather than transmit them to the framing. For best results, use drywall screws that are long enough to penetrate the drywall and the channel's flange, but not long enough to reach the stud.
Adding Mass
The more massive a wall is, the less sound it will transmit. The simplest way to add mass to a wall is to use 5/8-inch drywall instead of the 1/2-inch stuff that's standard for most remodeling projects.
But beware: While 5/8-inch panels transmit less sound, they're heavier to carry around and install. An alternative method is to install two layers of 1/2-inch drywall
Plugging Holes
Drywall is a good sound deadener -- until you put a hole in it. Even small holes weaken a wall's sound-deadening ability. After hanging your drywall, grab a caulking gun and obsessively plug gaps around electrical boxes, heating registers and plumbing fixtures. Use polyurethane or acrylic latex caulk, which will stay flexible.
Install electrical outlet plates over the same rubber gaskets you would use to stop drafts; they'll also block the passage of sound. Make sure your electrician staggers switch and outlet boxes on opposite sides of a wall; placing them back to back creates a direct path for sound to travel from one room to the next.
The biggest holes in a wall are the ones you walk through, so don't leave doors out of your soundproofing plan. As with walls, the more massive a door is, the less sound it will transmit, which means a solid-core door will be quieter than a hollow-core model.
And gaps around a door will pass as much sound as gaps in a wall. To make a super-tight doorway, fit door stops with rubber bulb gaskets and install a weatherstripped threshold.
Insulating
The key to a good insulation job is to pay more attention to detail than you think is necessary. The problem here is the same as with installing thermal insulation: You want to get the job over with, so you hastily stuff batts into place, leaving gaps behind wires and electrical boxes.
But tests have shown that leaving just 6 percent of a wall uninsulated reduces the insulation's sound-deadening properties by 35 percent. To get the most bang from your batts, cut them carefully to fit around and behind wires, pipes and outlet boxes (see "Batten the Hatches").
In deciding what type of insulation to use, remember: Sound-control batts are basically thermal insulation in a different wrapper. R-11 batts used to be standard for exterior wall insulation.
When most codes upgraded the requirement to R-13, manufacturers "simply repositioned R-11 batts for sound control and basement walls," notes Schmiedeskamp. There's nothing wrong with sound-control batts, but if you can't find them, ordinary batts will do.
Fiberglass, however, is not the only sound insulation to consider. Though it commands just a small share of the U.S. insulation market, rockwool is an excellent sound-deadening material.
Milton, Ontario-based Roxul, Inc., says that its rockwool Acoustical Firebatts have been tested by a Canadian government lab. In a perfect wall assembly, rockwool's sound resistance wasn't much better than that of fiberglass. But in a wall with an electrical outlet, its performance was dramatically better.
Why? Because the mass of a rockwool-insulated wall is a lot higher than that of a wall filled with fiberglass.
According to Roxul's John Evans, the fit of the insulation also comes into play. It's hard to fit fiberglass tightly around obstructions; instead, installers tend to stuff it in place, leaving gaps behind electrical boxes. Because it's so dense, rockwool is easier to cut into a shape that will fit around obstructions.
Acoustical Firebatts have an R value of 4 to 4.7 per inch and come in a range of densities. Batts are sized for 2 x 4 or 2 x 6 walls, and for 16-inch or 24-inch on-center framing. The company also makes special batts for steel framing.
Good Sources
There's a lot of good technical information out there on sound-proofing. The North American Insulation Manufacturers' Association, Alexandria, Va., publishes "Sound Control for Commercial and Residential Buildings," a booklet that discusses the effectiveness of different sound control strategies. To order it free, call (703) 684-0084.
Insulation makers are taking pains to educate consumers and contractors about sound control. Owens Corning's new marketing brochures outline "good," "better," and "best" sound control designs. Johns Manville gives dealers audiotapes that demonstrate sound transmission between rooms before and after insulating the walls -- a contrast marketing manager Scott DeShetlar likens to the difference between slamming the door of a Yugo and the door of a Mercedes.
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