Soundproofing strategies

How do you expand the virtual size of your home without increasing floor area? Soundproofing is the answer. By applying effective soundproofing strategies to various areas of your house you gain a lot more effective space because separate zones remain more private.

There are two things to consider in your noise reduction campaign: the elimination of all cracks -- even tiny ones -- extending between spaces. You'll also need to beef up the sound resistance of walls and ceilings. It's hard to beat the performance of drywall as a surface covering on both these counts. Its ability to absorb sound is considerable, thanks to high density. Drywall tape and joint compound also seal sound-transmitting, board-to-board cracks. My favourite type of drywall for workshop applications is 5/8"-thick "fire-code" material, the kind required for commercial applications. Besides being thicker than usual, the gypsum core includes glass fibres for extra strength. Fire-code drywall costs about 20% more than regular 1/2" stuff, but it fares much better physically, as well as acoustically.

Although there are several ways to measure sound resistance, the most appropriate yardstick for residential situations is a scale called the Sound Transmission Class (STC). This rates the minimum sound resistance of a floor or ceiling at all frequencies. The greater the STC, the more effective the blocking of sound. A 2x4 wall covered in 1/2" drywall, for instance, has an STC of about 30 -- not even enough to prevent loud speech from being understood from a neighbouring room.

As you'd guess, simply putting 1/2" drywall on the underside of the floor joists in an unfinished basement boosts STC of the whole floor structure to only about 36 -- not nearly enough. A better option is to install 5/8" drywall onto the floor joists, along with sheet metal strips call resilient channel. This separates the drywall from the floor joists, blocking sound transmission more effectively than direct drywall-to-joist contact. Adding fiberglass batts between the joists before drywall installation will boost the entire floor to a pleasant STC of over 50.

Your aim for a wall design that delivers at least 40 STC. A 2x4 wall with a double layer of 1/2" drywall on each side will do that. Far better, however, is a double-stud wall. It delivers a whopping 50 STC even without internal fiberglass batts that can boost sound resistance even further. Loud speech is not even audible through such a wall. The design is based on a 2x6 top and bottom plate, with staggered 2x4 studs spaced 8" apart. That sounds awfully close together, but don't forget that the studs are staggered. They actually provide nailing surfaces that are the standard 16" from centre-to-centre on each wall face. The idea of staggered studs is to eliminate the continuity of wood from one side of the wall through to the other. Sound is transmitted best through the vibration of solid materials, so the more you can eliminate this travel route the better.

Another feature of effective quiet-wall designs is the use of a sound-deadening layer underneath the drywall. The best material for this is the kind of fibreboard sheet materials sometimes used as exterior wall sheathing under brick veneer. Although it's usually coated in tar, to increase weather-resistance, it's also available uncoated for interior use. The combination of hard, dense drywall and soft flexible fibreboard underneath works well because it creates very different, side-by-side densities. Sound engineers have discovered that this kind of density diversity is a potent noise blocker. You'll need longer-than-normal drywall screws to secure such a combination to wall studs, but if you can't find any, don't worry. Regular, flathead wood screws work fine for securing drywall, and they're easy to find in longer lengths.