Most people install wall insulation to help keep the house warm. But insulation can also be used to slow the spread of sound from one room to another or from outside to inside. Though it commands only a small share of the U.S. insulation market, rock wool is particularly good at sound deadening. As a bonus, it's also noncombustible.

The Milton, Ontario-based Roxul, Inc. says that its Acoustical Firebatts have been subjected to several sound tests by a Canadian government lab. When installed in a perfect wall assembly, its sound resistance wasn't much better than fiberglass, but after an electrical outlet was put in the wall, the difference was dramatic. The reason is that drywall is a good sound deadener--until you put a hole in it. After that, the mass of a wall is the biggest contributor to its sound deadening properties. The higher the mass the less sound it will transmit. The mass of a rock-wool-insulated wall is simply a lot higher than one filled with fiberglass.

According to Roxul's John Evans, the fit of the insulation also comes into play. He says that tests have shown that a 6 percent defect will reduce the insulation's sound-deadening properties (as well as its R-value) by 35 percent. 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 denser, rock wool is easier to fit around obstructions.

Acoustical Firebatts have an R-value of 4 to 4.7 per in. and come in a range of densities from 1.7 to 12 pounds per cubic foot . Batts are available for 2x4 and 2x6 walls, and for 16-in. and 24-in. on-center framing. The company also makes special batts for steel framing. The company says that cost is equivalent to fiberglass.

Contact: Roxul, Inc.