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.