Degradation of wood stud wall sound isolation by electrical outlet boxes

Abstract

It is widely accepted that the presence of electrical outlets in party walls can degrade the fire resistance and sound isolation. However, a systematic study of the factors affecting the sound isolation have not been reported. This paper will summarize the key factors identified in a recent study conducted at the National Research Council of Canada which considered single, staggered, and double wood stud walls. The paper will show that box placement was very important for standard metal boxes (having many penetrations), but was not an important factor when the boxes were airtight (i.e., vapor barrier type). Wall cavity absorption and how it was installed in the wall cavity at an electrical box played a key role. Cavity absorption greatly reduced the effect of poorly located metal boxes. The benefit could be maximized by placing the absorption so that any direct line-of-sight was obstructed. It was also shown that a septum, separating back-to-back boxes, was not effective unless cavity absorption was present. The paper will also present simple and effective retrofit techniques that do not involve removal or relocation of either electrical box. These include draft stoppers, inserts, caulking, etc.a) Present address: Shuller International, 10100 West Ute Ave., Littleton, CO 80127.