Impact Sound Transmission Through Floor Assembly
Question:
A condominium project I am involved with is experiencing a sound transfer problem through the floor/ceiling assembly between the first and second floor. You can hear every footstep, from an adult to a child. The floor assembly consists of a carpet and pad, 3/4 in. OSB sub-flooring, 15 in. deep wood trusses at 16 in. on center, 9 in. fiberglass batt insulation and a 5/8 in. gypsum board ceiling. The trusses span 20 ft. with air/heat ducts between the trusses. Will an additional layer of 5/8 in. gypsum board attached to 7/8 in. furring strips oriented perpendicular to the trusses minimize the sound of the footsteps? If not, what would be the way to go with this problem?
Answer:
It appears your problem has more to do with impact sound transmission rather than airborne sound transmission. Impact sound transmission occurs when a structural element is set into vibration by direct impact (e.g., a footfall). The vibrating surface generates sound waves on both sides of the element. The Impact Insulation Class (IIC) is a method of rating the impact sound transmission performance of an assembly. The higher the IIC rating, the better the impact noise control of the element and assembly.
IIC values for various construction materials may be found in several publications, including SBCA’s Metal Plate Connected Wood Truss Handbook. The impact insulation characteristics of a floor or ceiling assembly can be roughly calculated by adding up the IIC values of the individual components. The IIC value of the assembly described above should be approximately 53-58, depending on the thickness of the carpet and pad. The addition of resilient channel and another layer of 5/8 in. gypsum board could increase the IIC rating of your assembly by 10 to 12 points. Whether this increase is enough to eliminate the impact sound problem is hard to say, but it certainly will help reduce it. You might also consider adding a sound-deadening product to the assembly, as this can significantly improve the impact sound performance of the assembly, depending on the type you choose.
Sound information on fire-rated assemblies is typically included in the Gypsum Association’s Fire Resistance Design Manual (GA-600-12). Other sources include specific gypsum manufacturers, manufacturers of sound abatement products and reports like this one by the National Research Council of Canada.