Sound Dampening With Insulation Between Floors Explained

Cellulose Answer Man: Your Source For Information.

Kelvin D asks:
In order to achieve the best sound dampen through installation of cellulose between my basement and first floor, should I dense pack the material to the tune of 3.5 lb/ft^3. Or do I install enough material in the enclosure cavities to fill the space between the floor joist, but keep it loose enough so that I can run any wires in the future? Will dense packing the material into these 2×10 cavities cause problems with drywall sagging or popping out drywall? The ceiling is finished with 2 sheets of 1/2? drywall.


Acoustics is a pretty complex subject. For instance, there is a difference between airborne sound, e.g. a cocktail party in the apartment overhead, and impact sound, e.g. a family of clog dancers living in the upstairs apartment. After two phases of their extensive acoustic testing of wall and floor assemblies the National Research Council Canada reported:

STC showed no significant dependence on the density of the sound-absorbing material; the three other airborne ratings used did. IIC and the other impact sound ratings all showed significant dependence on the density of the sound-absorbing material. The change in IIC predicted when cellulose fibre insulation is substituted for fiberglass batts of the same thickness is about 1.5 dB. Density was chosen as the variable to describe the sound-absorbing materials because it is known from other work that, for the fibrous materials used in this project, it correlates highly with airflow resistance, the material property that is expected to effect the sound attenuation.

Rock wool and cellulose tend to be somewhat better than fiber glass at impact sound attenuation. Assuming that air flow resistance is the causative factor I don’t think much would be gained by packing cellulose to a wall density in a floor/ceiling cavity.  Slightly compressed, or even at natural settled density, cellulose insulation is pretty resistant to air flow. I doubt that packing cellulose insulation to 3 pcf would be detrimental to the ceiling, but I don’t think it would add much to the acoustic performance of the assembly.

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