Maxx Minutes Blog

Solving the Floor/Ceiling Sound Puzzle -
Floor/Ceiling Isolation, Span Length and Floor Stiffness Considerations

As we wrap up our discussion of building an acoustically efficient floor/ceiling assembly, it is worth addressing proper floor isolation. We will also discuss the role joist span and floor stiffness play.

Proper Floor/Ceiling Isolation
It is essential that the gypsum board ceiling be resiliently mounted.  A properly installed, resiliently hung ceiling should not have direct contact with the joist or wall:

  • Screw length is important. If the screws are too long and carry through the resilient channel penetrating the joist, a “short circuit” develops. A short circuited screw rigidly connects the drywall to the joist making the resilient channel lose its flexibility. It takes very few of these short circuits to circumvent the acoustical value of the resilient channel. The UL fire rating and ASTM standards list the specific screw length to be used. Adhering to these guidelines will help to avoid short circuits.
  • A well performing resilient channel should be used. We recommend obtaining the acoustical data for the resilient channel from the manufacturer prior to its use on a project.
  • When installing the gypsum board ceiling, a 1⁄8" – 1⁄4" gap should be left between the wall and the ceiling. 
  • Once the wall and ceiling are in place, the gap is filled with an acoustical caulk. 

 Joist Lengths

  • If excessive noise complaints exist even though minimum IIC ratings have been met, it is possible that the recommended joist span has been exceeded. Excessive span can increase the transmission of low frequency footfalls, which can also be visually observed through ceiling deflection while someone walks across the floor above.   Additionally, span length can impact floor stiffness; generally a longer joist at the same depth will be less rigid.

 Floor Stiffness

  • The stiffer a floor (L/360 vs. L/720) the better the sound performance when a good ceiling and sound control mat are installed.