Quote:
Originally Posted by Bob Ross  FWIW, my dad, who was in the plastics industry for 20+ years (sales & marketing, not engineering, so you should take this with a big grain of salt) insists that closed-cell expanded polystyrene foam insulation -- the stuff that (often incorrectly) gets referred to as Styrofoam® -- does exhibit some soundproofing properties when used as the middle layer in a sandwich between sheetrock and/or plywood leaves. |
When one uses polystyrene in a cavity as partly filling you will DEGRADE the acoustic TL properties of that wall.
Since closed cell it will decrease the air cavity making that air spring stiffer thereby increasing MSM which not only influences the frequency of the resonant frequency in itself but the whole subsequent TL curve, which is related to that resonance frequency.
From that point of view your dad is right: it does exhibit some soundproofing properties: it makes that wall significantly worse (even when compared with an empty cavity).
If you make it into a glued/bonded sandwich panel, it acoustically starts behaving quite different and is not related to traditional drywall discussions here.