Natural Science Forum / Physics / Acoustics / June 2005

Been there. Done that.

I stuffed as much fiberglass batt as I could fit into my plenum without
compressing it, leaving just enough clear air space to allow airflow
without much static loss. My living room became much quieter
immeddaitely. At the bottom of the vertical duct drop by the floor, I
loaded some old acoustical tiles tilted in that elbow to also not impede
air flow. Under tha furnace fan itself, likewise, I laid foil backed
R-11 fiberglass batt, foil side up to prevent sucking random bits of
fiberglass into the fan. These bottom tratments attenuate the sound
right near the source.

These treatments reduced my hpusehold furnace fan noise considerably.
The plenum batts act as a lint trap, so I now lay an extra filter
element on top of the return grille to capture the bulk of house lint. I
clean lint from that front filter about twice a year by laying it on the
inlet of our tank vacuum cleaner, wiping it gently over the entire
filter face, both sides. Such a filter lasts about 5 years becoming
frayed by the repeating wiping actions. Been doing that for about 30
years. Have to replace the plenum batts about once a decade.

Angelo Campanella

I am not aware of any way to make the polyurethane foam safe enough for me.
It is not just the fire but the fumes that would be spread by the
ventilation system.

Owens-Corning has been marketing heavily to homeowners.  They sell
essentially the same material as the duct liner as a wall panel.  You also
could cut up some fiberglass ceiling panels from Armstrong, USG, or Capaul.
If the geometry permits it, you also may be able to use widely available
fiberglass batts as Ang suggested.