Fiberglass in duct

Just bought a house built in 1979. The house has a packaged HVAC unit outside. I went to replace the filter for the first time and noticed some insulation that sits in the “air intake” portion of the unit behind the filter. At first, I freaked out, worrying that it was asbestos. I took a sample to a local lab where they identified it as glass, not asbestos. That relieves my initial concern, but then I started thinking that having fiberglass sucked into the system is not the best thing either. Any thoughts on whether this should be removed, why it’s insulated in the first place, and whether I should replace it with another material.


Looks like what some call fiber ductboard. Easier to work with than metal; insulation is on the inside - foil covering on the outside. Here is one link:

No, it’s just duct insulation.

It is designed for this use (back in the good old days).

If you want to rip out something, change the whole Plenum box, it’s designed every which way from wrong.

Yep, see it at almost every house.


And if it’s covering is perforated or open mesh style, it suppresses fan or compressor noise, making the whole system much quieter.

Not in this case though, as we can see the evaporator coil from the living room! :wink: