My HVAC return ducts are run through the building cavities. In some places, this means a return duct is run through the joists in the basement ceiling (with a metal liner covering it).
In more than one place, I have found batts of fiberglass insulation inside the return duct. I have been removing them as I find them, with the idea that particles of fiberglass going into the HVAC is not a good idea, even if the furnace filter is probably going to catch them.
In other words, the batts of fiberglass are on the INSIDE of this pictured duct, not on the outside:
Today I had a HVAC company come and take a look at my other issues, and he told me that it's totally normal to have batts of fiberglass inside the duct, that it insulates the duct. I do not see how this could possibly insulate the duct.
Is this a common practice, or have you heard of it being done? Would you tolerate batts of fiberglass insulation in your own return ducts?