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: cavity run HVAC duct

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?

  • Fiberglass would be a no. The ducts do look like they could become drums, so maybe something in them might be okay, if the 'tin' as any movement.
    – crip659
    Nov 29, 2021 at 16:42
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    My guess is that it was done for sound deadening. It probably reduces furnace fan noise somewhat. I'm not aware of it being a common practice, though.
    – isherwood
    Nov 29, 2021 at 16:55
  • To clarify, they are just batts of fiberglass insulation and not duct board? Duct board would have an outer layer (shiny foil faced) and an inner layer to keep the surface smooth and importantly fiberglass out of the airstream. They are somewhat rigid and only about an inch thick. If its just batts like that go in your walls, then they should be removed, as you don't want fiberglass in your air or the restrictions in your ductwork.
    – Glen Yates
    Nov 29, 2021 at 17:50
  • Looks like galvanized sheet was attached to two joists to make a duct. I have done that to bring in outside air for combustion . I did staple foil face batts in the top of the duct which , in my case was the floor, for insulation. Nov 29, 2021 at 19:14
  • @GlenYates yes, they are just batts of fiberglass, not fiberboard.
    – negacao
    Nov 29, 2021 at 21:16

1 Answer 1


Yes, using the space between joists is quite common for the return air and is often sealed with duct board insulation and tape at all the joints.

See this layout as recommended by the NAIMA Insulation Institute on page 39 which is nearly identical to yours.


  • Yes, this is duct board (which is acceptable) as I was questioning in my comment, but OP stated that it was just fiberglass batts (which I would not deem acceptable)! Note: It is PDF page 39 but page 37 as printed on the actual document.
    – Glen Yates
    Nov 29, 2021 at 21:36
  • Fiberglass batts are acceptable on the exterior of the ducts (when acoustical treatment is not necessary). (See page 28.)
    – Lee Sam
    Nov 29, 2021 at 21:52
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    Of course batts are ok on the exterior, the concern here is the interior of the ducts.
    – Glen Yates
    Nov 29, 2021 at 22:19
  • I thought I answered this, but I’ll try again for @Glen Yates. The NAIMA recommends batts on the exterior of the ducts ONLY. (See page 28)
    – Lee Sam
    Nov 29, 2021 at 22:43

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