Inside my HVAC (past the horse hair filter) is a large gap before the air hits the radiator. The inside of this area appears to be lined with some kind of fiberglass insulation.

I asked maintenance why it looked the way that it did; I thought it was mold at first but was told that it's just dirty. It does appear dry and I agree that it's probably just dirty.

I'm also concerned that the fiberglass material looks like its deteriorating and the fact that its locates past the filter where it can get sucked up and into the ductwork.

My questions are: is this the right kind of insulation for this purpose and does it need to be replaced?

It looks like a very old unit but I can't find any indication of when it was installed.

enter image description here

enter image description here

enter image description here

  • 2
    What would lead you to believe that someone installed the wrong kind of insulation inside your furnace's air handler? Would they have done it at the factory? Was the previous tenant a malicious soul who left something evil lurking for the next inhabitant? Sure it's reasonable to think that it's dirty, but the wrong kind? I just don't understand what thinking could lead you (or anyone, not picking on you) to believe that's even possible...
    – FreeMan
    Commented Oct 23, 2020 at 10:46
  • 1
    In my head I imagine fiberglass getting sucked up and spat out through the air ducts. I don't know anything about HVACs which is why I'm asking a question.
    – Zhro
    Commented Oct 23, 2020 at 10:49
  • 2
    This is standard. Your evaporator coil appears to be unobstructed. If fiberglass insulation were getting pulled off, it would collect on the surface of the coil. Commented Oct 23, 2020 at 19:01

1 Answer 1


I don’t like internal insulation for several reasons the first is because if not foil faced it holds dust and can be a home for mold.

With that said it looks ok to me fiberglass and rock wool will not be affected by moisture (degraded other than reducing the insulation value).

Looking at the photo a very neat job was done on the outside blanket. the inside looks ok but it is hard to tell the orientation I would never insulate the bottom of the air handler with an AC coil in there but it’s hard to tell although it looks like it is attached not flopping around so other than my personal preference not to insulate inside ducts it is probably fine from what I see.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.