Opened up a fire-rated wall to install stuff and discovered blackened sections in the insulation (i think it’s fiberglass). marks have no distinct pattern.

Should i reuse it? Should i add something else before closing up?

It’s sandwiched between 5/8" drywall and a concrete wall to a common area in building. Opening is 40" high from finished floor to ceiling. I don’t think there could have been water there, other than maybe condensation. That stuff is 33 years old.

closeup one

closeup two

  • 1
    It does look to me like water infiltration bringing sediment with it.
    – keshlam
    Commented Jan 30, 2023 at 4:38
  • It could be moist/damp at times. Concrete is porous enough to wick water. How high above the ground is this?
    – FreeMan
    Commented Jan 30, 2023 at 14:00
  • You'd be hard-pressed to find such a wall without some discoloration due to occasional moisture or airflow.
    – isherwood
    Commented Jan 30, 2023 at 16:36
  • the bottom edge of the wall opening (in the second photo) is 42" (1meter) from the finished floor... hence why I doubt it's due to some flooding. The porosity argument makes a lot of sense.
    – init_js
    Commented Feb 7, 2023 at 4:05

1 Answer 1


Black patches like that are commonly seen in insulation like fiberglass when there has been air infiltration through that spot for an extended time. It's is just really fine dust caught in the mesh. This isn't causing any problems and isn't a bad thing, though it does indicate a (likely quite small) source of heat loss.

I cannot see quite well enough in your second picture, but it looks like the black line is aligned with an intersection of two blocks in the wall behind. Looking at that closely you might find there is a small gap or cracks. These could be filled with something, depending on the material and size details, like caulking. This would stop the air movement.

  • I think you're right, there are small cracks in the wall! There's bits of mortar that have fallen all over too.
    – init_js
    Commented Feb 1, 2023 at 2:09

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