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.
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.
