Dark staining on fiberglass insulation behind the siding - is this mold?

Nine times out of ten, those black streaks on fiberglass are just air filtration soiling. Since fiberglass is basicly spun glass, it doesn't provide a food source for mold on its own. If the wood framing behind and around it looks clean and isn't soft or stained, you're almost certainly looking at decades of dust, soot, and pollen that got pulled thru gaps in the sheathing. This is common near bottom plates and window openings where the air seal is ussually the weakest.

This happens because houses breathe through pressure changes. Air gets sucked in through small cracks, and the insulation acts as a giant filter, trapping all the fine particulates from the outside air. It’s particularly common in older homes where the air barrier wasn't as tight as what we do today. If it were actual mold, you whould usually see the wood turning black or white and fuzzy first, because mold needs the organic cellulose in the wood or the paper backing of the insulation to thrive. Since you've already got the wall open, you should defintely pull those dirty batts out and toss them. Fiberglass is cheap, and you don't want to trap that old gunk back in the wall. But the most important step isn't just replacing the material; it's **air sealing**. Get a few cans of high-quality spray foam or some exterior-grade caulk and hit every single gap where the studs meet the bottom plate and around the window framing. If you just stuff new batts in there without sealing those air leaks, you're going to have the exact same black streaks in another ten years. Even if it's not a health hazard, those air leaks are killing your efficiency and letting moisture-laden air into your wall cavity. Seal it up tight while you have the chance, then pop in some new **mineral wool** or fiberglass batts and you'll be in much better shape for the long haul.

I was wondering about that mineral wool suggestion. I’ve seen those bags at the hardware store and they’re way heavier than the pink stuff. Does it acutally stop the air leaks better than fiberglass, or is it just more dense?

I’m also worried about the air sealing part—do I need to worry about the foam expanding too much and bowing the studs? I’m trying to do this myself but don't want to mess up the structure while trying to save on my energy bill.

Mineral wool is definately more dense, but don’t let that fool you into thinking it's an air barrier. It still breathes. If you don't seal those gaps first, you’re just putting a heavier filter in your wall. For the foam, just make sure you grab the 'Window and Door' cans for the gaps around your framing. It’s low-expansion so it won't put pressure on your studs or window jambs. The regular stuff can definitely bow things out if you go overboard. Just a nice bead along the bottom plate and around the shim gaps shold do it without any drama.

If you're near a busy road or in a dusty area, that filter effect is way more obvious. I had the same thing in my old ranch—the fiberglass was pitch black near the electrical outlets and the bottom plates.

Since you have the sheathing off, check your house wrap. If it’s old and brittle, it’s basically doing nothing to stop that air. I switched to mineral wool mainly for the sound dampening and becasue it doesn't sag over time. It’s way easier to cut around boxes and wires too, which helps keep the fit tight so you don't get those air bypasses again.