Found some dark spots on my basement insulation, is this mold or just dust?

That dark discoloration is usually a mix of two things: air filtration and moisture migration from the foundation. Fiberglass is basicly a giant air filter. When you have temprature differences between the cold concrete and the warm basement air, it creates tiny air currents. As that air moves through the insulation, the glass fibers trap dust and particulates, which eventually turns the backside of the batts black or dark grey. The white crusty stuff is almost certainly **efflorescence**. Those are mineral salts that get pushed out of the concrete when moisture moves through the wall and evaporates. It's a clear sign that your foundation is wicking moisture from the soil outside. Fiberglass is a terrible choice for that enviornment because it doesn't stop the moisture; it just holds it against the wall like a wet sponge.

To check for mold, grab some high-concentration hydrogen peroxide and pour a little on the dark spots. If it starts foaming and bubbling vigorously, you're looking at organic growth. If it just sits there or soaks in without a reaction, it’s likely just accumulated dirt and soot from the air. Either way, you should absolutely be wearing an N95 mask. Even if it isn't mold, breathing in old fiberglass particles and mineral dust is going to make you miserable.

If you decide to rip it all out, don't just shove new pink batts back against the concrete. You need to break the thermal bridge. The best way to prevent this from happening again is to install **rigid foam board** (XPS) directly against the foundation wall and seal the seams with waterproof tape. This keeps the moisture in the concrete and prevents the warm indoor air from hitting the cold wall and condensing. Then you can build your stud wall and add more insulation if needed without worrying about it rotting from the back out.

That peroxide trick sounds like something I can acutally manage myself. I have those same dark spots up in my rim joists too. Does the foam board fix work for those areas, or is that a different situation? I looked up the price of those XPS sheets and they aren't exactly cheap for the whole basement. If I just patch the spots that look bad right now, will it just spread to the old stuff next to it? Also, does the thickness of the foam matter for stopping the moisture or is any layer enough to do the trick?

Rim joists are acutally the biggest leak point in most basements. If the full wall project is too pricey right now, definatly prioritize those joist pockets using 2-inch foam board cut to fit and sealed with canned spray foam around the edges. Don't bother patching just the 'bad' wall sections. Moisture is relentless and it’ll just migrate to the next path of least resistance in your old fiberglass. If you want a cheaper way to help while you save up, check your downspouts outside. If they’re dumping water right against the foundation, you’re fighting a losing battle regardless of what insulation you use.