Adding mineral wool over closed cell spray foam in my basement?

You're allready sitting at a total R-value of around 25 between the exterior rigid board and that inch and a half of spray foam. For a basement wall, that’s acutally fantastic. Most local codes are only looking for R-10 or R-15 in a subgrade application, so you’ve already exceeded the standard significantly. Adding more material at this point is realy hitting the wall of **diminishing returns**.

If you try to rip down R-15 batts to half thickness, you’re in for a miserable, itchy weekend for very little payoff. Fiberglass is a pain to split evenly, and if you compress it too much to fit in that small gap behind the drywall, you lose the actual R-value anyway. Plus, having that open air space is a massive benefit when you start pulling your **electrical wire** and mounting your outlet boxes. If you stuff that cavity full, you'll be fighting the insulation every time you try to seat a device. There's also a technical benefit to how the spray foam was applied. By leaving it an inch or so shy of the studs, the installers kept the "skin" of the foam intact. That cured surface acts as a better **vapor retarder** than foam that has been shaved flat to the studs. Shaving it opens up the tiny cells and can actually make it slightly more permeable. Since you already have the exterior board, your thermal break is solid and your condensation risk is basially zero. Honestly, the extra R-value you’d gain from half-thickness batts wouldn't even be noticable on your monthly heating bill. You’ve done the high-performance part of the job correctly with the closed cell. I’d save the money and the labor, leave the air gap for your wiring, and just get the drywall hung. You’re already in a much better spot than 90% of finished basements out there.

I was actually thinking about doing the same thing but maybe using that rock wool stuff instead of fiberglass becuase it's supposed to be easier to handle. But that point about the wiring makes a lot of sense—I didn't even think about how tight those outlet boxes woud be. If I leave the gap, does it make the room sound more 'hollow' once the drywall is up? I’m worried about the basement feeling like a drum if there’s just empty air behind the walls.

If you're worried about that hollow sound, honestly, once you get the drywall up and some furniture or area rugs in there, you won't even notice it. Basements only sound like caves when they're empty shells.

If you’re dead set on filling it for acoustics though, don't mess with fiberglass batts. Grab some mineral wool meant for sound dampening. It’s way easier to friction-fit than trying to split a pink batt in half. But for real, you’ve alredy won the thermal game. Any more is just for your own peace of mind or if you’re planning on turning the space into a high-end home theater.