Best way to dampen sound without the Rockwool price tag?

I’ve acutally done the "slice the wide batts in half" trick when I was finishing my basement workshop on a shoestring budget. It works, but it’s a total mess and takes twice as long as you think. If you go that route, get a dedicated insulation knife or a long serrated bread knife; a standard utility blade will just snag and tear the pink stuff. You'll end up with fiberglass dust everywhere, so wear a decent mask and long sleeves. For sound, those R19 batts are okay, but since they’re 6 inches thick, they won't fill the whole joist cavity if you have 2x10s or 2x12s. You’ll get some dampening, but leaving a big air gap can sometimes lead to resonance issues. I found that Johns Manville or Owens Corning R15 or R13 stuffed in there is often cheaper per square foot and easier to handle. If you realy want to save money, check your local Habitat for Humanity ReStore. I’ve found half-pallets of Rockwool Comfortbatt there for pennies on the dollar because someone over-ordered for a commercial job.

Honestly, I’d skip the fiberglass slicing altogether. I tried that once with some R19 Owens Corning rolls and the cuts were never clean enugh to get a tight friction fit in those narrow 12-inch bays. You end up with gaps along the joist edges, and for sound dampening, even a small gap lets all the noise trough. It’s alot of itchy work for a result that ussually underwhelms. Check out blown-in cellulose instead. Most big box stores will give you the blower rental for free if you buy a certain number of bags—usually around 20 or 30. You can staple up some poly mesh or even a thin garden fabric across the joists and blow the cellulose in behind it. It’s much denser than the pink fiberglass, which is what you actualy need to kill that foot traffic noise from upstairs. It fills every nook and cranny around the wires and pipes, which those sliced batts never do. The material cost for 900 square feet of cellulose is usually comparable to mid-grade batts, and the sound performance is night and day. Just wear a decent N95 mask becasue the dust is real, but at least you aren't covered in fiberglass shards for a week.

I'm super new to this, but how does blown-in even work if the drywall isn't up yet? Do you have to staple some kind of netting to the joists first to hold it all in? That sounds like it might be even more work than cutting the batts.

Also, I keep hearing about 'resonance' if there's a gap. Does that mean the sound actualy gets louder if I don't fill the whole space? I'm just trying to figure out if I shold be looking for thicker stuff or if just a thin layer of the cheap stuff is enough to stop the noise from upstairs.

I’ve spent forty years in crawlspaces and I'll tell you now, slicing 23-inch batts is a fool's errand. You'll end up with gaps that let noise through like an open window. Back when we did multi-family units in the late 80s, we’d hunt down 'slag wool'—the old-school mineral wool—in loose bags and hand-stuff it. If you’re on a budget, call a local supply house and ask for 'seconds' or overstock 12-inch batts. They’re usually gathering dust in the back. Just make sure you use 'tiger teeth' wire hangers to keep it snug against the subfloor. If there's air between the insulation and the floor above, those footsteps will still sound like a drum.

If you want the best bang for your buck without the mess, go get the Knauf EcoBatt R-15. They make a 12-inch wide version that is way denser than that cheap R11 or R13 stuff. I used it in a 900 sqft basement last summer and it’s alot easier on the skin than the pink stuff. It’s specifically designed to be a friction fit, so you won’t have those sound-leaking gaps you get from hand-cutting wide batts. It hits that sweet spot between basic fiberglass and the high-end Rockwool.

Have you ruled out whether insulation is actually the fix for your specific noise? If you're trying to dampen foot traffic, stuffing the bays might not do as much as you think. Fiberglass handles airborne noise—like a TV—but it does very little for the structural 'thud' of walking because that vibration travels directly through the wood joists.

Before you spend the money, have you looked into resilient channels? Even basic R-11 with a mechanical break usually outperforms a dense batt if the drywall is hard-screwed to the joists. Is the budget better spent on decoupling rather than the material inside the cavity?