Tips for choosing between rockwool and high density fiberglass that nobody talks about

Did my own garage conversion with the mineral wool batts last spring and your point about the bread knife is spot on. I found it cuts like cutting a firm cake, and I could get a snug fit around the old junction boxes without any fuss. It was just me working evenings so the lighter weight of fiberglass whould have been nice going up the ladder, but I managed the 3 inch stuff fine one bundle at a time.

The sound difference surprised me. My wife works nights and sleeps days, and with the wool in that shared wall plus the garage door insulated, she says she barely hears me running the table saw now. Not sure I could have gotten the same result with fiberglass since the walls are old and wonky, nothing square about them.

One thing I'd add for anyone doing it themselves is don't skimp on buying extra. I ordered right to the square footage and then wasted a couple batts learning how to cut around obstacles. Had to make a second trip to the supplier, and the delivery fee stung more than the extra batts would have cost up front.

Agree with the point about installation quality being the real decider. I had dense fiberglass put in a bonus room a few years back and even with a decent crew the fit around the kneewall corners was loose. I went back thru after them with offcuts and a utility knife stuffing every gap I coud find, took a whole Saturday. When I did the rockwool in the basement myself the next year it was honestly less fuss because the batts just stayed where you put them. Never understood the slump problem until I saw fiberglass slowly sag out of a sloped ceiling bay.

The itch factor is real too. I did fiberglass in an attic years back and was picking fibers out of my arms for two days even in long sleeves. When I did rockwool in the shed it barely bothered me at all. Sounds minor till you're halfway through a big job and dreading every bundle. Anyone checked prices lately? Around here the gap's shrunk alot since rockwool got more common at the big box stores. Ten years ago it was a no brainer on cost, now it's close enough that I'd just pay for the nicer stuff.

One wrinkle I ran into on my workshop build is that the two products don't fill a cavity the same way, so check what R-value is actually rated for your stud depth before comparing prices. The dense fiberglass batt I looked at was rated for a shallower wall than the mineral wool equivalent, and since my framing is plain 2x4, the wool fit the depth properly while the fiberglass option would have needed a thicker wall or left me under-insulated. Also worth measuring your on-center spacing before ordering, because my 1970s bays measured closer to 22 inches in places and the standard width batts needed a fair bit of surgery.