Is it okay to stack mineral wool scraps to fill narrow stud bays?

I've done this on plenty of jobs to save on waste, and as long as you aren't mashing it in there like a sardine, you'll be fine. Inspectors generaly just want to see that the cavity is completly filled with no **air gaps** or "holidays" where heat can escape. If those scraps are tight against the studs and each other, it’s going to pass. The main issue with using offcuts like that is over-compression. Mineral wool works beacuse of the air trapped between the fibers. If you’ve jammed so much in there that it feels hard to the touch, you’ve actually lowered the R-value of the wall. You want it snug enugh to stay put under its own weight, but not packed so tight that it’s losing its loft. If it’s just a little bit of a squeeze to keep them from falling out, you’re in the clear. Stability is the other thing to watch for. Since you have a bunch of horizontal seams, those pieces have a tendency to slump or shift before you get your vapor barrier or drywall up. If you don't have horizontal blocking in those narrow bays, you might want to throw a few pieces of tape or some poly over the bay just to keep everything indexed where it belongs. Usually, it’s easier to just cut one long vertical strip from a batt for those weird 2-inch or 3-inch gaps, but since you’ve already got the scraps in there, I wouldn't bother pulling them out. Just take a scrap piece of wire or a thin stick and poke around the seams to make sure there aren't any hidden pockets. As long as it's a continuous thermal layer, it’ll perform just as well as a single piece.

I was thinking about doing this with my leftovers too, but the slumping part has me nervous. If I stack them like that, do I need to worry about the seams between each scrap piece letting air through more than a single vertical strip would? Also, how can you really tell if it's too compressed? I'm worried my snug might actually be too tight since I'm new to this. Is there a trick, like if you can still poke a finger into it easily or somthing?

A good trick for the compression check is the 'two-finger poke.' If you can't easily push your fingers an inch into the material without hitting a brick wall of resistance, you've packed it too tight. It shoud feel like a firm sponge, not a piece of wood.

For the seams, you can 'knit' the edges together to stop air gaps. Just take your fingers and lightly tease the fibers at the joints where the scraps meet so they interlock. It basicly turns those seperate pieces into one continuous mat. I’ve done this on my last few basement jobs and never had any issues with cold spots later on.