Are butt joints okay for Rockwool when furring strips won't come off?

Those butt joints aren't going to ruin the project as long as they are tight, but those gaps along the edges are a diffrent story. In an attic, even a small gap acts like a chimney for heat. You really want a **friction fit** everywhere. If you can't pull the furring strips, try to tuck the Rockwool behind them from an adjacent bay instead of cutting every single piece. It’s a bit of a workout, but it saves you from having dozens of seams that might leak air. For the wider gaps where the rafters are wonky, you absolutely need to fill them. Don't leave any visible wood or daylight. You can slice thin strips of the mineral wool to stuff in there, but don't compress them too tight or you lose the insulating properties. If the gaps are really narrow and annoying, a can of **low-expansion spray foam** is a solid fix. It’ll seal the air leaks better than a loose scrap of wool ever coud. Cutting this stuff is usually the most frustrating part if you don't have the right tool. Forget a utility knife; you need a long serrated bread knife or a dedicated insulation knife. Mark your lines about 1/2" wider than the actual space so the piece stays in place by its own tension. Use a scrap piece of 2x4 or a level as a straight edge to keep your cuts square. It makes those butt joints much cleaner and less likely to sag over time. Just make sure you aren't skipping the **air sealing** phase. Insulation stops heat transfer, but it doesn't stop air movement. Before you finish those baffles and cover everythign up, check that any wire penetrations or top plates are sealed with foam or caulk. Also, keep a mask on. Even though Rockwool is better than fiberglass, breathing in those mineral fibers all day is no joke.

I’m definitely switching to a bread knife because my utility blade is just chewing up the edges. When you cut it a half-inch wider, does it actualy stay up there on its own? I’m worried about pieces falling on my head while I’m trying to get the next one positioned.

Also, for those tiny gaps, does the wool hold together if you cut it into super thin strips? I’m finding that whenever I try to jam a small piece into a wierd corner, it just crumbles into a mess. I'd love to know if there's a trick to keep those little slivers from falling apart.

If you're worried about batts falling on your head, grab some insulation hangers—usally called "tiger teeth." They're stiff wire rods you wedge between the rafters to pin the wool in place. Even with that extra half-inch for friction, gravity can still win, and those wires keep everthing tight. For those shredding strips, they usually fall apart if they’re under an inch wide. I've had better luck using a serrated knife to "scallop" the edge of a full batt to fit around protrusions. Also, wear goggles. Mineral wool dust in the eyes is miserable when you're looking up all day.

If those thin strips are shredding on you, try hand-stuffing the gaps with 'fluff' instead. Just pull apart some scrap pieces and pack it in like pillow stuffing; it fills those irregular voids way better than trying to make a perfect cut.

If the wire hangers feel like overkill or you're struggling to get them to bite into that old-growth lumber, stapling some construction twine in a zig-zag pattern across the rafters works wonders. It holds everythign up while you're positioning the next piece and you don't have to worry about metal rods poking you while you work in those tight spots.