Tips for preventing sagging batt insulation in ceilings

I went ahead and tried that compression method with the R49 batt insulation over my last patch, and I noticed a definite difference compared to my first attempt where I just gently nudged the stuff in. When you really wedge it tight, it feels like it locks into the framing, which is what I think is causing the natural shift you mentioned. I used a pry bar just a few times along the edges to really seat the material between the studs and the rafters. It was a lot of extra effort, and frankly, I thought it would make no difference because of the sealing advice. However, once I finished taping every seam and cut the batts to fit perfectly without any gaps, the ceiling felt completley solid. The drywall layer didn't just cover it; it felt like it was anchored against the whole assembly. I think the heat expansion and contraction cycles in my attic are way less likely to cause visible sag now.

I'd recommend taking the time to get those edges perfectly snug, because even a tiny gap where the batt meets the wood felt like it would give way eventually. The total effort is defintely higher, but knowing the ceiling won't start moving in a couple of winters makes the extra work worth it.

I think you gotta think about the wood itself too. If your joists are alredy warped or bouncy, stuffing the insulation in tight won't change that much long term. You're fighting the structure, not just the batt. I had this same issue once and figured replacing those few joists that were already bad was cheaper than dealing with constant micro-sagging every year. Honestly, if the framing's not solid, you're just applying a bandage. You might want to check your stud integrity before you go through all that extra effort on the bedding.

I don't think sealing the edges perfectly is going to prevent the problem entirely, like the last post suggested. Even if you get the material fully seated and the seams taped up perfectly, the entire structure is still going to shift. I've dealt with older construction where the house frame itself moves quite a bit with seasonal temperture changes. The insulation being tight only addresses compression; it doesn't stop the larger movement of the joists or rafters. If the frame shifts, the insulation is just shifting with it anyway.

I had this exact problem with a section that had some uneven framing. Even after I got the R49 pressed into the space, it seemed like it could still shift a little over time because of the gaps around the edges. What stopped the movement for me was adding thin foam tape around the perimeter before I covered it with the drywall. It filled in the tiny voids the batt couldn't reach and prevented the entire layer from creeping away from the framing. It's a small step but made a big difference in preventing that slow sag you're worried about.