Blown-in or batts when the eaves are basically impossible to reach?

I went the batt route on my last project, and I really think the concerns you are having about coverage are valid, but they don't align with how batts perform in those tight spaces. My attic has those same low, irregular joists, so fitting batts was a real wrestling match, but once it was set, I didn't have to worry about localized gaps or settling later.

When you use blown in material, you are essentially filling a volume with air, which is why it seems like it's covering everything. But you often end up with big pockets of under-filled spots behind vents or around the corners of the rafters, expecially if the blower doesn't reach that tight gap perfectly. Batts force you to address each area individually, ensuring the full 30 inches of thickness is actually present from end to end. Plus, you can just lift a section up to tuck insulation into those gaps around the ductwork, which is much cleaner than trying to blow material into a narrow void.

In my experience, the argument about future access often outweighs the immediate inconvenience of installation. I had a similar project on a ranch where the ceiling height was only maybe six inches, and tried forcing batts in between irregular joists. Even when I got the material in, the framing geometry prevented a truly tight seal along the edges. Eventually, the material shifted slightly, and I felt noticeable drafts, especially near the electrical panel on the far wall. That’s why I lean toward the blown-in option, specifically cellulose. The idea that it will become a mess during maintenance is a genuine concern, but you have to weigh that against the fact that a poorly installed batt system might be completely ineffective right from the start. When you use blown-in material, it expands to fill the voids between the structural members, creating a seamless thermal envelope that batts just can't replicate in tight, non-standard cavities.

For that reason, I prioritized air sealing and consistent R-value right behind the joists, even if it meant the attic became much harder to work in later. If the primary goal is achieving the maximum possible thermal performance in a difficult location, the blown option handles the irregular framing better and provides a more consistent performance.

I think the access argument is a common one, but for me, the key was when I had to run lines for security cameras and speakers in an older crawlspace. I found that using the specialized nozzle setting on my blower made all the difference. It wasn't the bulk of the material, but how focused the flow was. It pushed and sealed around those thin utility gaps perfectly. When I was dealing with a really tight joist space, that high-pressure stream really held its own without leaving a lot of voids behind the fixtures.

I went the batt route on a project where the joists were pretty irregular. What I learned is if you just shove the material in, it’s going to shift and settle down unevenly. You really need to push the batt into the gap between the wood a little bit as you fit it. The key for me was cutting strips and wrapping them around the perimeter of the rafters first. This stops the big sheets from sliding around once they are up there. Blown in material always felt like it needed another pass to make sure it wasn't just sitting loosely.

I used rock wool since I had some leftover, but the physical fitting process was easier that way than trying to blow it perfectly into every single corner. It made a huge difference in how solid the ceiling felt when I walked under it.

I agree with the point that batts are better because they require you to address each space individually. When I did my insulation job, I ran into a tricky situation where a plumbing penetration went right through a joist. Even after carefully shoving the batt around the pipe, there was a small gap around the edges. I ended up using some canned spray foam just to fill that tiny void around the perimeter of the insulation and the pipe. It just made the final coverage look way more solid.

I'm trying to figure this out for a project that's a bit diferent since my ceilings are pretty low and the subfloor is rough. I've been reading about air sealing, and that's what I'm worried about most. Even if the material covers the whole area, I worry about drafts coming right through the gaps between those irregular joists. Do you think the blown material is just pushing air through the spaces, or is it actualy creating a complete thermal break along the perimeter? I'm hesitant to just assume the coverage is enough.