How do you seal wood ceilings to prevent dust from coming through?

I know this is an old thread but want to put this up for the benefit of anyone struggling with this problem. Our house is hundreds of years old. There are hand hewn chestnut beams and plank ceilings of tulip poplar. Above that, someone added blown insulation and then bats on top (versus cleaning out the loose fill first) in the attic and the rooms below are dusty beyond belief. We attempted caulking and I will tell you this is a *very* bad idea. Wood shrinks and expands with the seasons, and the caulking will fail. You will be left with a major clean up at some point and it won't keep the dust out. The only way to tackle this is to get above the ceiling and cover the area, then lay your insulation on top of that. You are likely getting dirt and insulation fibers as mentioned above and these are not healthy to breath. I attempted other "sealing" approaches, as well, and very sincerely, you need to tackle the problem from above, not below.

I am struggling with the same question in an old multistory building we have renovated for apartments (to brig life again to an old downtown). The floors above have been cleaned and sealed and the ceilings sealed with varnish but dust still comes through between the beams . I am looking for a paintable sealing solution on the ceilings. Any ideas?

I've chipped the plaster off my lath due to broken keys, (main floor living room, ceiling. Discovering that my lath was red oak, I decided to get them off the joists, pressure wash, sand and stain them. With the intention of putting them back up. Same issue major dust. I am taking this opportunity to shove some insulation on the exterior wall sides, in between the wall and plaster cove and what I this is called the sil joists. The space between the joists and wall...

I've decided to cut up old wool blankets to give a natural layer of fire retardation to run along the refinished lath. I'll throw some moth traps up there and I'll seal up the joists at the wall as well. I have used a wire brush on the joists vacuumed with a power spinning brush attachment and a regular brush. Most of my dust came from the plaster removal. However check any ducts for cracks, that would cause loads of dust. I also got bats... fun times.