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?