"finding z0" wrote:

The repair was combined
with re-siding several months later, and the insulation was never
replaced (my bad). I can confirm this in both winter and summer by
reaching up and feeling the temp differential in that one 16"? run in
the ceiling. I can touch the ceiling from the loft. I'm thinking about
cutting a wide hole through the ceiling sheetrock and blowing some
cellulose into that run. How doable is this (inside the house), and
what alternatives are there?

You might want to consider just buying a couple of cans of expanding foam and
drill some small holes in the drywall. Read the label to figure out how much you
need - overfilling is not a good idea.