How do I protect and insulate a ceiling built against the roof?
It really depends on a lot of factors...namely your climate and how much space you have in the ceiling rafters. Ideally, you'd insulate with spray foam. That will give you the best r-value and act as a vapor barrier. You should be able to do it separate from the new roof but it may be a lot easier to just do it all at once. (It's a lot easier to toss lathe and plaster out the roof than it is to haul down bucket by bucket.