We have a 138 year old home with dropped plaster ceilings probably added in the 1950's. The ceilings are in excellent condition and have about 6" of blown-in cellulose between the joists. I want to add more but a roofer I know suggests to do so is worthless because there is no ventilation directly to the outside. The ceiling cavities are indirectly open to the unheated attic above, which is properly vented and are a cold space during the winter months.

Is the roofer nuts? A similar area I insulated a few years ago appears to be doing just fine. There is no vapor barrier and installing one retroactively is out of the question. The cellulose seems to wick water vapor away adequately.