Blown-in cellulose insulation for a flat roof

Okay, thank you very much! I had another idea yesterday: how about opening the ceiling from the inside across the beams by about one meter each (maybe less would be enough, it would be a question of handling) and inserting insulation wool into the existing lower shell of the ceiling? It would be a kind of horizontal insulation between the rafters. Even with 20cm insulation thickness, there would still be 10cm of ventilation above it, so no vapor barrier would be necessary. It would definitely be much less invasive than removing the entire lower shell to install a vapor barrier. Additionally, in my opinion, it is the technically cleaner alternative.