If you have a house with a massive Leca structure and then wooden trusses with battens in the ceiling, how do you make the plastic airtight against the outer walls made of stone? In wooden houses, the plastic goes down the walls and you tape the seams, but you probably can't tape against a stone wall?
You won't get it as clinically sealed as when you have taped against plasticized walls, so I understand your question. Mount the plastic on the ceiling and pull it down a bit on the wall. Then clamp it against the stone with the battens and trim away the excess. Once you have installed plasterboard and plastered the walls, it is as good as it can be with that construction solution, and it works well. You want to prevent humid air from getting into the attic/roof construction, and you have achieved that. I don't know what the rest of the house looks like, but if you have a ventilation system with a calibrated negative pressure, the plastic acts more as a brake to prevent cold air from entering from outside (more expensive heating) rather than as a barrier to prevent humid air from damaging your roof construction.