Damp on bedroom ceiling

All roofs need ventilation to stop your timbers rotting,vaulted ceilings will require solid board insulation leaving a 50mm gap between insulation and the felt for air flow. A good loft is a well insulated draughty loft.it stops your heat escaping & keeps your timbers healthy Good luck Alex

If the curved part of the ceiling is on the external wall, it sounds like the insulation installers haven’t pushed the insulation into the eaves. This creates a cold spot on the ceiling, with a black spot mould appearing on the ceiling below

I can understand the view that a good loft is a ventilated one. However; if you insulate between the rafters instead of laying material on the loft floor you won't be separating two environments. Icynene is patented and BBA Certified. The important thing is it has been approved by the Energy Saving Trust to encapsulate the total roof area without the need for a ventilation gap. This is unique, Icynene will let moisture pass through it and leave the building envelope naturally. Although it is already being applied to many new builds the general public and many trades people are unaware of the healthy alternatives on offer.

The answer offered created more questions and didn't directly answer his question. Answer is. You have to much insulation at the edges of your lift. The exsess insulation has stopped airflow. At the edge of your curved ceiling use a thiner layer of insulation allowing for airflow.

Excellent answer from ADR!