The picture shows the inside of a flat roof which has been fibreglassed on the outside.
I removed the old plasterboard as part of gutting the whole of the top floor to re-do it from scratch. There was lathe and plaster below the plasterboard but no insulation between the rafters.
Should I now insulate this area in between the rafters or just re-platerboard it. If I insulate it with loft roll or sheet insulation I won't have any room to leave a ventilation gap between the insulation and the top of the roof. If there was a gap it would be 10-20mm max due to the restrictions I have with the current rafters. I can't extend the rafter depth due to the level of the ceiling and the window.