I have a flat roof cold roof.
Celotex PL4000 insulation board (60mm insulation 12mm plasterboard ceiling > roof joists (12" tapering void) vented via facia front and back for cross breeze > 18mm water bonded plywood > two layers of roofing felt.
The room obviously refusing to hold any heat. What would be the easiest, most cost effective way of insulating the roof space?