Having watched many of your excellent videos, I decided on a cold roof because of thermal bridging issues. I must disagree with a comment made at the end of this video based on theoretical knowledge only i.e. no access to thermal imaging equipment. The joist hangers would still be a source of thermal bridging on a warm roof because the wall they are attached to is an external one. As hot goes to cold, the room heat would travel into the plasterboard, then the joists, then the joist hangers and then to the brick and then out to atmosphere. The house extension I have constructed has 150mm of XPS insulation in between the joists, 50mm of EPS insulation underneath the joists, an unvented 50mm air gap and then the plasterboard. There is also no thermal bridging from the block walls as they are all covered with 50mm EPS bonded to the wall and then bonded to the plasterboard with PU adhesive. In my opinion, this is about the only way you can cut thermal bridging to its minimum value with todays construction methods. Just as a FYI, I chose XPS because its LTTR (Long Term Thermal Resistance) figures remain fairly constant after decades of use whereas PIR boards e.g. Celotex, Kingspan etc start dropping from the day you buy them and start to match XPS/EPS after 5 years or so. In short, in less than a decade, your U values would NOT pass todays building regulations. On the plus side for XPS it has twice the compressive strength as PIR boards, not affected by water and is cheaper in the long run.