DTM

hi mate, Im looking at similar calcs for my roof extension. I've been planning to approach it similar to your thick option B. but biasing most of my insulation to outside to insulate the studs better from cold bridging. And @john0wingnut and @PhilC is having PIR in a cavity any different to running it externally on a timber frame? I would expect with proper humidity control for it not to be a problem? Im planning to be adding a MVHR system to the house as a part of my extension. @southcoastseagull i like that you have presented multiple options but they differ in thickness and U value? makes it a bit odd to compare them.