thanks for the explanation - I have not heard of anybody doing that technique before on the walls, its a bit like a warm roof on its side, no thermal bridge... but yeah, agree it doesn't seem to be text book for damp control.I made the timber frame with 95mm x 47mm studwork, 50mm celetex cut 8mm smaller in between then foamed up the gap (makes it totally air tight and structure rigid and gets over probs of bowed studs etc)
Then I foil taped to create a complete vapour tight structure, then used 18mm OSB board, then a continuous layer of 50mm celetex. 25mm battens and featheredge. I realise it's perhaps not correct as it puts the 18mm OSB in between celetex, but it allowed me to create an insulated wall with no thermal bridging.
I built a 500mm overhang over the front which has French doors and windows - I'm really pleased I did that, almost no rain reaches that elevation, very handy when going in or out.