I am hanging my head in despair Shavey. Okay, to be fair, the assumption was that you were doing the roof, and we then find you're doing the walls, but the principles the same. You need the joists the same depth as the celotex - end off. You've got 72mm joists, or as it's a wall, the uprights, but you're using 100mm celotex, so you need 100mm of wood. As it's a wall, you don't need to use the ali tape, as you're not bothered with heat loss from one room to the next.