Hi Tom from Tom
Not that I don't want OSB, just not on the inside. My proposed build-up is:
extg 6x2 rafters, blown-in Warmcel or sprayed expand-in-place Icynene between. Pbd and skim beneath, 9OSB over. On top of the OSB, 100 EPS, breather felt laid down-slope. laps clamped by down-slope battens (Mark Siddall's good idea - but not sure now that stands with BBA on watertightness), battens and slate.
No principal airtight membrane, because I'm suspicious of relying on a single membrane and the perfection of its taped joints forever. Instead, a series of failry-airtight things - the blown-in Warmcel with its gapfilling ability, the OSB allowing for imperfection in many of its joints, and the trapped-edge breather felt. Seems long-term robust.
The question is, the water vapour resistance gradient from inside to out. The main resistor, the OSB isn't on the inside but half way through the sandwich. At least it's not on the outside. Matbe I do need a stronger resistor on the inside - perhaps leaky vapour check plasterboard? I'm wondering whether a simple interstitial condensation check would be reliable to OK this build-up. So far I've found no solid technical guidance on the breathing wall principle, even from Excel, just rule-of-thumb/hearsay.
