My extension ceiling will be 20mm celotex and plasterboard below joists and mineral wool full height between joists. The roof is open up to nearly the apex. Think flat roof on an anlge (12.6 degrees) Where in this should wiring for lights be run ? Will the BCO mind the celotex being pierced and slotted for wiring and light fittings ? Insulation as described gives U values of 0.165. Should I reposition the celotex above the joists, thus tending toward a warm roof ? (I'd then need to add vapour barrier under the plasterboard) Any thoughts ? Simon.