I'm after some advice. I've built an outbuilding that has insulated cavity walls, and I now want to insulate the (pitched) ceiling between the rafters with celotex - then plaster board over that. On top of the rafters I have a breathable membrane, then battens and tiles. Under the membrane I believe I need to leave an air gap. However, there's going to be wiring running around the building, lighting and sockets etc. (I'm going to use an electrician) My question is this: Do I employ the electrician first - so that the wiring runs in the air gap, or do I fit the celotex first, leaving 50mm or so, enabling the electrician to then run the wiring under the celotex - and then plasterboard over that?