I am currently performing the mental planning of a single-storey kitchen extension which will have a pitched roof and ceiling. With reference to the following graphic showing Celotex insulation between rafters where and how should lighting and other ceiling-related cables should be run?

http://www.encon.co.uk/sites/default..._rafters_0.jpg

Specifically, am I right in thinking that they should be run on warm side of the insulation? If so, above or below the vapour barrier? Should you ensure that there is a small gap between the insulation and plasterboard or could this air gap undermine some of the insulating properties of the setup?

Presumably recessed downlighters would be out of the question given the heat/space issues without building a new ceiling suitably spaced from the structural ceiling/roof?

On a related note, what are the benefits of adopting an insulation between AND under the rafter approach?

http://www.encon.co.uk/sites/default..._rafters_1.jpg

Is it simply that more insulation can be used for a given rafter depth?

Mathew