I am about to install 2 downlights (GU10s) in a pitched roof/ceiling that is almost fully filled with Kingspan insulation board. How far back should the insulation board be cut for a GU10 fixture? Obviously it's a trade off between thermal performance of the ceiling and the amount of space needed to cool the fixture. Anyone know any figures for this? The ceiling is plasterboard/40mm Kingspan over rafters/90mm Kingspan between rafters/50mm air gap/felt/clay tiles. I actually only want to fit energy saving GU10s, but I guess I'll have to make the fixture ok for 50W GU10s to satisfy Mr Building Inspector!