Extension roof and electrics

ds1980

We've decided on having ceiling joists fitted in the new extension (pitched roof) as it made sense for us. We can insulate a number of ways but have mainly been thinking rockwool between and over the ceiling joists. BC have indicated this at 270mm. There seems to be so much differing info between cold and warm deck roofs that I begin to switch off but wanted to ask on people's preference and how we would go about fitting down lighters (probably led) with the above scenario as it seems pretty pointless insulating then leaving big gaps around them to prevent overheating. The ceiling joists are 6" so in theory there will be about a 3 inch gap between the ceiling and start of insulation if I insulate 100mm to top of joist then roll the other 170mm over the top but not sure that's enough?

There will be velux going into the pitch with a light tunnel from each. I will be insulating around the velux but is there any other insulating I need to do within the tunnel as it were and anything I need to make sure is done to prevent "cold bridging" when I sulating as a whole?

Ta