Flat roof adding insulation from inside

[Berniethebolt]

I have a small extension which had a flat roof I had a pitched roof fitted some years back after a leak. The old flat roof was left in place but a small section was cut away to allow the timbers for the pitched roof to be installed. I would like to add more insulation as am have the ceiling skimmed I thought I could cut some plasterboard away and feed some more insulation in from inside. I have cut away a section to see what is already there and there is 65mm loft insulation which is attached to the old roof timbers above joist level. I was wondering what best to add between the joist should I put in more loft insulation or would it be better to feed in some kingspan insulation boards. I am also wondering about air gaps should I fit new insulation so it sits on the plasterboard leaving an air gap above or up to the old insulation leaving an air gap below. Please advise.

 

[Londoner2]

You dont say if its a cold roof or a warm roof, if cold roof then it needs ventilation gap, 65mm is insufficient, i would aim for 150mm but only if its a warm roof.