Currently have double layer plasterboard on ceiling in garage but bedroom above is freezing cold, so I’m going to put some insulated plasterboard 35mm thick on top and get it skimmed again I’m going to board it out but what size screws should I use please? and is the GTEC XP thermal plasterboard 35mm a good option?
You would be better off pulling the lot down, insulating between the joists properly and boarding the ceiling with one layer of fireline plasterboard, then skim.
I did think about that but it's a very big job and more expense getting rid of the waste, there is 100mm insulation between the joist but it's still very cold Just thinking insulated plasterboard will be my best approach just wasn't sure on screw size