Insulated plasterboard on ceiling

It wouldn’t meet regulations I don’t think, insulated plaster board maximum thickness is 75mm, I believe but don’t quote me PIR insulation is 2 times as effective as glassfibre insulation/rock wall, I know to meet regulation on a flat warm roof you need 150mm which is equal to 300mm of glass fibre. Also each sheet of 75mm insulated plasterboard is £85, that without fixings, plaster, and paint and labour it’s a very expensive way of insulating the loft, and very disruptive to the home. I would suggest 100mm glassfibre/rock wall insulation, loft legs then doing a cold roof insulation between the roof rafters, but make sure you leave 50mm air gap, you can over board with 25/50mm PIR to get better U value, if your rafters aren’t thick enough to accommodate enough insulation, and air gap, this would get you up to regulation, and be a bit cheaper and less disruptive to the home, then your first idea. To do the cold roof properly you need to install fascia/soffit vents and dry ridge system, or ventilation tiles personally I’d go with fascia/soffit vents & dry ridge system. You need this to help combat condensation build up in the cold space be hide the insulation. I’d advise booking a visit from a specialist in the field, who can help you figure the most cost effective, least disruptive solution to meet your needs and get you to regulation. Hope this helps Regards Frankie BT Roofing Services Limited