loft insulation question

Hi George Foil Backed Plasterboard utilises a thin foil backing to provide a vapour resistance and is not designed for insulation. The foil acts as a barrier that limits moisture diffusion. If you intend using your loft as a room, you really need to consider a properly designed roofing solution which will allow for vapour transference and insulation, ultimately this may involve re-tiling. If you're simply leaving your loft as a loft space and provided you have no water ingress and your ceilings are well insulated there should be no need to re-tile your roof. hope this helps.

From the description it sounds like insulated plasterboard. The roof is made up of tiles with a membrane breathable or felt underneath. I have seen these before and unless it’s a room I would not bother and just use regular loft insulation on the ceiling / floor.