The choice of board you will use to insulate your roof/ceiling, is normally driven by the the U value that you are required to achieve . At a U value of 0.2 this equates to thickness if in excess of 120mm of PIR insulation board. This is normally achieved by placing a thickness say 80-100mm between the joists and the rest under laying the joists. The benefit of this is that it reduces, by insulating; the cold bridging that will occur from the cold timbers of the joists.
If you are insulating a cold roof, care needs to be taken to ensure that there is a gap between the insulation and roof felt to avoid the possibility of cold bridging and condensation.
Kingspan TP10, TW55 and TF70 (all the same insulation board) are high-performance, rigid PIR panels with a thermal conductivity of conductivity 0.022 W/mK. They are an ideal home insulation product for use as wall insulation, floor insulation, and pitched rooves and ceilings, and are the equivalent of Celotex's GA4000 range.