The choice of board you will use to insulate your roof/ceiling, is normally driven by the the U value which 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 form 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.
