You can buy a product called Kingspan, that comes in 8'x4' sheets it is a dense foam sandwiched between foil sheets and is used in insulating between the rafters. it can be cut to fit nicley in betwen you're rafters easily with a handsaw or sharp knife. Also make sure you leave a 50mm ventilation gap between the tiles and the kingspan to allow air flow and prevent condensation. If there is not enough room for the air gap then batten out the rafters to give you the adequate space required.