The energy savings of a building roof integrated with a phase change material (PCM) and different insulation strategies are presented in this paper. The proposed roof structure includes a concrete slab with a PCM layer and an air cavity insulation, which can be adjusted according to certain strategies. The adjustable insulation is devised to enable a reduced total heat gain throughout 24 h in summer days, thereby improving the performance of the PCM. The heat gain/loss through the roof with the PCM layer and adjustable insulation is compared with that of the roof with the PCM layer and fixed insulation during a typical year in Hangzhou, China. The effects and optimization of the melting temperature of the PCM are also explored. The simulation results show that, overall, the adjustable insulation can reduce the daily heat flux through the roof by approximately 20% compared with the conventional fixed insulation.