Nowadays building energy efficiency is one of the prime objectives of energy policy at regional, national, and international scales. Among building services, the growth in energy consumption of heating, ventilation, and air conditioning (HVAC) systems is particularly significant. A promising approach to reduce the energy consumption of the HVAC system is the reduction of heat fluxes through the building envelopes. In this study, a design of double roof is proposed to reduce the heat transferred into the building. Computational fluid dynamics (CFD) is used to investigate the heat transfer at the building roof. First, it is determined how the key parameters, such as the roof inclination angle, the distance, and the environmental wind speed could affect the behavior of the roof design. Then, based on the simulation results, the optimal design which could save the most energy is determined. Finally, a building experimental model with double roof design is established to validate the simulation results. This study shows a well-designed double roof could greatly reduce the energy consumption of the building.