One third of the building's energy is lost from the windows, not only through the window glass, but also through the heat conducting window frame. However, through the proper and reasonable design of the window frame structure, the energy loss can be reduced to the minimum. The windows with rigid polyurethane foam plastic window frame can keep 2 times the thermal insulation performance of the traditional wooden frame windows. If the north to west window is changed into single frame and double glass, the outer wall will be directly sprayed with 30mm thick rigid polyurethane foam insulation. After this treatment, the energy efficiency of the building can be increased from original 2.4-2.9 to 5.8-6.0.