The heat loss of the building enveloped with the electrochromic device was measured in an acrylicenclosed chamber with cooling/heating water recirculation. The setup of this experiment is described in Figure . The surface temperature is controlled by the top heater. To eliminate the lateral heat loss, the temperature guard heater is heated to have the same temperature as the top heater. Both top and guard heaters are connected to a DC power supply (Rigol, DP821A). The temperatures of two heaters are measured by two thermal couples, collected by the DAQ module (MC Measurement Computing, USB-2408). Such a process is achieved by the PID control system, coded by LabView. By doing so, the heat will transport vertically in a 1D direction. The building envelops sample, heaters, and vacuum insulation panel are attached by thermal paste. The ambient temperature was controlled by recirculating water pipes. To simulate the real building in all seasons, we choose five ambient temperatures: 10 °C, 15 °C, 20 °C, 25 °C, and 30 °C. For these ambient temperatures, we set the surface temperature to be 23 °C, 27 °C, 30 °C, 33 °C, and 37 °C. Since the building surface temperature is always higher than the ambient temperature, the building will lose heat to the environment. The measurement is conducted under the thermal equilibrium (wait 10 mins after the initial temperature raise peak), so the heat loss through the building envelope is equal to the power density of the top heater.