Airflow windows (AFWs) help to stabilize the thermal environment in the perimeter zone and also allows for switching between solar-shielding and solar heat acquisition operations. This is based on the method of channelizing the air discharged from its cavity layer, either by using a total heat enthalpy exchanger and choosing to exchange heat or otherwise. It has a certain thickness owing to the blinds that are installed in its cavity layer. The purpose of this research is to develop a window system that uses electrochromic (EC) glass, instead of blinds, as a solar-shielding device. This study also provides the specifications of the window system of reduced thickness. The performance of the window system is assessed by CFD analysis. The solar heat gain coefficient (SHGC) of the proposed window system is found to range from 0.083 to 0.268 and its solar radiation properties can be controlled in a flexible manner. These characteristics contribute to an improvement in the building environmental performance in the form of climate-adaptive building shells (CABS) in places with four distinct seasons such as Japan that require solar-shielding to reduce the cooling load in summer and solar heat gain to reduce the heating load in winter.