This paper describes a basic design method which can apply in the first stage of planning the natural ventilation system. This method is aiming to design a thermal specification of building, such as thermal capacity and insulation, for achieving both of the energy conservation and the comfortable indoor environment by natural ventilation. At first, the temperature difference between indoor and outdoor under natural ventilation was modeled. Next, the effectiveness of the model was verified by numeric calculation and measurement results. And the coefficient of the model was statistically predicted using numeric calculation results. Finally, based on the examination of the total available time of the natural ventilation by the model, we discussed the relation between the effectiveness of the natural ventilation and the thermal characteristics of a building. It was shown that the proposed model was effective as the method to presume a thermal specification for achieving the aimed indoor thermal environment at an initial stage of system planning.
