On the Pacific side of Japan, we can take advantage of rich solar energy in winter times to design the low cost passive solar heating system by the direct solar gain through glazing. To keep the room air temperature warm and comfortable only by the passive way, it is important to design the balance among heat gain, heat loss, thermal mass, and thermal mass area. First, we focused on PCM (Phase Change Material), light enough for Japanese wood structure houses to have greater heat capacity, and proved its performance by the actual space experiment. Based on this result, we made the numerical model to simulate the room air temperature with PCM. Finally, we proposed an easy evaluation method to find the better combination of the four, using Southern window area to Q-value charts with adaptable time rate estimation.
