This paper describes a natural ventilation system that incorporates a pitched roof constructed of Breathing Walls for use in a passive solar house. The Breathing Wall is expected to solve the inherent contradiction that exists for a building that is open to the outside air, in which a balance in the indoor air/moisture conditions is maintained, but energy efficiency is low. In other words, the use of Breathing Walls can provide a healthy and comfortable environment at higher energy efficiency in a passive solar house. In the present study, we experimentally evaluate the effects of roof pitch an the heat recovery capability and the occurrence of internal condensation within the Breathing Wall, using a simulation of the ventilation rate of a natural ventilation system. The effective opening area per unit area required to provide sufficient air change capability, i.e., 0.5 AGH, even for zero wind velocity, was determined to be 10 and 5 cm(2)/m(2) for pitches of 26.5 degrees (Pitch 1) and 45 degrees (Pitch 2), respectively. Furthermore, at the maximum predicted wind velocity of 10 m/s for Pitch 1/2, even though the outdoor air temperature is 5 degrees C, the temperature of the indoor-most air reaches 13.2/12.5 degrees C, corresponding to a heat flux of;approximate to 128/158 W/m(2). No internal condensation occurs, even at the maximum pressure differential, which corresponds to a wind velocity of 10 m/s across the wall of the Pitch 1/2 model, when a Breathing Wall having an opening area per unit area of 11.3 cm(2)/m(2) was installed in the ventilation system.