Several researches related to airflow within wood-frame wall assemblies have been conducted in order to assess their hygrothermal behavior. However there are very few reports which investigate boundary conditions such as pressure difference and/or leakage area to predict the airflow rate. This paper describes the pressure difference that causes the airflow for vertical direction within the wall assembly. The pressure difference between the crawl space and the attic on an actual wood frame house were measured in the winter season. Additionally effects of several factors, such as air-tightness of connections of the wall, outdoor wind velocity, and temperature difference between indoor and outdoor air, were experimentally investigated. The experimental results finally indicated that the pressure difference can be approximately predicted using the temperature difference between indoor and outdoor air when outdoor wind velocity is less than 1.5m/s.