Parts 2 and 3 of a series of articles. Describes office buildings with natural and mechanical ventilation systems mentioned in part one, where the mechanical ventilation plus humidification and cooling is used only during the coldest parts of the winter and hottest parts of the summer. Provides comparative cost analysis for the building. Discusses design criteria to take account of fullor part time natural ventilation. Treats maximum room depths, window types,external and internal doors, furniture, room layouts. Gives examples of fresh air flows in different room configurations, control of operating conditions throughout the air with auxiliary ventilation.