A building’s natural ventilation can be based on a variety of different ventilation principles. A ventilation principle shows how natural ventilation will function based on the design of the building, internal thermal loads and the positioning of openings (typically windows).
All natural ventilation principles are based on the principle of ensuring healthy and comfortable indoor climate through minimal energy consumption and at minimal cost.
In general, our ventilation systems are designed around the following three fundamental principals:
The following illustration shows a typical situation in an office with single-side ventilation, i.e. a room with windows on only one side. The example shows the room during the winter months. The surrounding air is often cold in winter, meaning that windows cannot be opened for longer periods. To overcome this problem pulse ventilation is used. Windows are opened for short periods of time ensuring that the air in the room is replaced quickly.
Because cold air creates draughts even at very low wind speeds, the windows are quickly closed again after a set period of time. High wind speeds and low outdoor temperatures further limit the amount of time that the windows are open.
The following illustration shows the cross-ventilation principle. Cross-ventilation is achieved using windows on both sides of the room, creating a current of air across the room. If the windows on both sides of the room are open, the overpressure on the side of the building facing into the wind and/or low pressure on the opposite sheltered side will create a current of air through the room/rooms from the exposed side to the sheltered side. To ensure optimal airflow with as few draughts as possible, the windows on the side of the building that is facing the wind are not opened as much as the windows on the sheltered side.
The following illustration shows the stack effect that arises as a consequence of temperature differences. Warm air rises because it is less dense than cold air. When warm air rises to the roof of a building it creates a slight vacuum in the building's lower levels, which in turn pulls fresh air in through windows in the ground floor. This creates a natural airflow. This physical process depends on the height difference between the windows that are used to let outdoor air in and the windows used to exhaust 'used' air.
Windows in the roof are used to let the 'used' air escape and the windows in the lower levels are used to let fresh air into the building. In the illustration, the stack effect is combined with the wind direction. Wind direction determines which windows are used to let air in and which windows are used to exhaust air from the building. The ground floor windows on the sheltered side are opened more than the windows on the wind-exposed side, whereas only the windows in the sheltered side of the roof are opened.