The principles of natural ventilation in buildings are relatively few and straightforward, relying on wind, thermal buoyancy, or both as driving forces. There is, however, a whole range of subtle and sophisticated ways to take advantage of the natural driving forces to promote the ventilation principles. This is exemplified in a number of both new and old buildings that utilise natural driving forces for ventilation.

The utilisation of natural ventilation in modern buildings is almost without exception done in conjunction with a mechanical driving force that assist the natural forces in periods when they do not suffice. The combination of natural and mechanical driving forces is most commonly referred to as hybrid1 or mixed mode2 ventilation in the literature. We have, however, decided to use the term natural ventilation in this paper, even if auxiliary fans are installed in the buildings we deal with. The reason for this is that our focus is on the "natural part” of the ventilation system, and the consequences and possibilities this part has for the architecture.

We use three essential aspects of natural ventilation to describe and classify various concepts. The first aspect is the natural force utilised to drive the ventilation. The driving force can be wind, buoyancy or a combination of both (Figure 1). The second aspect is the ventilation principle used to exploit the natural driving forces to ventilate a space. This can be done by single-sided ventilation, cross ventilation, or stack ventilation (Figure 2).

The third aspect is the characteristic ventilation element used to realise and/or enhance the natural ventilation principles. These elements are characteristic for natural ventilation, and distinguish natural ventilation concepts from other ventilation concepts. However, natural ventilation can be realised without the use of dedicated ventilation elements. The building itself then doubles as a ventilation element. With such a building integrated element we understand that the building as a result of its design is capable of harnessing the natural

driving forces and of directing the ventilation air through its spaces without the need for dedicated ventilation elements. In this sense, a building integrated ventilation element is really not an element, but rather the absence of one. As the "ventilation system” and the occupants share the same spaces (rooms, corridors, stairwells et cetera), and windows and doors are utilised as part of the air-paths as well, the most characteristic feature of a building integrated element is that the building appears not to have a ventilation system at all. The main advantage with a building integrated element is that the ventilation system represents no additional use of space in the building. Ductworks, ventilation plants, and related components are avoided. The B&O Headquarters building is a good example of this approach.

Most naturally ventilated buildings do, however, make use of dedicated ventilation elements to harness the natural driving forces and to support the airflow through the building. An overview over the various elements, together with the ventilation principle the various elements is most likely to be associated with, is provided in Tablel.

In addition to the principles and elements above, the nature of the supply and exhaust paths is crucial to the architectural consequences and possibilities associated with a natural ventilation concept. With supply and exhaust paths we understand the air path the ventilation air travels between the outside and the occupied spaces inside a building, i. e. not the airflow path within the occupied zones. The supply and exhaust paths can be divided into two categories: local and central.

A central supply path means that one or several occupied zones are serviced by the same path. The ventilation air can be given different treatments along this path. It can be filtered, heated and cooled, and fans can be installed to surmount pressure drops in the airflow path. Thus, one single filter unit, one single heat exchanger, and one single fan can service the entire supply airflow. A central exhaust path means that used air from one or several occupied zones is collected and exhausted at the same point. When both supply and exhaust paths are central, heat recovery is easier to implement. An embedded duct and an atrium are examples of central supply paths. A staircase that serves as a stack is a central exhaust path.

As opposed to central supply and exhaust paths, local supply and exhaust paths have no distribution system associated with them. The air is taken into and exhausted out of an occupied space directly through openings in the building envelope. Openable windows and hatches in the fagade are examples of local supply and exhaust paths.