Exchanging heated internal air with cool outside air is an obvious source of summertime cooling. In many climates, however, the outside air temperature can often be close to or slightly higher than the current the indoor temperature.

This does not necessarily preclude the use of natural ventilation as increased air velocities past the body can actually increase the effectiveness of sweating, cooling the body down more than if surrounded by still air at a lower temperature. As usual there are limits to this and in very hot climates it is often necessary to prevent any form of ventilation during the heat of the day. However, there are ways of increasing the effectiveness of natural ventilation and cooling the air before it enters the building.

Figure 1 - Designed for natural ventilation. Section through Simpson-Lee House, by Glenn Murcutt.
Drawn by craines.

In order for air to move through or around a space, there needs to be some driving force. One often sees the obligatory blue and red arrows indicating the movement of air through an architect's latest design. Unfortunately, these arrows seldom make it into the final building, leaving the air confused and unsure of where to go next. It is then left up to natural forces to direct the air through the building.

The building form and construction determines the relative strength of these natural forces. This basically comes down to the size and location of air inlets and outlets as well as any ability to capture or funnel prevailing breezes. The building form can be designed to enhance ventilation, using atria, narrow building depths, open plan environments, massive concrete structures, sun-assisted chimneys, wind-wings and twin facades. In hybrid systems, motorised windows can be used as active regulators to achieve control over air change rates and heat load reduction.

Inducing Air Movement

Air will move only when it is pushed, pulled, heated up or cooled down. In a passive design, the pushing and pulling has to be done by the prevailing wind, whilst the heating and cooling can be done by solar radiation, evaporation and/or thermal mass.

Figure 2 - Principle passive ventilation pathways in buildings.

Wind-Driven Ventilation

Air can only be pushed and pulled by producing localised areas of high or low pressure. Thus building form is fundamental to any wind-driven natural ventilation system. Anything that diverts or changes the path of the air will act to impede its flow. This impedance is significantly higher if the air is forced to move upwards or downwards to navigate a barrier without any corresponding increase or decrease in temperature.

Wind causes a positive pressure on the windward side and a negative pressure on the leeward side of buildings. To equalise this pressure, outside air will enter any windward openings and be drawn out of leeward openings.

In summer, wind is usually used to supply as much fresh air as possible while in winter ventilation is normally reduced to levels sufficient only to remove excess moisture and pollutants.

Stack-Effect Ventilation

Buoyancy results from differences in air density. The density of air depends on temperature and humidity. Cool air is heavier than warm air at the same humidity and dry air is heavier than humid air at the same temperature. Thus, heat and humidity given off by occupants and other internal sources tend to make air rise. The stale, heated air escapes from openings in the ceiling or roof, drawing fresher air in through lower openings to replace it.

Thermo-Syphon Effect

Operating in much the same way as the stack effect, a thermo-syphon makes use of direct sunlight to warm the air in a building. This requires a large amount of equator-facing glass. Dark surfaces beneath the glass absorb the direct sunlight, increase in temperature and re-radiate long-wave infrared radiation heat back into the enclosed space. As glass is opaque to long-wave infrared radiation, the heat energy is trapped within the space and eventually absorbed by the air. This is basically the greenhouse effect at work.

If allowed to vent out the top, the heated air will rise - drawing new cooler in at the bottom. This causes quite a strong convection current within the building. By not venting the warm air out the top, but letting it move through internal vents, this convection current can be used for heating in winter, even on a relatively cloudy day.

Variations of this technique include solar chimneys and atrium spaces.

Cooling the Inlet Air

If the inlet air can be cooled before it is drawn into the building, natural ventilation can become an option even in climates with relatively high daytime temperatures. This cooling can be achieved passively by:

• Evaporative Cooling
  If the inlet air is taken from the shaded pole-ward side of the building, and is drawn first through large areas of vegetation or over a cooling pond, it can sometimes be several degrees cooler than the ambient outside air temperature.
• Geothermal Cooling
  It is also possible to cool the inlet air by drawing it through underground pipes or through an underground plenum. The idea is that the air loses some of its heat to the surfaces over which it passes. Underground, these surfaces tend to be at roughly the annual average temperature, providing cooling in summer and possibly also warming in winter.

Many early versions of geothermal cooling used rock stores or gravel beds for their thermal storage capacity, however the pressures required to draw air in sufficient quantities through these systems was quite high, often requiring a powered fan or pump. However, more recent examples have used large open plenums with only minimal obstruction to similar, though slightly less effect.

Night-Purge Ventilation

In a building with high levels of exposed internal thermal mass, it is possible to open up natural ventilation pathways throughout the night in order to cool down the thermal mass. Early in the morning, the building is closed and sealed up throughout the day. The cool internal surfaces absorb heat from any infiltrating warm air and act to lower the internal mean radiant temperature.

To work effectively, the mass must be exposed. This means not obscuring floors with carpets and coverings, or walls with cupboards and panels, or ceilings with acoustic tiles or drop-panels. It also means a relatively unobstructed interior to promote flow.

Such systems are only suitable for climates with a relatively high diurnal range, where night-time temperatures are below about 20-22°C but not too cold as to be uncomfortable. There are also the obvious issues associated with security when windows or vents are left open at night, however these can usually be overcome.