Passive Cooling

Passive cooling

To be comfortable, buildings in all Australian climates require some form of cooling at some time of the year. There are many ways you can design or modify your home to achieve comfort through passive (non-mechanical) cooling, as well as hybrid approaches which utilise mechanical cooling systems.

The most appropriate passive cooling strategies for your home — including orientation, ventilation, windows, shading, insulation and thermal mass — are determined by climate, so first identify your climate zone by reading Design for climate. You can then apply the more detailed advice here and in Passive solar heating.

All Australian climates apart from tropical (Zone 1) require some form of heating in winter, and this affects advice relating to cooling. The balance between summer cooling and winter heating should be adjusted for climate through appropriate passive design. Tropical climate buildings, which require year round shading and are subject to very different passive cooling principles, are discussed separately below.

Heat waves can affect large regions at the same time, causing combined household demand for cooling energy to ‘peak’ for a few days or weeks each year due to increased use of air conditioning or heat pumps for cooling during these periods. However, with careful design for passive cooling we may delay or eliminate this peak demand.

What is passive cooling?

Passive cooling is the least expensive means of cooling a home in both financial and environmental terms. Some level of passive cooling is required in every Australian climate at some time of the year. As cooling requirements are dictated by climate, distinctly different approaches to passive cooling are required for:

• hot humid climates (Zone 1) where no heating is required
• temperate and warm climates (Zones 2−6) where both heating and cooling are required
• cool and cold climates (Zones 7−8) where heating needs are more important.

Each climate is discussed separately below.

Cooling people

Factors affecting comfort for people (human thermal comfort) are outlined in Design for climate and include both physiological and psychological factors. To be effective, passive cooling needs to cool both the building and the people in it. Evaporation of perspiration is the most effective physiological cooling process. It requires air movement and moderate to low humidity (less than 60%). Radiant heat loss is also important, both physiologically and psychologically. It involves direct radiation to cooler surfaces.

Conduction contributes to both types of comfort and involves body contact with cooler surfaces. It is most effective when people are sedentary (e.g. sleeping on
a water bed).

Cooling buildings

The efficiency of the building envelope can be maximised in a number of ways to minimise heat gain:

• shading windows, walls and roofs from direct solar radiation
• using lighter coloured roofs to reflect heat
• using insulation and buffer zones to minimise
• conducted and radiated heat gains
• making selective or limited use of thermal mass to avoid storing daytime heat gains.

To maximise heat loss, use the following natural sources of cooling:

• air movement
• cooling breezes
• evaporation
• earth coupling
• reflection of radiation.