Understanding Passive Cooling
A quick look & key points
- Passive cooling means using design choices to reduce heat gain and increase heat loss.
- Buildings in all Australian climates require some form of cooling at some time of the year to be comfortable. Passive cooling is especially useful in hot and humid or hot and dry climates.
- Passive cooling can significantly increase your comfort and reduce your energy bills.
- It is best to use passive cooling design principles when building or buying a home. The main thing to decide is whether you will include any air-conditioning. In some climates, adding air-conditioning later may require fundamental changes to the home design.
- Many aspects of the principles (for example, shading, increased insulation, and window design and placement) can be used in home renovations.
- The main methods to reduce heat gain are to include good insulation levels, and shade windows and thermal mass in summer.
- The main methods to increase heat loss are to place and design openings to allow good ventilation, add ceiling fans or whole-of-house fans, and ensure any air-conditioning works well with building design and insulation. In climates with a temperature difference of 6°C or more between day and night, thermal mass can also be used to cool a home.
- Landscape and garden design can also play an important role in passive cooling
A closer look at the different areas that can support passive cooling designs.
What is passive cooling?
Passive cooling is where the building design and materials are used to control temperature in hot weather. To be comfortable, buildings in all Australian climates require some form of cooling at some time of the year, and this need is increasing with a warming climate. There are 2 basic components to passive cooling: cooling the building, and cooling people.
Cooling buildings is about:
- reducing heat gain (for example, by installing insulation and shading windows, walls and roofs)
- increasing heat loss and access to cooling sources (for example, by using earth coupling and encouraging air movement).
Cooling people is about:
- physiological comfort (the physical factors necessary for comfort; for example, encouraging breezes to evaporate perspiration and increase body cooling)
- psychological comfort (psychological factors that affect our perception of comfort, for example, levels of acclimatisation and air movement, radiation and conduction).
Why is passive cooling important?
Passive cooling is the least expensive means of cooling a home, especially in environmental terms. There are many ways you can design or modify your home to achieve comfort through passive cooling.
Passive cooling is becoming more important as our climate changes. Climate change will see our average temperatures increase, and extreme events such as heatwaves occur more often. With careful design for passive cooling, we can keep our homes comfortable and reduce energy costs.
The following advice on passive cooling applies generally in any climate zone. For specific design advice for your climate zone, refer to design for climate.
Achieving passive cooling
With passive cooling, building envelopes are designed to minimise daytime heat gain, maximise night-time heat loss, and encourage cool breeze access when available. Considerations include:
- designing the floor plan and building form to respond to local climate and site
- zoning living and sleeping areas appropriately for climate
- locating any air-conditioned rooms in thermally protected areas (ie highly insulated, shaded and well sealed)
- maximising convective ventilation with high-level windows and ceiling or roof space vents
- designing ceilings and positioning furniture for optimum efficiency of fans, cool breezes and convective ventilation.
Cooling requirements are dictated by climate, so different approaches to passive cooling are required for:
- hot humid climates (Climate zone 1), where no heating is required
- temperate and warm climates (Climate zone 2−6) where both heating and cooling are required
- cool and cold climates (Climate zone 7−8) where heating needs are most important.
All Australian climates apart from tropical (Climate zone 1) require some form of heating in winter, and this affects advice relating to cooling. Refer to passive heating to balance the heating requirements in your home.
Reducing heat gain
Heat enters and leaves a home through the whole building envelope – the roof, walls, floor and glazing. The internal layout — walls, doors and room arrangements — also affects heat distribution within a home.
Insulation
Insulation is critical to passive cooling. The National Construction Code requires minimum insulation levels for roofs, walls and floors, according to your climate zone and other building features. Choosing appropriate insulation products and paying careful attention to installation will help to maximise thermal comfort and prevent condensation.
You can also explore additional, less conventional, insulation options. For example, green roofs and walls can provide both insulation and shading.
Roof space ventilation
Well-ventilated roof spaces contribute to passive cooling by providing a buffer zone between internal and external spaces in the most difficult area to shade: the roof.
Ventilators such as whirlybirds can reduce the temperature differential across ceiling insulation, increasing its effectiveness. The use of foil insulation and light-coloured roofing limits radiant heat flow into the roof space. Always ensure that the ceiling is sealed against any draughts.
Shading of glazing
Shading of glazing is a critical element in passive cooling. Glazing is the main source of heat gain (through direct radiation and conduction), and of cooling (through cross, stack and fan-drawn ventilation; cool breeze access and night purging).
The following diagram shows why shading is so important, especially when sun shines on glass at a low angle, such as through east- and west-facing glass in the morning and afternoon.
Choosing windows with good thermal performance (for example, double glazing) will reduce the heat gain caused by sun hitting the window. But preventing sun from hitting the window in the first place will have a much larger effect.
In most climates:
- use horizontal (for example, correctly sized eaves) or adjustable shading on north-facing windows to block high-angle summer sun and allow in winter sun.
- use deep overhanging shading, or vertical shading if close to the window, for east- and west-facing glass.
Where adequate shading is not possible, such as in close proximity to boundaries, it may be necessary to specify glass with a solar heat gain coefficient (SHGC) as low as possible, and certainly no more than 0.20 (this means only 20% of the solar radiation will pass through).
Double glazing can assist in passive cooling, as its low conductivity reduces the heating effect of the hot outside air contacting the glass. The frame must also be considered, with light colours having less heat absorption, and frames made of uPVC, timber or thermally broken aluminium providing better insulation than conventional single extrusion aluminium or steel frames.
Landscape design
Landscape plays an important role in keeping neighbourhoods and homes cool. Its impact on managing urban heat is becoming increasingly recognised as the climate changes and urban development intensifies.
Outdoor spaces around your home can be a source of heat for your home. Gardens and green plants, rather than hard surfaces, will help to reduce the temperature of air moving over those surfaces and in and around your home. Plants and soil provide a cooling effect through the process of evapotranspiration, and plants can also be used to provide shade and funnel cooling breezes. Green roofs can also provide additional insulation to roofs.
Shaded areas around earth-coupled slabs can help to keep surface ground temperatures lower during the day and still allow night-time cooling. Poorly shaded surrounds can lead to earth temperatures exceeding internal comfort levels in many areas. In this event, an earth-coupled slab can become an energy liability.
