Passive cooling

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.

Photo: Simon Wood Photography

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.

Earth coupling

Ground and soil temperatures vary throughout Australia. Earth coupling is where concrete floors (and sometimes walls) are in direct contact with the earth. In a well-insulated properly shaded house, this ‘draws up’ the stable deeper ground temperatures to the surface of the floor, which gives the house a head start in regulating temperature.

Earth-coupled concrete slabs-on-ground are effective for passive cooling where deep earth temperatures (at a depth of 3 metres or more) are low, such as in most of southern Australia. This strategy should be avoided in climates where deep earth temperatures contribute to heat gain, such as in most northerly latitudes where suitable ground temperatures do not occur at useful depths. In these regions, use open vented floors with high levels of insulation to avoid heat gain. Obtain expert building design advice to determine deep ground temperatures.

Note that slab edge insulation (shown on the right of the sketch) improves the coupling effect by keeping the slab temperature controlled right to the edges.

Thermal mass

‘Good thermal mass’ usually describes a block of material that has high thermal mass and long thermal lag times.

Thermal mass is usually used to help heat your home in winter (by allowing the sun to heat up the mass during the day and release that heat at night). But it is important to ensure that thermal mass in your home is protected from summer sun. Badly positioned mass heats up and radiates heat well into the night.

Shade thermal mass in living areas during the day in summer, and avoid or limit thermal mass in upstairs sleeping areas. In climates with little or no heating requirement, low mass is generally the preferred option.

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.

Source: Adapted from COOLmob

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.

Source: Adapted from Association of Building Sustainability Assessors

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.

For hot humid climates (Climate zone 1), maximising shading for all aspects will be useful.

 

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.

Increasing heat loss

Sources of passive cooling can help to remove unwanted heat from the home. These sources are more varied and complex than passive heating, which comes from a single, predictable source — the sun. Additional mechanical cooling may be required in hot humid climates and in extreme conditions in many climates, especially as climate change leads to higher temperatures during the daytime and overnight.

The key to most sources of passive cooling is air movement. Air movement cools buildings by carrying heat out of the building and replacing it with cooler external air. Moving air can also carry heat to mechanical cooling systems where it can be removed by heat pumps and then recirculated. Air movement also cools people by increasing evaporation of perspiration.

Cool breezes

Where the climate provides cooling breezes, maximising their flow through a home is a key component of passive cooling.

Cool breezes work best in narrow or open-plan layouts and rely on air-pressure differentials caused by wind or breezes. They are most effective in:

• buildings with narrow floor plans or open-plan layouts
• locations without long periods of high external temperature (ambient or conducted heat gains above 35–40 watts per square metre)
• locations where windows can be left open (that is, secure, quiet locations with good outdoor air quality).

 

Design and locate planting, fences and outbuildings to funnel breezes into and through the building, filter stronger winds, and exclude adverse hot or cold winds.

 

Sea breezes flow as the day’s heat warms the air further inland which rises, drawing in cooler air from the ocean. Common examples are the famous ‘Fremantle Doctor’ and the Sydney ‘nor-easter’. These can be very localised, sometimes extending only a few hundred metres from the coast, but at other times extending hundreds of kilometres inland.

In mountainous or hilly areas, cool breezes often flow down slopes and valleys in the late evening and early morning, when heat radiating to clear night skies cools the land mass and creates cool air currents. These ‘katabatic drafts’ can be used to help cool buildings, but must also be held at bay in cooler weather.

Thermal currents (‘thermals’) are common in flatter, inland areas, created by daily heating and cooling. They are often of short duration in early morning and evening, but can yield worthwhile cooling benefits with good design.

 

Cool night air

Regardless of daytime temperatures, if the night sky is clear of cloud, then the air, ground surfaces and buildings will radiate heat simply because the sky is now cooler than the ground – this is called ‘night sky radiation’. Cloudy skies at night hamper this effect.

Cool night air is a reliable source of cooling in inland areas where cool breezes are limited and diurnal temperature ranges usually exceed 6−8°C. Hot air radiating from a building fabric’s thermal mass is replaced with cooler night air, drawn in by the internal−external temperature-pressure differential. Full-height opening windows are ideal for this purpose.

Convection

The rule of convection is that warm air rises and cool air falls. Stack ventilation relies on the increased buoyancy of warm air relative to the higher density of cooler air. The lighter warm air rises and if allowed, will escape the building through high-level outlets (windows or vents), drawing in lower-level cool night air or cooler daytime air from openings in shaded external areas (typically on the south side).

 

Stack ventilation can increase cross-ventilation and overcome many of the limitations of unreliable cooling breezes. Even when there is no breeze, convection allows heat to leave a building via controllable openings such as high clerestory windows, roof ventilators or vented ridges.

Convection produces air movement that can cool a building but usually has insufficient air speed to cool people.

Evaporation

As water evaporates, it draws in heat from surrounding air. Evaporation is therefore an effective passive cooling method. It works best with low relative humidity (70% or less), because air with lower humidity has a greater capacity to take up the evaporated water vapour than air with high humidity. Rates of evaporation are increased by air movement – thus breezes or fans can increase evaporative cooling.

Large bodies of water close to windows and doors can pre-cool warm air entering the house. Carefully located large water features can create convective breezes. The surface area of water exposed to moving air is also important – small water features may not provide a noticeable effect. Fountains, mist sprays and waterfalls can increase evaporation rates in dry climates, but in humid coastal climates their effect will be less noticeable.

Mechanical evaporative coolers have been common in drier climates and inland areas where relative humidity is low. They typically use less energy than refrigerated      air-conditioners, but their water consumption can be considerable. They work better with doors and windows left open and can therefore contribute to poor indoor air quality if external air quality is low (for example, during periods of bushfire smoke). They are also prone to becoming clogged with mineralisation in hard-water inland areas. Generally, high-efficiency reverse-cycle air-conditioning has proved more effective and efficient than evaporative coolers.

Cooling methods

Windows

Design windows to maximise beneficial cooling breezes by providing multiple flow paths and minimising potential barriers; single-depth rooms are ideal in warmer climates.

Most sites have a limited range of directions from which beneficial cooling breezes blow – these should be identified before design decisions are made. Breezes can be deflected or diverted, so perfect orientation to breeze direction is less important than the actual design of windows and openings to collect and direct breezes within and through the home.

Wind does not actually ‘blow’ through a building — it is sucked towards areas of lower air pressure. To draw the breeze through the house, use larger opening areas on the leeward (low pressure or downwind) side of the house and smaller opening areas on the windward (high pressure or upwind) side. Openings near the centre of the high-pressure zone are more effective than openings on the edge, because pressure is highest near the centre of the windward wall and diminishes toward the edges as the wind finds other ways to move around the building.

 

The design of openings to direct airflow inside the home is a critical but much overlooked design component of passive cooling. Size, type, external shading and position of any openings (doors and windows) is critical.

Position windows (vertically and horizontally) to direct airflow to the area where occupants spend most time (for example, dining table, lounge or bed).

In rooms where it is not possible to place windows in opposite or adjacent walls for cross-ventilation, you can place projecting fins on the windward side to create positive and negative pressure to draw breezes through the room, as shown in the following diagram.

 

Consider installing a louvre or awning window above internal doors to let breezes pass through the building while maintaining privacy and security. In climates requiring cooling only, consider placing similar panels above head height in internal walls to allow cross-ventilation to move the hottest air out of the building.

Ceiling or personal fans

Fans provide reliable air movement to cool people and supplement breezes during still periods.

In high humidity, low fan speeds are very effective: 0.5 metres per second at 50% relative humidity reduces the perceived temperature by 3°C. As temperature rises and humidity falls, higher fan speeds are required. However, this may increase noise and some people find high-speed fans unsettling.

In a lightweight building in a warm temperate climate, the use of ceiling fans in bedrooms and all living areas (including kitchens and undercover outdoor areas) significantly reduces cooling energy use.

In air-conditioned buildings, using ceiling fans in conjunction with the air-conditioning will increase its effectiveness.

Fans should be located centrally in each ‘use area’ of a room. Because air speed decreases with distance from the fan, position fans over the places where people spend the most time. Large areas, such as a large combined lounge and dining area, may need 2 fans – one over each space. In bedrooms, locate the fan close to the centre of the bed.

Source: Adapted from Ballinger 1992

Whole-of-house fans

A whole-of-house fan is a single fan unit installed in a circulation space in the centre of the house (hallway or stairwell) to draw cooler outside air into the building through open windows and out through the roof space. It exhausts the warm air through eaves, ceiling or gable vents. This also helps to cool the roof space and reduces any temperature differential across ceiling insulation.

Whole-of-house or roof fans are good for cooling buildings, particularly where cross-ventilation design is inadequate. They do this by exchanging all of the air in the house many times every hour, assuming there is sufficiently cool outside air to provide cooling. However, they do not create sufficient air speed to cool people.

Source: Breezepower

Whole-of-house fans can be noisy at full speed but are generally operated in the early evening when cooling needs peak and households are most active. If run at a lower speed throughout the night, they can draw cool night air across beds that are near open windows, provided doors are left open for circulation. On still nights, this can be more effective than air-conditioning for night-time sleeping comfort.

These systems are used when external temperatures are lower than internal temperatures and should have controls to prevent the fan operating when external air temperatures are higher than internal. These systems work well as long as all openings are air-tight in cooler seasons. Drawing large volumes of humid air through the roof space can also increase condensation, therefore this should be considered when specifying and installing insulation.

Air-conditioning

Air-conditioning lowers both air temperature and humidity and provides thermal comfort during periods of high temperature and humidity. Air-conditioning can be useful in extreme heatwaves, and when passive ventilation techniques are hampered by poor external air quality such as bushfire smoke.

However, air-conditioning is expensive to install, operate and maintain, and has a high economic and environmental cost because it consumes significant amounts of electricity unless high-efficiency equipment is used in a high-performance building envelope.

Set your air-conditioning thermostat to the warmest setting that still achieves comfort – each degree cooler will increase your energy needs by 10%. In general, summer temperatures should be set between 25°C and 27°C.

There is usually no need to air-condition all rooms in your home. Decide which rooms will receive most benefit, depending on their use, and try to reduce the total volume of air-conditioned air space (room size, ceiling height) in your home.

Design conditioned rooms with high levels of insulation and lowest exposure to external temperature influences; rooms usually found in the centre of the house. Avoid conditioning rooms that have high levels of indoor−outdoor traffic. Alternatively, use airlocks to minimise hot air infiltration or install an automatic switching device to the doors leading to the air-conditioned room that allows operation only when the door is closed.

Adjoining passively cooled living spaces can provide a thermal buffer to conditioned spaces. These spaces should be well ventilated, with fans to encourage acclimatisation.

You will need to address condensation in externally ventilated rooms surrounding conditioned rooms. When insulated walls surround an air-conditioned space, install a vapour barrier between the warm humid air and the insulation material to prevent condensation.

 

Hybrid cooling systems

Hybrid cooling systems are whole-house cooling solutions that use a combination of cooling options (including air-conditioning) in the most efficient and effective way. They take maximum advantage of passive cooling when available and make efficient use of mechanical cooling systems during extreme periods. For example, running an air-conditioner in a closed room for about an hour at bedtime often lowers humidity levels to the point where air movement from ceiling fans can provide sufficient evaporative cooling to achieve and maintain sleeping comfort. Clever electrical design using master circuits or solenoids can be used to ensure the fans are running before power is available to the air-conditioning.

Hybrid cooling solutions require a decision early in the design stages about whether air-conditioning is to be used and how many rooms require it. Inefficient air-conditioning installations can occur when they are added to a home designed for natural cooling as an afterthought.

Source: Envirotecture

Solar chimneys

Solar chimneys can be a passive form of cooling for a home. In a solar chimney, a tall chimney is placed to be heated by the sun. This heats air within the chimney and causes it to rise. This draws in air from the rest of the house, which in turn draws in air from the outside or from underground pipes.

Solar chimneys must be carefully designed to ensure that the home draws in cool air. If using an underground pipe system, the system will need to be extensive to give the air enough time to cool as it travels through the pipe. Special precautions will also need to be taken to prevent mould in the pipe, which can result because the cooling air causes condensation.

 

Adapting lifestyle to climate

‘Adapting lifestyle’ means adopting living, sleeping, cooking and activity patterns that respond to, and work with, the climate rather than using mechanical cooling to emulate an alternative climate.

Lifestyle adaptions can be a significant factor in achieving thermal comfort:

• Acclimatise your body to slightly warmer temperatures. If using air-conditioning, adjust your thermostat to between 25°C and 27°C – each degree cooler will increase your energy needs by 10%.
• Vary active hours to make best use of comfortable temperature ranges at different times of the year (for example, do outside work in the early morning).
• Live outside when time of day and seasonal conditions are suitable — particularly in the cooler evenings.
• Cook outside (for example, on barbeques) during hotter months to reduce heat loads from cooking inside.
• Consider using sleep-outs to achieve sleeping comfort and provide low-cost additional space for visitors during the summer holiday period.

Photo: Simon Wood Photography

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Learn more

• Read Australian climate zones and Design for climate to find out the best design for your region
• Read the Passive heating to learn how your home can be designed to work well in summer and winter 
• Explore Heating and cooling to discover how to save energy when cooling your home

Authors

Original author: Chris Reardon

Updated: Dick Clarke 2013, 2020