Passive Cooling Your Home Additions

Amid rising temperatures and energy costs, it’s more important than ever to consider passive cooling design principles when planning a home addition. In this article we walk you through the basic principles of designing home additions that keep you cool when the heat is on.

Passive cooling can be broken down into two main elements – the cooling of buildings and the cooling of people. A well designed addition takes both into account.

The key elements of design for effective passive cooling are:

• Ventilation
• Orientation
• Windows
• Shading
• Insulation
• Thermal Mass

Let’s take a closer look at each of those and how they help with the cooling of the building and the people in it – that means you!

Ventilation

Air movement is perhaps the most important cooling mechanism. Moving air causes the moisture on our skin to evaporate, giving an immediate cooling effect. To gain this benefit good design is required to maximise air flows and harness any available cooling breezes.

Cool breezes in Sydney are often from an on-shore direction i.e. north-to south-east, so designing for breeze capture from these directions can be beneficial.

Cool night air can work wonders in cooling down buildings as well as the people in them. Convective currents draw in cool night air, replacing warm air and cooling down any thermal mass. Upstairs, full-height double-hung windows or clerestory windows with flyscreens are ideal for venting this warm air while downstairs windows with flyscreens and security if required, can be left open to draw in the cool air.

For times when breezes are not forthcoming, ceiling fans and whole-of-house fans are effective and low-energy forms of cooling via air movement.

Orientation

While your existing home’s orientation is fixed, you can consider the orientation of your addition for maximising comfort in hot weather. West-facing rooms will cop the full brunt of the afternoon sun, which slants in low and really heats things up.

If your addition includes a bathroom, positioning it on the western side will spare your bedrooms from this onslaught. Good use of windows, shading and insulation will help a lot too, as we discuss below.

Windows

Your windows are your main source of heat gain and your most effective means of cooling, so it’s important to get them right. Radiated heat from east- (morning) and west- (afternoon) facing windows is the main culprit. North-facing windows reflect a lot of summer heat due to the higher angle of incidence, and are often well-shaded by overhanging eaves. They also let in light and warmth during winter months when you need it most.

As discussed in the Ventilation section above, windows are your main means of passive cooling through enabling of air movement, the intake of cool air and venting of warm air.

Air movement through your home can be increased by placing smaller windows on the upwind side (facing the cool breezes) and larger windows on the downwind side. This uses air pressure differentials to suck cool air in and move it through the home faster.

Different window types and placements can also improve breeze flow. For example, casement windows can be used to catch and deflect external breezes from different angles, louvres can direct air flows vertically and clerestory windows can allow cross-ventilation without letting in too much solar heat.

Finally, positioning windows where they will provide airflow across commonly-used areas like living rooms and bedrooms will allow that lovely moving air to get to work evaporating and cooling your skin.

Shading

There are a number of ways that well designed shading can help keep your home cool. By preventing the sun hitting windows and areas of thermal mass, and by keeping ground temperatures lower with shading, the building remains much cooler and requires less effort to keep heat to a manageable level.

Overhanging eaves are vital to minimise the sun’s heat hitting windows and walls. West-facing windows and doors will benefit from external shade devices such as awnings and shutters.

Well thought-out plantings can also be of benefit – large trees can shade walls and roofs, while smaller plants can help keep the ground cooler and allow for cooler air to be brought in when it’s needed. Deciduous trees will also have the benefit of allowing light and warmth in winter. v

Insulation

Roofs and walls copping direct sun should have the best insulation that your budget will allow. In addition to insulating batts and reflective foil insulation, ventilation of the roof cavity will minimise the temperature differential across the bulk insulation, helping to prevent that heat from entering the home.  This can be achieved under the eaves but additional devices such as whirly-birds can make a big difference too.

Lighter coloured roofs reflect more heat and will keep your roof space cooler in hot months.

Thermal Mass

Thermal mass stores heat collected during the day and slowly radiates it out over a long period of time. Put your hand on a west-facing brick wall at midnight and you’ll get the idea. It’s a great way of warming a home in winter, but it’s not what you want in a Sydney summer.

Upstairs sleeping and living areas are best built using low-mass materials to prevent them from becoming slow cookers.

When high-mass materials are to be used, good shading, insulation and even a low-mass wall on the western side can make a huge difference to the heat-giving potential of the thermal mass effect.

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