The cheapest way to keep an Australian house cool is to stop the sun reaching it in the first place. Air conditioning has its place, but shade, roof treatment and moving air do much of the work for free once they are in. This article sets out what matters most, roughly in the order a British owner tends to discover it.

Stop the sun before it reaches the glass

In a British house, sunshine through a window is welcome for most of the year. In much of Australia, a large unshaded west-facing window can turn a room into an oven by mid-afternoon. Glass lets solar heat in readily and is poor at letting it back out, so the most effective shading sits outside the window, not inside it.

Internal blinds and curtains help a little, but by the time the sun hits them the heat is already inside. External options work far better:

• Eaves and awnings over north-facing windows, which block high summer sun while letting lower winter sun in.
• Adjustable external blinds or shutters on east and west windows, where the sun is low and hard to block with a fixed overhang.
• Pergolas with deciduous vines, which shade in summer and drop their leaves for winter.
• Trees and shrubs on the west side, planted with enough clearance from foundations and gutters.
• Window films or performance glazing where external shading is not practical, such as in apartments.

Remember that in the southern hemisphere the sun sits to the north. British habits of favouring a south-facing garden need reversing here: a north-facing living area with well-sized eaves is the classic recipe for comfort all year round.

Eaves: why older houses got it right

Plenty of older Australian homes have generous eaves, wide verandahs and deep porches for exactly these reasons. Some newer project homes have very narrow eaves or none at all, often to fit as much floor area as possible on a smaller block. Those houses tend to rely more heavily on air conditioning.

The right eave width depends on latitude, window height and orientation, so there is no single figure. Designers work it out from sun angles, and the energy efficiency provisions of the National Construction Code, published by the Australian Building Codes Board, take shading into account when a new home is rated. If you are extending or renovating, ask your designer to show how the shading will perform in midsummer and midwinter.

The roof is the biggest heat source

On a hot day, a roof absorbs a great deal of solar energy and passes it down into the ceiling space, which can become far hotter than the outside air. Without good insulation that heat radiates into the rooms below well into the evening.

Three changes make the most difference. Bulk insulation in the ceiling slows heat flow in both directions and is usually the best-value upgrade. Reflective foil sarking under the roofing, typically installed when a roof is built or replaced, reflects radiant heat. A lighter-coloured roof absorbs less heat than a dark one; many councils and some state planning rules now favour lighter roof colours in new developments for this reason. Whirlybirds and powered roof vents can also help clear hot air from the roof space, though their effect is more modest.

Moving air through the house

Australian houses were traditionally designed for cross-ventilation: windows on opposite sides, high ceilings and doors that can be left open to let a breeze pass right through. Whether that works depends on where you live. In coastal cities, an afternoon sea breeze can be used well. Inland, where nights can stay hot during a heatwave, the approach is often to close up the house during the day and open it at night when the air outside is cooler.

Ceiling fans are cheap to run and make a room feel several degrees cooler by moving air across the skin. Most Australian homes have them, and if yours does not, installing them requires a licensed electrician. Fly screens on windows and doors let you ventilate without inviting in mosquitoes and flies.

Know your climate

The balance between shading, ventilation and insulation shifts across the country. Humid tropical areas favour lightweight, airy houses with maximum shade. Hot, dry inland areas can benefit from thermal mass, such as a slab floor or masonry walls, combined with night cooling. Temperate cities need a house that copes with both a heatwave and a cold winter. The Bureau of Meteorology’s climate data gives average temperatures, humidity and heatwave patterns by location, which is useful before planning any renovation.

Many of the same measures also keep the warmth in during the colder months, which is covered in our piece on why Australian houses feel cold in winter.

Questions readers ask

Is air conditioning a sign a house is poorly designed?

Not necessarily, as most homes in hot regions benefit from it during heatwaves. A well-shaded, insulated house simply needs it less often and runs it more cheaply.

Can I add eaves to an existing house?

Sometimes, depending on the roof structure, but it can be costly. Awnings, pergolas and external blinds are usually simpler ways to add shade.

Do I need approval for an awning or pergola?

It depends on size, height, position and your state and council rules. Check with your local council before building.