How to keep your house cool in a heatwave

If overnight temperatures are due to fall below your inside temperature, open the house as much as possible from late afternoon. Image from shutterstock.com

Should you open or close your house to keep cool in a heatwave? Many people believe it makes sense to throw open doors and windows to the breeze; others try to shut out the heat. Listen to talk radio during a hot spell and you are likely to hear both views.

In a modern house the best advice is to shut up shop during the heat of the day, to keep the heat out. Then, throw open the windows from late afternoon onwards, as long as overnight temperatures are lower outside than inside.

But our research shows that opening and closing doors, windows and curtains is just one of the factors at play. To really stay cool when the heat is on, you also need to think about what type of house you have, and what its surroundings are like.

The traditional “Queenslander” house has long been seen as ideally suited for hot weather. Such houses have great design features for cooling, including shady verandas and elevated floors. But the traditional timber and tin construction provides very little resistance to heat transfer.

If uninsulated homes are closed up during a heatwave they would very likely become too hot. This has led people to opening up their house, to stop them getting much hotter inside than outside.

But in temperatures of 40C and above, one could argue that both strategies (opening and closing) in an uninsulated house would result in very uncomfortable occupants. Such houses would also not meet current building regulations, as insulation has been required in new houses since 2003 (or earlier in some parts of Australia).

Our research explores the role of design and construction on occupant comfort in hot weather. We have looked at brick and lightweight houses, as well as those made from less common materials such as structural insulated panels, earth, straw, and advanced glass and roof coatings.

We found that three factors influence the comfort of people inside a house: whether is it opened or closed; its urban context; and its construction materials. Having a better understanding of these factors could help you to keep cool this summer – or prepare for the next one.

To breeze or not to breeze

Whether they have air-conditioning or not, we found that people usually approach hot weather in the same way: by opening doors and windows to capture breezes.

People in both groups also tended to shut up the house if it gets hot outside, or if there is no breeze, or before switching on the air-conditioner if they have one. Most participants in our survey, which looked at homes less than 10 years old, also used ceiling fans to create air movement.

Occupants tape foil to the inside of windows to try to stop their home from overheating in Queensland.

But our research showed that many people failed to take advantage of cooler overnight temperatures, meaning their homes were hotter than the outside during the night. This may mean that houses have not been designed to get rid of daytime heat. Or that people aren’t opening the windows overnight to allow the house to cool down.

The impact of context

The research shows that occupants first try natural ventilation for achieving comfort. But the success of this strategy depends on the urban context of the house. This includes factors such as housing density, street scape and microclimate.

For example, during a hot spell in 2013 an Ipswich estate experienced minimum and maximum temperatures that were 3-4C hotter than the local weather station. Restricted air movement due to nearby buildings, and radiant heat from hard surfaces such as concrete, can both drive temperatures up.

Built for comfort

Both the housing industry and occupants seem to have little understanding of the impact design and construction have on the temperature inside the building. As a result, air-conditioning is now seen not as desirable, but as a necessity. This does not have to be the case.

Most houses are built to minimum regulations (5-6 stars out of 10). There is also evidence that, with poor construction practices and virtually non-existent compliance testing, many would fail to meet even this level.

What does this mean for comfort year-round, and in a heatwave?

In inland southeast Queensland, a 6-star home will have an internal temperature of 18-28C for 80-85% of the time. In a typical year, its temperature will be above 30C for between 300 and 350 hours (3.5% of the time). Heat-wave conditions would result in more hours above 30C.

In contrast, a 9- or 10-star house in the same climate would deliver more “comfort” hours (85-95%) and would be above 30C less than 2% of the time. These houses are designed to slow down the transfer of heat, meaning they naturally stay cooler for longer. And there is no (or little) need for air-conditioning!

This 9-star home uses 48% less electricity than the south-east Queensland average.

A wide variety of design and construction techniques and materials can be used to achieve such high performance houses in every climate zone in Australia.

Open and shut case

So when facing a heatwave, should we open up our houses or close them up? The answer is… it depends.

If your home is well insulated and shaded, it should be able to resist several days of extreme heat. Closing doors, windows and curtains during the heat of the day can help the house stay cooler than outside. Ceiling fans provide air movement to make you feel cooler.

Opening the house as much as possible from late afternoon to early morning is beneficial if overnight temperatures will fall below your inside temperature.

Air conditioning a poorly insulated house with little shading is expensive and futile. In a well-insulated and shaded house, air-conditioning can be used quite efficiently by using the same strategies as above. A higher thermostat setting (perhaps 26-28C), combined with ceiling fans, can provide comfort with lower running costs. This can also reduce strain on the electricity network.

Whether air-conditioned or not, houses can be designed specifically for their climate, to limit the flow of heat between the outside and inside. The higher the star rating of the house, the more effectively it stops unwanted heat from entering the house. Different strategies are required for different climates.

Of course, the knowledge that you might be more comfortable in a different house is likely to be cold comfort as you swelter through this summer. But perhaps you can prepare a “cool comfort” plan for next summer.