How to Keep a House Cool in Summer - Why Australian Homes Overheat and What Works
Australian homes overheat because heat enters faster than it can leave. The main causes are unshaded windows, poorly insulated roofs and walls, dark roofing, air leaks that let hot air in, and no easy way to flush heat out at night. The most effective approach is to block heat first with orientation, shading, glazing, insulation and airtightness, then ventilate smartly, and use air conditioning as a back-up. Structural insulated panels (SIPs) support this because continuous insulation slows heat moving through the walls and roof, though they work best when the rest of the design does its part.
We’ve already delved into why Australian homes are so cold in winter. Now, with summer approaching, we explore why homes get so hot, what helps and how to design a new home that stays comfortable through our increasingly hot summers and heatwaves.
Why Australian homes overheat
Heat gets into a home in several ways, and each one has a different fix.
Windows. Sun shining through glass is usually the biggest source of unwanted heat. Unshaded west and east windows are the worst offenders.
Roof and ceiling. A dark roof absorbs a lot of sun, roof spaces get very hot, and that heat radiates down into the rooms below, especially where ceiling insulation is thin or patchy.
Walls. West-facing walls take the afternoon sun, and heat travels through framing such as timber studs and steel.
Air leaks. Hot outdoor air seeps in through gaps, while the cool air you have paid for leaks out.
Internal heat. Cooking, appliances, lighting and people all add warmth.
Trapped heat at night. Heat stored in the building during the day has to go somewhere. Without cross-ventilation, bedrooms stay warm long after sunset.
Australian summers are getting hotter, and a high NatHERS rating is a strong foundation for staying comfortable through them. The rating measures how much energy a home needs to heat and cool across a typical year, and a SIPs home, with continuous insulation and airtight construction, slows heat moving through the walls and roof. NatHERS doesn’t currently include a dedicated overheating check, though, so comfort on the hottest days also depends on design details such as shading, glazing and night ventilation. According to the Bureau of Meteorology, In January 2019, Adelaide reached 47.7°C - the hottest day recorded in an Australian capital city. Designing for heat as well as the annual average is what keeps a home comfortable through extremes like that.
Can a well-insulated home overheat?
Yes, a well-insulated home can overheat if it lacks good ventilation and a well thought out summer design. This surprises many people. Insulation slows heat moving in both directions, so if heat gets in through windows or open doors, a sealed and insulated house can be slow to let it out again.
The good news is that a well-insulated home designed for summer as well as winter can stay comfortable through the hottest months. Insulation and airtight construction slow the heat moving through the walls and roof and stop hot air leaking in, so the home starts each hot day cooler and its temperature changes more slowly. The following help it stay that way - shading to keep the sun off the glass, a way to release heat when the day cools down, and ventilation planned into the design.
Ventilation is the part that often gets overlooked. Opening windows and doors on a cool evening or overnight lets the heat built up during the day escape, and cross-ventilation and ceiling fans keep the air moving. Some mechanical ventilation systems also have a summer bypass that draws in cooler night air, and they bring in fresh, filtered air when you would rather keep the windows shut. At Smart SIPs, ventilation is planned as part of the home design from the start, so the airtight shell is paired with the right approach for your climate, layout and household. For more detail, see our post on airtight homes, ventilation and air quality.
The cool home checklist
1. Orientation and layout
During the design phase, it’s important to plan out the orientation and layout of your home to optimise comfort in both summer and winter. Put living spaces on the north side with properly sized eaves, keep west-facing glass to a minimum, and place bedrooms away from the afternoon sun where possible. Plan the layout so breezes can pass through the house from one side to the other.
2. Shade windows from the outside
External shading stops heat before it reaches the glass. Eaves, awnings, external blinds, shutters, pergolas and deciduous trees on the north and west sides all work. Internal curtains and blinds help, but they act after the heat has already come through the window.
3. Choose glazing carefully
Double glazing, low-e coatings and a low solar heat gain coefficient (SHGC) on east and west windows reduce heat entry. Frames with thermal breaks help too. In an airtight, well-insulated home, glazing is often the weakest link, so it deserves as much attention as the walls. Our NatHERS 7-star guide covers the glazing basics.
4. Insulate and seal the envelope
Continuous insulation in the roof and walls reduces the heat passing through them. The roof matters most, because that is where the sun hits hardest, and a lighter roof colour reduces heat absorbed. Sealing air leaks keeps hot outdoor air out. This is where SIPs fit in.
5. Ventilate smartly
Open windows when it is cooler outside than inside, usually late evening and overnight, and close them in the morning when the day heats up. High-level openings let hot air escape, and ceiling fans keep air moving at low energy cost. If your home has a heat recovery ventilation system, a summer bypass lets it bring in cooler night air.
6. Use thermal mass with care
Concrete slabs and masonry can even out temperature swings in climates with cool nights. In places where nights stay hot, or where the mass sits in direct sun, it can store heat and release it into the evening. Your designer can model whether it helps on your site.
7. Size cooling to the home
A home with low heat gain needs a smaller reverse-cycle system, which costs less to buy and run. Pairing it with solar panels and ceiling fans means you can cool the rooms you are in rather than the whole house.
How SIPs help keep a home cool
SIPs are factory-made panels with an insulating core between two structural boards, and they form the walls, roof and floor. In summer, they help in several ways:
Continuous core insulation: Solid foam cores provide unbroken R-value coverage across walls and roofs without settling or sagging over time.
Minimised Thermal Bridging: Replacing repetitive timber studs with continuous panel insulation removes the structural heat pathways that carry summer heat inside.
Roof panel: Smart SIPs supplies roof panels as well as wall and floor panels. A SIPs roof insulates directly where much of the sun's heat arrives, and can reduce or remove the hot roof space found in a conventional roof, depending on the design.
Airtight construction: Hot air stays out and cooled air stays in, so cooling runs less often.
Stable temperatures: The indoor temperature rises and falls more slowly over the day.
Reflect radiant heat: Neopor cores contain graphite particles that actively reflect radiant heat, preventing high summer solar loads from warming indoor walls. Learn more on our panel details and tools page. This applies to Neopor cores only, so confirm the core type for your project. You can compare options in our post on Neopor and polyurethane SIPs.
In this 9-star Smart SIPs home, it is estimated to have cooling bills about 85% lower than a 6-star home of the same size in the same area. You can see more of our work on our projects page. For winter performance and airtightness, our SIPs vs timber frame side-by-side page summarises a Canadian study that compared two homes over 12 months, and our SIPs testing page covers the structural and wind testing our panels have been through.
While SIPs are a great way to improve your home’s airtightness and make it less prone to overheating, they work best when paired with optimal shading, good windows and ultimately a carefully considered design. The best results come from combining the panels with the checklist above, which is why our home design service looks at orientation, glazing and shading together with the panel layout.
What suits your climate?
Summer looks different from Brisbane to Hobart, so the priorities change.
Climate | Summer challenge | What to prioritise | How a SIPs home helps |
Tropical and humid north (for example Darwin and Cairns) | High humidity and warm nights | Deep shading, cross-ventilation, light roofs, ERV or dehumidification | Insulated, airtight roof and wall panels limit heat gain and keep cooled air inside, so air conditioning works less. Moisture and ventilation need careful planning in the design |
Subtropical coast (for example Brisbane and the Gold Coast) | Hot, humid summers | Shading, breezes, good glazing, a well insulated roof | SIPs roof panels insulate where the sun hits hardest, and an airtight shell limits heat and humid air getting in while planned ventilation uses the breeze when conditions suit |
Hot dry areas, Adelaide and Perth | Very hot days, cooler nights | Ventilation, external shading, high insulation levels, thermal mass used with care | Continuous insulation slows heat through walls and roof during the day, and night ventilation then flushes out what has built up. Cement-sheet panels are available for bushfire-prone sites |
Temperate cities (for example Sydney and Melbourne) | Heatwave spikes | Shading, glazing, ventilation and insulation working together | Steadier indoor temperatures through short hot spells, as in our Gowanbrae project in Melbourne |
Cool and alpine areas (for example Canberra, Hobart and the high country) | Short but hot summer spikes alongside cold winters | Adjustable or deciduous shading that blocks summer sun and lets winter sun in | The same insulated, airtight envelope that holds heat in winter slows heat gain in summer, as in our Canberra 9-star project |
Smart SIPs designers tailor each home to its site and climate Australia wide. Our offices across Victoria, New South Wales, Tasmania, South Australia, Western Australia, Queensland and the ACT can help with local conditions. For more on how the system suits different regions, read why SIPs homes suit the Australian climate.
Heatwave tips for the home you live in now
You do not need a new build to make a difference this summer.
Close windows, doors and external blinds in the morning, before the heat builds.
Shade west-facing windows from the outside with shade cloth, awnings or external blinds.
Open the house up at night once the outside air is cooler than the inside air.
Use ceiling fans, and cool the rooms you are using rather than the whole house.
Cook and run heat-producing appliances early or late in the day.
Check on older neighbours and relatives, who face the greatest heat risk, and seek medical advice if someone seems unwell in the heat.
Home design for a hotter future
Hot days are expected to become more frequent. A home built today will be lived in for decades, so it makes sense to build it to handle hotter summers, as well as colder winters. To see what year-round comfort looks like in a completed home, read about living in a SIPs home year-round.
If you are weighing the costs, our posts on how to reduce heating and cooling costs and how a SIPs home reduces your energy bills go into the numbers, and designing an energy efficient home covers the design side. Building in a bushfire-prone area? Read about bushfire-rated SIPs.
What to remember when planning your build
Homes overheat when heat enters faster than it leaves, mostly through windows and the roof.
Insulation and airtightness help, but only when shading, glazing and night ventilation are part of the design.
The order that works: block heat, ventilate smartly, then cool with air conditioning as needed.
SIPs provide continuous insulation and airtight construction, which reduce heat gain and cooling demand.
Climate, site and orientation shape the right design, so plan these early.
Ready to Build a Home That Stays Comfortable All Year?
At Smart SIPs, we supply SIPs and design services for new homes across Australia, using building technology that works with the climate rather than against it. If you want a home that handles both cold winters and hot summers, our team can help you work out what suits your site.
Contact us today for an obligation-free consultation and ballpark quote.
Frequently Asked Questions About Keeping a House Cool
Do SIPs homes stay cooler in a heatwave?
SIPs help keep your home cool in a heatwave. The panels slow heat moving through the walls and roof and limit hot air leaking in, which keeps indoor temperatures steadier and reduces how hard cooling has to work. They work best as part of a design that also includes shading, orientation, good glazing and planned ventilation, because sun through windows and trapped heat still affect comfort on the hottest days. At Smart SIPs, these elements are planned together with the panel layout as part of the home design.
Can an energy efficient or airtight home overheat?
Yes, it can, if it lacks good ventilation and a summer design. Insulation slows heat in both directions, so heat that gets in through windows or open doors can be slow to leave a sealed home. Shading, glazing with a low solar heat gain coefficient, night ventilation and, where fitted, a heat recovery system with a summer bypass all reduce the risk. A well-insulated home designed for summer as well as winter is much easier to keep comfortable, because hot air stays out and cooled air stays in.
What is the best way to cool a house without air conditioning?
Block heat before it enters, then let heat out. Use external shading, a light roof colour and good glazing, and seal and insulate the building. Plan ventilation so you can flush the house with cool air in the evening or overnight, using cross-ventilation and ceiling fans, then close it up in the morning on hot days. Cooking and running appliances at cooler times of day also keep internal heat down.
Do I still need air conditioning in a SIPs home?
Often a smaller system is enough, and it runs less. It depends on your climate, orientation, windows, ventilation and how you use the home. Many households still keep cooling available for extreme heatwaves. An energy rating on your design gives a good estimate of the cooling load, and Smart SIPs can provide one as part of our design service.
