Light-coloured roofs are one of the oldest cooling techniques in the book, but a new generation of materials is promising to be even better at beating the heat.
From the white-washed houses of the Greek islands, to Bermuda’s white limestone roofs, to corrugated light Colorbond on Queenslanders, homes in hot climates have long relied on light colours to keep cool.
And with good reason. On a hot summer day, the sun can heat a black roof to as high as 90°C, while a white roof might reach only half that temperature. This can easily translate to as much as 5°C difference inside the house, increasing comfort and cutting cooling costs.
When it comes to roof coatings, there are two main factors affecting how much heat it gains from sun – solar reflectance (or albedo) and thermal emittance.
Most black roofs absorb 95 per cent of solar radiation, giving them a reflectance of 0.05. Traditional white paints, with titanium dioxide pigment, have a reflectance of between 0.8 and 0.9.
Of course, more than half the solar spectrum is made up of invisible near-infrared (NIR) light. By using pigments and binders that are highly reflective of NIR light, coloured coatings can be made much cooler. For instance, there exist cool black coatings with reflectances five times higher than traditional black paints, meaning they are heated significantly less by the sun.
Tapping into the universe
But where it starts to get interesting is when attention turns to thermal emittance.
Material scientists and engineers across the globe are developing technologies that offer passive daytime radiative cooling (PDRC), where a roofing material can direct enough radiation into the great heat sink of the sky to decrease surface temperatures below the ambient temperature.
Materials that can do this combine some special properties:
- Extremely high solar reflectance (approaching 1), minimising solar gain
- Extremely high thermal emittance of infrared wavelengths between eight and 13 μm, the window in the atmosphere that is relatively transparent to thermal radiation
- Extremely low thermal emittance in the rest of the thermal infrared spectrum, maximising radiative loss
This allows roofs to absorb heat from their environment and redirect it into space, staying significantly cooler than the ambient temperature, even in direct sunlight.
While the reduction in interior temperature is highly dependent upon building design and insulation, PDRC has been found to cool interiors by up to 10°C.
Material reality
A wide range of approaches to creating PDRC materials have been explored, including polymer coatings and films, glass-polymer hybrids, nanoprocessed silk and even delignified wood.
Creating a material with the right properties for radiative cooling is not overly technically challenging – in fact, it can be done at home by putting washing soda, calcium chloride (often sold as a de-icing agent) and citric acid in a blender.
What is significantly more challenging is designing a suitable roofing material with these properties; one able to withstand extremes in temperatures, high amounts of UV radiation, rainstorms, and the accumulation of dust and grime.
To be effective, it needs to be more than durable. It must be cheaply and easily made
and installed. Many polymers degrade rapidly with prolonged UV exposure, with the resultant yellowing reducing the material’s reflectance.
One promising material uses titanium dioxide nanoparticles in a suspension with air-filled pores, and can be sprayed like paint.
Crucially, this material is not only resistant to UV degradation, but also highly hydrophobic, making it resistant to soiling.
Research conducted by China’s Jianing Song and Wenluan Zhang found that when subjected to the equivalent of three years of natural soiling and sunshine, the material’s solar reflectance declined by just 0.4 per cent, whereas a reflective white paint’s declined by 9 per cent.
Read more: How Australia can leverage Passivhaus housing design for the net-zero revolution
Global benefits
Widespread adoption of cool roofs – whether radiative or not – promises broad benefits. By reducing interior temperatures in the summer, reliance on air conditioning can be significantly reduced, cutting energy costs and emissions, as well as peak loads. Cool roofs can even boost the efficiency of solar panels; every 1°C reduction in roof-surface temperature increases photovoltaic efficiency by up to 0.9 per cent.
A 2022 University of NSW report found that cool-roof technology, combined with reflective pavements, could reduce citywide ambient temperatures by up to 2.3°C. This reduction in the urban heat-island effect would also further reduce the load on air conditioning.
The UNSW report also estimated up to 30 per cent fewer deaths in heat waves.
While the technology behind radiative cooling is not yet mature, widespread adoption of tried-and-tested white roofs offers enormous benefits. A 2009 Berkeley study estimated that increasing the average solar reflectance of all roofs in tropical and temperate regions by just 0.25 would offset the heating effects of 24 Gt of CO2, equivalent to removing 300 million cars from the roads for 20 years.
For now, though, the most popular roofing colour in Australia remains a deep charcoal.
This article was originally published in the May 2025 issue of create with the headline “Cool roofs”.
