Queensland University of Technology (QUT) researchers have found that a special roof coating could lead up to a 30 per cent reduction in air conditioning load and offer relief for Queenslanders in the summer by reducing the amount of heat entering a building through its roof.

The two-year study, which focused on a sample of houses, schools, offices and retail spaces in Townsville and Brisbane, revealed that the special reflective white paint reflected 88 per cent of the sun’s energy. In comparison, a standard light coloured roof reflects less than 65 per cent of the sun’s energy, while a dark roof reflects less than 25 per cent.

This high rate of reflectance of the painted roof led to a two degrees Celsius temperature drop in internal spaces that weren’t air conditioned, such as classrooms, which in turn, generated “significant savings” in energy usage and electricity costs.

“We saw average energy savings of between five and 30 per cent across the range of buildings, with most buildings showing savings above 15 per cent,” said Professor John Bell, head of QUT’s School of Chemistry, Physics and Mechanical Engineering.

“For an air conditioned block of two classrooms in Springfield to the west of Brisbane, for example, we reduced electricity use by 1,144 kilowatt hours per year, which is roughly 1 tonne of CO2 per annum reduction for these two classrooms alone. Both electricity consumption and peak demand were significantly reduced, resulting in lower electricity bills for the school.”

Professor Bell added that the results indicated a cool roof could be a viable and far cheaper alternative to air conditioning classrooms.

“At Nundah we had a reduction of roof temperature of around 15 degrees during the hottest part of the day,” he said.

“This resulted in a reduction of between one and three degrees in the internal temperature. That meant it rarely got above 27 degrees which is arguably cool enough to eliminate the need for air conditioning, especially if ceiling fans are used.

“It really is an extraordinarily simple solution. Twenty years ago it was cheaper and easier to put in an air conditioner than to paint your roof, but not anymore.

“Unlike air conditioners this is a passive cooling solution that does not require ongoing investment in the electricity network or in housing. Because of this, it would be a worthy candidate for inclusion in the building regulations much as they have done in the California energy code since 2005.”

Colour and Chemistry a simple way to lower bills

The tests, which involved onsite monitoring of the temperature and electricity use of 16 buildings across 10 sites both before and after the application of a cool roof coating, found that the coating led to temperature reductions in three areas – the roof surface, roof cavity, and non-air conditioned internal spaces.

According to QUT Senior Research Fellow Dr Wendy Miller, who ran the project with Professor Bell, it is a combination of colour and chemistry that made the roof coating effective.

Generally, light coloured materials are more likely to reflect more sunlight and absorb less energy and heat than dark coloured materials. Low thermal mass materials also emit heat quicker than high thermal mass materials.

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Pointing out that the study proves that reducing the costs of operating a business, or running a house or a school, is as simple as painting a roof, Dr Millers says the trials indicate the economic benefits of cool roof coatings would be greatest for single storey buildings, buildings with aged, dark or medium coloured roofs, buildings with no or low levels of roof insulation, and buildings with high air conditioning use.

The project was run under the Guided Innovation Alliance, a collaboration between QUT, electricity distributor Ergon Energy, and SmartGrid Partners.

Geraldine Chua

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