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The outdoor air temperature in major Australian cities can be reduced by 2.1- 2.5°C with solar reflective roofs – light coloured or cool coloured - which additionally reduce the cooling energy consumption of buildings.

Project Aim and objectives

The aim of this project is to understand the applicability and costs & benefits of using cool roof technologies on Australian buildings and to identify any barriers to adoption.

1. To evaluate the current climatic conditions in major Australian cities, understand the characteristics of urban overheating, and develop detailed climatic data through advanced mesoscale climatic modelling.
2. To evaluate the magnitude and spatial variation of the mitigation /cooling potential of cool roofs when implemented at the city scale, as well as how their application affects the urban ambient temperature and the other main climatic parameters. 
3. To investigate the impact of cool roofs on the cooling/heating energy needs and indoor air temperature for different building types of buildings in all capital cities. 
4. To understand how specific building characteristics affect the performance of cool roofs and the advantages of applying cool roofs in the different Australian climates. 

Project Findings

A city-scale deployment of cool roofs reduces the maximum peak ambient temperature by 2.1°C - 2.5°C

Mesoscale climate simulations demonstrated that the use of cool roofs on all buildings reduces the maximum peak ambient temperature by 2.4°C, 2.1 °C and 2.5 °C in Sydney, Melbourne and Brisbane, respectively compared to the existing conditions. The maximum decrease in the sensible heat flux is of 292.8 W/m2. Further effects of an increase in the urban albedo are an average reduction of the planetary boundary layer heights over the city by up to 1608 m and a maximum decrease in wind speeds by up to 3.9 m/s.

Cool roofs applied at the city scale can reduce building cooling energy needs

• In existing (pre-code) buildings without or with low insulation levels, the cooling energy savings achieved with cool roofs are significant. For instance, the annual energy savings in a low-rise office building without insulation are 22.2-39.9 kWh/m2 (34.7-42.3 %) in Sydney, 4.0-9.7kWh/m2 (12.3-27.6%) in Melbourne, and 34.2-52.2kWh/m2 (33.8-39.4%) in Brisbane.
• In new buildings with high level of insulation (NCC 2019 DtS levels), the cool roofs savings are relatively less than that in old buildings. For instance, the annual energy savings in a new low-rise office building is 1.6-8.3 kWh/m2 (4.6-18.2 %) in Sydney, 0.2-1.0 kWh/m2 in (1.2-4.6 %) Melbourne, and 3.4-5.2kWh/m2 (4.6-6.0%) in Brisbane.
• In residential buildings: Indoor air temperatures in houses are also reduced by up to 4°C in new houses with high insulation (NCC 2019 DtS), with the number of hours exceeding 26°C reduced by even 100 hours per month (summer only) compared with a conventional solar absorptive roof.

Reduction of ambient temperature at (a) 06:00 LT (b) 14:00 LT, and (c) 18:00 LT: The maximum decrease of the ambient temperature during 18:00 LT is 1.9°C over eastern Sydney and the average decrease of summer months is 1.1°C

In conclusion

Reflective roofing can both mitigate the outdoor air temperature and reduce the cooling energy consumption of buildings, in all Australian cities. The use of cool roofs is associated with heating penalties – i.e., higher heating energy needs – which are largely outweighed by the cooling energy savings, already in the present climate context. Cooling energy savings are not limited to poorly insulated low-rise buildings. Also high-rise insulated buildings benefit from a city-wide application of reflective roofing, because of the reduced outdoor air temperature.

Findings of interviews with industry partners and experts

• Most common material type in cool roofs: Among various cool roofing product types available on the Australian market, spray, paint, membrane, and metal sheet account for 44%, 69%, 75%, and 25%. Other material types include precoated concrete, asphalt, brick, timber, canvas, PVC, polycarbonate, and fiberglass.
• Most common material colour: Although white products are the majority, products of any colour are available on the market.
• Estimated Service Life of cool roofs: On average, the estimated service life declared by manufacturers (in reference or standard application conditions) is 28.5 years for metal cool roofing products and 22.5 years for field applied coatings.
• Product conformity & certification: Consumers should check the certification of the products. The certification has to be provided only by accredited laboratories, in compliance with the NCC.
• Cost of cool roofs: We collected the standard unitary price of 14 cool roofing products. The highest price collected is 32.5 AUD/m2 while the lowest one is 2.5 AUD/m2, and the average 13 AUD/m2.
• Cool roofs market: The largest market of cool roof is in North America, followed by Europe, while the market in the Asia- Pacific region is expected to grow at the fastest rate of 6.6 % per year from 2020 to 2027.
• Can cool roof technology be applied to any type of roofing material? Based on our survey, among the 16 collected cool roof products, 38% can be applied both to roof retrofitting and replacement; 50% can only be applied to retrofit projects while 12% can only be used to replace the original roof.
• Are there cool roofs benefits with rooftop solar panels? Yes, for two reasons, 1) because the roof reflects more solar radiation back towards the panels and 2) because it decreases the ambient temperature and thus increases the performance of the photovoltaic modules

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