Cool roofs and the heat island effect: the evidence is in

Dark coloured and bare metallic roofs are common and make up a sizeable portion of the urban fabric. And while they usually do a great job of protecting the building beneath, they can also contribute to higher energy costs and the “urban heat island effect”. The solution may be a simple black-and-white decision.

White roofs work because of their properties in reflecting sunlight. While dark roofs typically absorb more than 80 per cent of solar energy, some white roofs can reflect 75 per cent or more of it away. This makes a white-roofed building cooler and saves money on air conditioning.

On a hot day, a standard black roof can be more than 40 degrees Celsius hotter than the ambient air temperature, while a cool roof would typically be less than 10 degrees Celsius warmer. That translates to a cooler building and less air conditioning.

Given that Australians spend about $5.2 billion annually – and use one-sixth of all electricity produced in Australia – on air conditioning, the potential savings are enormous.

The US-based Lawrence Berkeley National Laboratory has research indicating that painting 63 per cent of the roofs white in 100 large cities would provide approximately the same climate benefits as taking all the world’s cars off the road for 10 years.

While the reflected solar radiation from a white roof is often advantageous, in some situations the white roof may create some downside risk. Where the roof is visually prominent white membranes can be hard on the eyes. Significant reflected solar radiation directed back to other materials prone to increased light and heat load may be affected.

Despite these considerations, cool roofing is a trend that’s here to stay.

Until recently, selection of a cool roof colour was literally a black and white choice. But recently, thanks to innovations in coatings technology, the solar reflectance of coloured roofs may be increased without impacting on colour.

Companies such as BlueScope Steel have adopted this technology into COLORBOND® steel to provide cooler-coloured roofs that are available in a wide range of standard colours.

Within the Cooperative Research Centre for Low Carbon Living (University of New South Wales), the urban micro climate team is working on mitigation strategies and measurements of the urban heat effect in several Australian cities.

One of the current projects is investigating the surface temperature with the satellite Landsat, launched by the National Aeronautics and Space Administration (NASA). The satellite is measuring the surface temperature of the same location every 16 days. Surface temperature changes over time are measurable and provide a great opportunity to detect the impact of land cover changes including surface colours.

Land use changes, especially urbanisation, can have an enormous effect on the land surface temperature. Water, forest and grassland have the lowest temperature. Paved areas and built-up areas store the most heat and contribute to a much higher land surface temperature compared to the unpaved surroundings. Our fast-growing Australian cities with their densification and urban spread has led to an increasing urban heat effect.

The CRC-LCL urban micro climate project is investigating the surface temperature of different paving materials and roof colours for the greater area of Brisbane.

Using cloudless satellite images of Brisbane on summer days with a low humidity and low wind speed in the atmosphere, it’s possible to detect the land surface temperature of different roof colours from space – a staggering 705 kilometres above ground level.

This Landsat summer image (of a location 25 kilometres north-east of the CBD and west of the Airport of Brisbane) shows land surface temperature ranging from water bodies (lakes and the Brisbane river) below 15 degrees Celsius up to paved areas (e.g. the Brisbane airport rolling field) of more than 55 degrees Celsius. Vegetation has a high cooling potential but deforestation west of the Brisbane airport within the last decade has resulted in an increase of the surface temperature of more than 20 degrees Celsius.

A second example shows similar findings. This Landsat image was taken in February 2013 (19 kilometres south of the CBD and 29 kilometres southwest of the Airport of Brisbane) and shows land surface temperature ranging from water bodies below 15 degrees Celsius up to paved areas (e.g. parking lots) of more than 55 degrees Celsius.

New paved areas and development zones lead to an increase of the surface temperature compared to the unsealed areas before the development. However, it’s important to take into consideration which material and colour is used to minimise the local heating within a new development zone.

The installation of a white roof – a roof made from COLORBOND® Coolmax® steel (see the purple point [example 1] and arrows [example 2] within the images below) – is seen to lead to a lower surface temperature.

On warm summer days the Landsat images show white roofs decreasing the surface temperature up to 24 degrees Celsius. That is an enormous effect compared to black roofs, highlighting the impact of roof material selection on the roof micro-climate and surrounds.

Further investigations in Sydney, Melbourne, Adelaide and Perth with measurements through satellites techniques will help to develop a better understanding of the urban climate situation under different climate conditions for several roof colours.

Dr Conrad H. Philipp

Post doctoral research fellow
Project coordinator ‘urban micro climates’, CRC for Low Carbon Living (Sydney)

Project leader, China-Australia Centre for Sustainable Urban Development (Adelaide)