Cool roofs reduce heat transfer into buildings by reflecting solar radiation. Less heat transfer means less demand for air conditioning. Cool roofs can have a payback period of a few years, depending on the air-conditioning configuration and building use. In a US example, applying a 60% reflective paint to the roof of an existing low-rise medical office building cost about US$2.20/m2 and reduced energy consumption by 18%; incorporating an 85% reflective coating to the roof of a new mixed-use building reduced electricity consumption by 18%.1
Until recently, white or light-coloured paints, coatings and tiles were the only roofing surfaces that could achieve high albedo (solar reflectivity). Today, high-albedo coatings and tiles come in a variety of colours although white still gives the best results.
Cool roof efficacy depends on a number of factors:
- local climate (temperatures and the amount of incident sunlight)
- the proportion of the year when heating and cooling are needed; white roofs can be a negative in winter or in colder climates
- the proportion of roof area to building volume; white roofs are most effective on single storey buildings
- roof insulation, sarking and ventilation which all reduce the amount of solar thermal radiation that passes through the ceiling of the building
- local building regulations/planning laws etc (white roofs may not be allowed in some zones)
- roofing materials, which affect the costs of recoating and its durability
- roof angles, which affect how much of the roof is shaded at certain times of day, and the angle at which direct sunlight strikes the surface.
Rooftop gardens also help to keep roofs cool by absorbing and using solar radiation. University of Melbourne research shows a 48% decrease in cooling load for spaces under a green roof, while winter heating loads were also reduced by 13%.2 The garden roofs can also provide a more comfortable microclimate.
Some issues to be considered when planning a green roof are maintenance costs, irrigation needs, personal safety, structural pressure and pest infestation.