Lawrence Berkerley National lab reported November last year on some fantastic research into how “cool roofs” can help slow global warming. White surfaces reflect rather than absorb radiation, and can be effective in re-radiating heat back into space. I’ve only just come across this research today, and the potential greenhouse gas savings are enormous.

Most roofs are dark in colour, the research by Akbari, Menon and Rosenfield calculated the CO2 offset achieved by increasing the solar reflectance of urban surfaces. For a 100 m2 roof making a dark roof white (with a long term solar reflectance of 0.60 or more) will offset around 10 tonnes of CO2 per year.

A 10 tonne saving per 100 m2 is a large saving. In hot climates white roofs also reduce air conditioning loads. So called “cool coloured” surfaces apparently have only half the benefit.

In California its been law since 2005 that flat roofs be painted white. We should have the same laws in Australia, and should also be legislating that sloped roofs should be white, or at least “cool coloured” as has been the case in California since July.

Assuming it costs $1,700 to clean and paint a 100m2 tiled roof white, and thus save 10 tonnes of carbon, this one measure will provide more climate benefit implementing all of the following:

• Replacing you gas hot water system with a solar hot water heater (gas boosted)
• Installing a 2 kW solar PV system on your roof
• And implementing energy conservation measures that save 16 kWh per day

* Assuming an emissions factor of 1 kg CO2/kWh.

If you don’t have an air conditioner “geo engineering” by painting your roof white won’t save you any money. But in terms of tonnes of greenhouse gas saved per dollar invested painting your roof white - whether at home or at work - could be one of the least expensive ways of cutting greenhouse gas emissions. And it may help you avoid the need to get an air conditioner.

If you have a low carbon footprint to start with, based on this research, painting your roof white could actually neutralise your other emissions. And someone with a white roof is doing more to slow global warming than someone with a 5 kW PV system on their dark roof.