What’s in a roof? Well apparently a whole lot. There’s a fast growing movement around the world to paint roofs white or at least a very light shade of grey for it’s light reflecting qualities. Sited by the British ‘Institute of Mechanical Engineers’ in 2009 as one of the top three measures we can take to mitigate global warming effects, and it has gained attention from some of the world’s most influential organisations and leaders including President Obama. The European Union funds Cool Roofs Europe, and the Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system requires buildings roofs to comprise of at least 75% reflective materials.
According to whiteroofs.org.nz, there is a fascinating ‘real life’ example in the provence of Almeria, Spain, where large scale greenhouse agriculture is the predominant industry. There the farmers paint their greenhouse roofs white in summer to stop the plants from becoming too hot. The effect is that each decade the area’s temperature has been dropping by 0.8 degrees. It is now on average 1.5 degrees cooler than the surrounding regions.
I had a look for myself on google maps and the view is indeed quite amazing! Here are a few screenshots I took.
Here’s an excerpt from The White Roofs Project NZ to explain why this is such an important idea:
‘The White Roofs Project NZ is promoting white and off-white roofs as simply one of the fastest and lowest cost ways of helping reduce global warming, by mimicking the way the polar icecaps reflect sunlight back into space and help cool our planet. If your building has air conditioning a white roof may result in lower electricity use and reduced carbon emissions, and if your building does not have air conditioning you may benefit from a cooler building. In many cases it does not involve additional costs when a roof needs maintenance anyway.
When sunlight falls on a white roof, about 55% is reflected and much of this goes back into space, whereas a dark roof absorbs most of the sunlight and re-radiates it as heat, which gets absorbed by greenhouse gases in the atmosphere and does not make it back into space. And that is why a white roof helps cool the planet and a dark roof warms the planet.’
In our sharing, caring community of Kakariki Street + neighbours, friends and relatives, the idea of painting our roofs white or light colours was floated via a generously broadcast email. It was timely for some, while others just missed out having already bought the paint with which to re-colour their roof. However three of us are able to act on this advice and will do so in the next few months. Two of us are Kakariki Street-ers – the other house owner is a neighbor in our communication loop. When I look around our neighbourhood I see a total of 4 light/white roofs from my garden.
The website also says that: ‘When new roofs are chosen or when roofs come up for regular maintenance, the choice to go white or off-white comes at no extra cost in many cases, and gives the opportunity to do good without it hurting your pocket.’
I have seen some comments that say the benefits of this approach are cancelled out or even negative in colder climates where a building’s energy usage is higher in winter for heating than it is in summer for cooling. Upon further analysis this would appear to not be entirely true as the winter sun is low in the sky, so a roofs thermal gain in winter is far less than in summer. The need for heating our houses also occurs predominantly outside of the main sunlight hours in the late afternoon and evening. A well insulated wooden house (average houses in New Zealand are wooden, though not always insulated), receives most of it’s thermal gain from sunlight entering through its windows.
Our house, like the majority of New Zealand houses, has a corrugated iron roof. Unlike some houses, it also has a thick layer of insulation in the ceiling cavity. The roof, having virtually no thermal mass, is not designed to store heat and slowly release it during the night to keep us all warm. The insulation offers a further barrier to the heat entering our house from above -which becomes quite apparent on a hot day when the inside temperature is lovely and cool. Our house does however have a dark green roof that converts light into radiant heat -exactly the sort that gets trapped in the earth’s atmosphere by greenhouse gasses, so I think we’d be well advised to go for a lighter roof colour.
Wouldn’t it be cool if in a few years time Paekakariki -or even Kapiti,
could be spotted on Google Earth from a great height like Almeria in Spain, because it was covered in white roofs!