Eco Living
White is the New Black in Eco-Friendly Roofs
How one roof colour helps reduce global warming – and keeps your house cool in summer
We know ourselves that sitting out in the hot sun is more bearable in a white T-shirt than a black one, right? The same can be argued for our buildings – more and more scientific research is revealing how a white roof is good for the planet, reducing the heat build-up in cities and in our homes. Fortunately, no aesthetic sacrifices are necessary if you’re opting for a white roof. As these projects reveal, a home with a white roof can be the envy of the neighbourhood.
This process gives us the wonderful scenes we see of places such as the Greek Islands, but also helps explain why it’s more than just relaxed holiday vibes that help us feel cooler.
How it works
Buildings with white roofs, or ‘cool’ roofs as they are sometimes called, reflect sunlight back through the atmosphere into space. Sunlight on a dark roof is mostly absorbed and converted into heat, which can’t pass back into space because it is blocked by the greenhouse gases in the atmosphere.
The atmosphere is transparent to sunlight but opaque to the much longer wavelengths of heat, which is why a white roof helps cool the planet and a dark roof warms the planet. This is a concern because as farms or forests get replaced by buildings and roads, the local temperatures rise in what’s known as the urban heat island effect.
Buildings with white roofs, or ‘cool’ roofs as they are sometimes called, reflect sunlight back through the atmosphere into space. Sunlight on a dark roof is mostly absorbed and converted into heat, which can’t pass back into space because it is blocked by the greenhouse gases in the atmosphere.
The atmosphere is transparent to sunlight but opaque to the much longer wavelengths of heat, which is why a white roof helps cool the planet and a dark roof warms the planet. This is a concern because as farms or forests get replaced by buildings and roads, the local temperatures rise in what’s known as the urban heat island effect.
Think globally
White roofs increase energy efficiency because cooler roofs reduce electricity consumption used for air-conditioning, reducing household power bills and also the emissions from power generation that lead to global warming.
Research on white roofs has been led at the University of California by Arthur Rosenfeld. Back in 2009, Rosenfeld estimated that making roofs and pavements around the world more reflective could offset 44 billion tonnes of CO2 emissions. Another study estimates that cool roofs could offset the emissions of roughly 300 million cars for 20 years.
The Global Cool Cities Alliance shares science and action on cool roofs, cool pavements, and a wide range of urban heat island-related issues.
White roofs increase energy efficiency because cooler roofs reduce electricity consumption used for air-conditioning, reducing household power bills and also the emissions from power generation that lead to global warming.
Research on white roofs has been led at the University of California by Arthur Rosenfeld. Back in 2009, Rosenfeld estimated that making roofs and pavements around the world more reflective could offset 44 billion tonnes of CO2 emissions. Another study estimates that cool roofs could offset the emissions of roughly 300 million cars for 20 years.
The Global Cool Cities Alliance shares science and action on cool roofs, cool pavements, and a wide range of urban heat island-related issues.
In the US, the non-profit White Roof Project (tagline: ‘We’re going green by painting white’) uses private donations and teams of volunteers to coat the roofs of buildings of low-income residents for free.
As environmental and energy awareness has grown, so has the white roof movement. In a recent study, Rosenfeld and colleagues compared white roofs with ‘green’ vegetated roofs, concluding that while both do a good job at cooling the building and the air in the city, white roofs are three times more effective at countering climate change than green roofs, and are more cost effective.
However, green roofs also do other jobs, such as limiting the amount of stormwater run-off, providing insulation, and looking more natural.
10 reasons to love a green roof
As environmental and energy awareness has grown, so has the white roof movement. In a recent study, Rosenfeld and colleagues compared white roofs with ‘green’ vegetated roofs, concluding that while both do a good job at cooling the building and the air in the city, white roofs are three times more effective at countering climate change than green roofs, and are more cost effective.
However, green roofs also do other jobs, such as limiting the amount of stormwater run-off, providing insulation, and looking more natural.
10 reasons to love a green roof
Act locally
In New Zealand, our glaciers and snow lines have retreated significantly over the last 100 years. Six years ago, Ian Montanjees, who has degrees in engineering and architecture, and has worked in physics and as a hands-on builder, had a big goal – to make a positive contribution towards reducing global warming. He launched the New Zealand White Roofs Project to share the worldwide movement locally, backed by a depth of science.
He found that the majority of commercial buildings built in the last 20 years in Auckland have white or off-white roofs, including the airport, major shopping centres and warehouses.
In New Zealand, our glaciers and snow lines have retreated significantly over the last 100 years. Six years ago, Ian Montanjees, who has degrees in engineering and architecture, and has worked in physics and as a hands-on builder, had a big goal – to make a positive contribution towards reducing global warming. He launched the New Zealand White Roofs Project to share the worldwide movement locally, backed by a depth of science.
He found that the majority of commercial buildings built in the last 20 years in Auckland have white or off-white roofs, including the airport, major shopping centres and warehouses.
While Montanjees unfortunately didn’t get funding to keep the organisation running full-time, his goal is to see the white-roof trend spread to houses. His calculations conclude that 100 square metres of flat white roof cancels the global warming of roughly 10 tonnes of CO2 emissions, so if one third of New Zealand roofs were white, then that would equate to taking up to 70,000 cars off the road for the 20-year lifetime of the paint.
A study by Adam Taylor of Auckland University of Technology’s school of engineering, in the International Journal of Sustainable Building Technology and Urban Development, looked at how the roof colour affected the warmth and comfort inside an insulated building. He found that while both red- and white-roofed buildings had similar temperatures during the cooler parts of the day, during the hotter parts of the day there can be up to a 10°C temperature differential in the roof, and that white roofs do help to reduce the internal temperature of a structure.
Taylor says the effect of solar heating is a function of solar intensity and the insulation between the roof and the occupied space, and that much of Australia and the US have significantly differing solar energy levels to New Zealand. “The impact of painting a single structure’s roof white on a city heat island is quantifiable locally,” he says, “but the magnitude of the local effect depends on the amount of solar energy received, the size and location of the roof, and the density of structures nearby.”
Researchers from the University of South Australia have also found that while cool roofs are a good solution for much of the country, in regions where heating is regularly needed a cool roof may not be appropriate, as heat absorption via the roof may be of benefit in winter. In warmer climates, Energycut reports that a white roof can reduce household energy bills by 4-8 per cent.
Researchers from the University of South Australia have also found that while cool roofs are a good solution for much of the country, in regions where heating is regularly needed a cool roof may not be appropriate, as heat absorption via the roof may be of benefit in winter. In warmer climates, Energycut reports that a white roof can reduce household energy bills by 4-8 per cent.
Get out the brushes
Because white is the cheapest paint colour, it makes financial sense to opt for a white roof. White paint typically lasts longer than dark colours because it has less thermal expansion and contraction, meaning less wear and tear on the actual paint and substrate below it.
Some paint companies now also offer a paint range that absorbs light but reflects heat to keep surfaces cooler (Resene’s CoolColour range, for example). These colours are a valuable alternative to white in districts where town planning regulations allow only dark colours that blend in with the bush (such as in parts of Auckland’s Waitakere Ranges and some coastal and mountain zones).
See more photos of white houses
Because white is the cheapest paint colour, it makes financial sense to opt for a white roof. White paint typically lasts longer than dark colours because it has less thermal expansion and contraction, meaning less wear and tear on the actual paint and substrate below it.
Some paint companies now also offer a paint range that absorbs light but reflects heat to keep surfaces cooler (Resene’s CoolColour range, for example). These colours are a valuable alternative to white in districts where town planning regulations allow only dark colours that blend in with the bush (such as in parts of Auckland’s Waitakere Ranges and some coastal and mountain zones).
See more photos of white houses
Pre-coloured roofing materials, such as longline roofing from Coloursteel, come in a huge variety of colours and there are light tiles available for those who want a tiled roof.
Make it work for heritage too
Don’t think that it is only new builds that can get away with a white roof. Traditionally, the roofs of villas and bungalows were painted in much stronger colours than the neutrals popular today. Why not go for a pale roof and feel good about reducing our carbon footprint?
Don’t think that it is only new builds that can get away with a white roof. Traditionally, the roofs of villas and bungalows were painted in much stronger colours than the neutrals popular today. Why not go for a pale roof and feel good about reducing our carbon footprint?
Or why not go the whole hog and pop solar panels on top while you’re up there? Multiple studies suggest that white roofing improves the effectiveness of solar panels, by preventing a rooftop from heating up beyond the optimal temperature at which panels perform.
TELL US
Have you painted your roof white? How has this affected your energy bills? Tell us in the Comments below.
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Have you painted your roof white? How has this affected your energy bills? Tell us in the Comments below.
MORE
Browse more house exteriors
For centuries, white or light-coloured roofs have been cooling buildings in Mediterranean and Middle Eastern countries. Traditional Mediterranean buildings were made of volcanic stone and, before paint was so easily accessible, they were whitewashed to help deflect heat from the dark stones.
The whitewash was made quite easily and inexpensively with lime produced from burning limestone (mainly coral and shells that have been squeezed over aeons into a solid mass of calcium carbonate). The calcium oxide or ‘quicklime’ left behind was then mixed with salt and water before being slopped on.