Paint it white to combat global warming

White paint has been used to keep buildings cool in places such as Greece and the Middle East for centuries and many owners of commercial buildings have adopted the practice as a way to cut their air conditioning bills. Now white paint is being used on the roofs of residential buildings around the world to cool down our cities, and as a clever way to fight global warming.

White roofs mimic the effect of the polar ice caps, which reflect some of the sun’s rays back into space before they can be converted to heat. This reflective quality of polar ice plays an important part in keeping the planet cool, but man-made global warming is melting these natural ‘mirrors’ at an alarming rate, cutting down their size and creating a global warming vicious cycle.

It’s hoped that as the white roof movement grows, the earth’s cities will start to do some of the work of these vanishing ice sheets. So choosing a more reflective colour when you next paint the roof is a proactive way to help tackle global warming – and it can even save you cash.

How does it work?

Light from the sun passes through greenhouse gases in the atmosphere, but once it strikes the surface of the earth and is partly converted into heat these same greenhouse gases then trap the energy and warm the planet.

However a white roof reflects roughly 55 per cent of the sunlight that hits it as light, not heat, which can pass back through the greenhouse gases again without warming the earth. In contrast, a traditional black roof reflects about 20 per cent as light, and the rest is converted to heat.

During the middle hours of a summer’s day a typical New Zealand residential roof of 140m2 gets energy from the sun at a rate of roughly 140kW – that’s the same rate of energy as 1400 regular 100W light bulbs blazing simultaneously. Studies by US government labs have found that painting a roof of this size white has a cooling effect that cancels out the global warming effect of roughly 14 tonnes of CO2 per year.

To put that in perspective, 14 tonnes of CO2 is roughly what an average car releases in 3.5 years of driving. That’s a big impact – and these savings can be made every year for the 20-year life span of the paint job.

How is it cheaper?

Here’s the beauty of the scheme – it doesn’t cost you anything extra and can actually save you money. Most people need to re-paint their roof around every 20 years, so all you’d be changing is the colour. White paint is often cheaper because you don’t have to pay for it to be tinted – which can range between an extra $11.50 and $50 per 10L of paint – and white paint typically lasts longer because it doesn’t expand as much in the heat and there is therefore less wear and tear.

As an added incentive, Resene has got on board with the White Roofs NZ project and is offering anyone with a White Roofs NZ letter a 30 per cent discount on white roof paint or the CoolColour range and accessories. To get your hands on the discount letter, all you need to do is email [email protected] and request it.

Will it make my house colder in winter?

You might think that having a dark roof would keep your house warmer in winter, and a white roof would make it colder, but in winter it doesn’t make that much difference. For starters, much of the heat from the surface of the roof rises away before it can warm your house. And the times when most people want their house snug and warm – at the beginning and end of the day – are generally dark in winter, meaning sunlight plays an even smaller role in the temperature of a home in the darker months. In fact, a study by the University of Columbia found that houses with light-coloured roofs lose heat more slowly than darker coloured buildings.

Do I have to use white paint?

No. In fact, some councils – such as those in Waitakere and on Waiheke Island – require you to have darker roofs in order to blend in with the bush, so make sure you check with your local council’s guidelines. However that doesn’t mean you can’t do anything. More and more paint companies are creating reflective dark colour ranges that greatly increase how much light the roof reflects. Examples include Resene’s CoolColour range and Dulux’s Infracool paints.

Who’s behind the idea?

Ian Montanjees started White Roofs NZ in October 2010 in order to raise awareness of the effect simply painting a roof can have. “I wanted to do something to make a significant contribution to help global warming because I see it as the biggest issue we’ll face this century,” Ian explains.

He wasn’t aware at the time that the idea was already gaining traction worldwide: in California all new flat commercial roofs must be painted white, and sloping ones must at least be painted with cool colours; in January 2012 the New York City Council made it compulsory for all new or replacement roofs of a particular slope to be “cool” roofs (painted white or cool colours with reflective materials). The European Union is also funding an education programme around cool roofs that spans the UK, Greece, Italy and France.

Catching on

Large businesses already recognise the benefits – roughly 75 per cent of all commercial buildings built in New Zealand in the past 20 years have white roofs – and Ian says some residential home owners are really keen, but at this stage they are small in number. This is affected partly by awareness and also by factors such as whether they own the house they live in.

Ian is currently trying to raise the money to launch an 18-month awareness campaign to promote white roofs to businesses, councils and government.

Hot metal

To show the difference in the heat generated by different roof colours, White Roofs NZ took measurements of colour samples laid out in the midday sun in mid February 2011. The standard ColorSteel samples were mounted on a polystyrene sheet in order to simulate an insulated flat roof and laid flat in the midday sun. The temperatures were then read using a non-contact infrared thermometer.

The results were:

• White 39°C
• Cloud 47°C
• Titania 48°C
• Gull Grey 53°C
• Unpainted galvanised 63°C
• Pioneer Red 64°C
• Permanent Green 70°C
• Black 74°C

The results showed clearly that roofs painted white stay coolest in sunlight.