A growing awareness of the role buildings play in the urban heat island effect has seen a widespread installation of white roofs in cities.
Conventional darker roofs are being converted to white or, in some cases green with the roof surfaces covered in vegetation in a bid to lower city temperatures and energy consumption. This results in reduced building cooling costs while preventing carbon dioxide emissions.
Cities are becoming warmer due to reduced vegetation and the heat absorbed by and retained in buildings and pavements, particularly dark surfaces.
According to the International Energy Agency, every 100 square metres of roof area that is white instead of black cancels the warming effect of 10 tonnes of carbon dioxide over a roof’s lifetime, which is typically around 20 years.
With roofs accounting for roughly 25 per cent of urban surface area and with cities occupying one to two per cent of the world's land area, converting most flat roofs in warm cities to white would cancel warming from more than one gigatonne of carbon dioxide per year. In terms of emissions, it’s akin to taking half the world’s cars off the road for 20 years, the agency states.
In a bid to battle the heat, commercial building owners are looking to raise their buildings' solar reflectivity, also known as albedo, by lightening roofs and to shade the roof membrane through the application of green roofs.
Dark roofs absorb up to 95 per cent of sunlight and convert it into heat, escalating energy costs, increasing the potential for energy shortages in the summer months and mostly creating discomfort for building occupants.
White roofs have been criticised for their lack of heat retention in winter, but studies have found the savings from white roofs in summer generally outweigh the costs of heating the same building in winter.
The US has led the trend toward roof cooling with the Cool Roof Rating Council (US) adding “cool roofs” to their building codes as a mitigation strategy for the urban heat island effect.
New York City, one of the world’s darkest and most dense cities is one of the first cities to move towards brightening urban rooftops.
Stuart Gaffin, a research scientist at Columbia University and lead author on a 2012 paper detailing roof study believes New York’s efforts to convert its roofs will lower the city's temperatures and energy costs.
"City roofs are traditionally black because asphalt and tar are waterproof, tough, ductile and were easiest to apply to complex rooftop geometries. But from a climate and urban heat island standpoint, it makes a lot of sense to install bright, white roofs. That's why we say, 'Bright is the new black,'" he says.
The study found that even the least expensive white roof coating reduced peak rooftop temperatures in summer in New York City by an average of 43 degrees Fahrenheit and noted that one of the most immediate, cost effective methods is to simply paint roofs white.
In a community initiative in New York, the White Roof Project is demonstrating that a painted white roof can have a significant impact.
With rooftops covering approximately 12 to 13 per cent of the land in New York City, White Roof Project founder Juan Carlos is working to create awareness of the concept on a national and global scale with an ambitious goal:
“By painting 5 per cent of the rooftops every single year, we’ll save 24 billion metric tonnes of CO2, that’s exactly the same amount of CO2 created in 2010 so essentially by the year 2030, we’ll have turned of the world for an entire year,” he says.
In Australia, legislation for cool roofs, or roofs that feature a certain solar absorption rating in commercial buildings, remains in discussion. The movement as a whole has begun with an incredible amount of research, paint products and initiatives beginning to respond to the concept.
Many Australian suppliers have introduced light coloured paints and heat reflective paints that have been suggested to reduce energy absorption by up to 50 per cent or more.
Working closely with the Green Building Council of Australia and the University of Wollongong’s Sustainable Building Research Centre, BlueScope Steel is contributing to research on white roofs and has introduced a roofing product called Colourbond Coolmax that comes in the colour Whitehaven.
The “whiter than white” product is said to reduce the annual cooling energy costs of a commercial and office building by up to 7.5 per cent compared to other colours.
Melbourne, a city that aims to be carbon neutral by 2020, is promoting cool roofs through its 1200 Building Projects initiative, which aims to see two thirds of the municipality’s commercial stock retrofitted for environmental reasons.
Last year, Melbourne City Council commissioned the University of Melbourne to undertake research on white roofs with councillor Cathy Oke, chair of the Future Melbourne (Eco-City) Committee, certain that commercial buildings would benefit from this tool.
“White roofs can cool commercial buildings by three per cent on hot days, which helps reduce the urban heat island effect and improve the health of city users,” she said.
Dr Dominique Hes, a senior lecturer at the University of Melbourne in sustainable architecture and lead author of the research, reiterated the ability of white painted roofs to reduce energy and cool buildings.
“Melbourne’s CBD has over 3,500,000 square metres of lettable commercial space,” Hes explained. “If the roofs of these buildings were painted white, the city could in theory reduce its CO2 emissions by 4.5 million MJ per year, 1.5 million kilos of CO2 or 3 million black balloons.”
“White roofs are a low cost solution in making buildings more sustainable, particularly for our older buildings. And if our air conditioners are not working as hard, there are financial benefits for buildings owners as well.”
Australia has performed well at implementating green roofs, which provide direct shading to the roof membrane, acting as a thermal mass to keep buildings cool in warmer months and warm in cooler months.
Green roofs also offer the opportunity of food production, storm water management and the double layer of substrate and vegetation can reduce noise pollution.