New York City proves that white roofs are just as cool as green roofs
Public Works looks at public space, urban design, and city-building innovations from around the world, and considers what Toronto might learn from them.
This past weekend, Pride gave Torontonians an excuse to show off their most colourful clothes, from electric green heels to rainbow wigs. But if you’ve ever worn a black T-shirt on a sunny day you’ll know that dark colours absorb the heat. That same principle applies to cities.
As paved surfaces and tall buildings have replaced vegetation in urban areas, cities around the world have become home to what is known as the urban heat island effect. Cities are increasingly composed of materials—such as asphalt, brick, and concrete—that absorb rather than reflect the sun’s energy. This means that during the summer urban areas are typically several degrees warmer than their suburban or rural surroundings. Although the average difference varies between 1 C and 2 C, in some cases the gap can reach up to 12 C.
Higher temperatures have an impact on everything from public health to personal comfort. Cranking up the air conditioning results in higher energy bills and greater greenhouse gas emissions.
But not all parts of the city are hit equally hard. The urban heat island effect is most pronounced in places that don’t have a mature tree canopy, which are also more likely to be low-income areas.
There are many ways to deal with the spike in summer temperatures and keep people cool without penalizing residents of the hottest neighbourhoods. Solutions range from planting more trees to building shade structures.
One group in New York City has taken a DIY approach to the problem. As its name suggests, the White Roof Project equips communities with the know-how to create cool roofs and bring down temperatures in some of the city’s hottest neighbourhoods. The idea behind cool roofs is to cover the tops of buildings with light colours that reflect sunlight and radiate much of the heat that they absorb.
Whereas a conventional black roof reflects 20 per cent of the sun’s rays, a cool roof reflects 90 per cent. This can reduce internal building temperatures by up to 30 per cent and cut summer cooling costs anywhere from 10 to 40 per cent. Cool roofs save money for tenants and contribute to a more moderate urban climate for everyone.
In practice, there are two ways to create cool roofs. The first is to cover a roof with specially developed, highly reflective paint. The second is to install a reflective membrane over the existing surface. Both options extend the life of a roof by reducing warping and cracking from extreme heat—another advantage from both a financial and environmental perspective.
Cool roofs are gaining traction in New York City beyond the work of the White Roof Project. In 2008, cool roofs were written into the city’s building code, with the result that 75 per cent of the roof area of new buildings must be covered with white or other reflective surfaces. The City of New York also runs a program that aims to create nearly 100,000 square metres of cool roofs each year. Since 2009 more than 550,000 square metres have been converted to cool roofs.
Back at home, the City of Toronto’s Eco-Roof Incentive Program subsidizes the cost of installing both green and cool roofs. The program was introduced in 2009 to support the construction of green roofs. Green roofs offer a host of benefits, from managing rainfall to providing habitats and even space to grow food. But the program was expanded in 2011 to include cool roofs on industrial buildings. The lower maintenance needs of cool roofs makes them well suited to large, industrial buildings.
Although cool roofs are a better fit for some buildings than others, what’s most exciting is the evidence that light colours and reflective surfaces can keep cities cool, reduce energy use, and help deal with climate change. If roofs can be painted, then all of Toronto becomes a canvas of surfaces, from parking lots to streets, that could be adapted to reduce heat absorption and create a more livable city—during patio season and all year long.