The Case for White Roofs

The Case for White Roofs


by Tony Douglas

Why, on a hot summer day is it more comfortable to walk barefoot on a white concrete sidewalk than a black asphalt street? And why does it feel cooler when you get in a white car that’s been parked in the sun than a black one?

It’s as simple as black and white. Black absorbs the sun’s energy while white reflects it. The same principle applies to the roof over your head. Buildings with white roofs stay cooler than those with black ones. But there are a lot of black roofs in North America. Apartments, offices, hospitals, schools, libraries, warehouses and many urban homes have flat or low-sloped roofs, and 80 per cent of them are covered with either black asphalt or a black rubber sheet product known as EPDM. That’s like having a hot blanket on your roof — it keeps everything underneath very warm.

Research has shown that blacktop roofs can soar to 77C on a 27 C day and remain as high as 60C after dark. On the same 27 C day, a white roof will reach just 28C. So how do you cool off a black roof? Coat it white!

But not any coating will do. Regular white paint, for example, won’t stick to some roofing material, wont remain flexible, and is not effective at sealing out water. Brentwood Polymer Coatings of Barrie, Ontario, Canada, manufactures Safe-T-Plex, a white reflective, elastomeric coating that adheres to black EPDM rubber membrane, steel roofing, and sprayed polyurethane foam insulation. In addition , the product adheres well to wood, concrete, pvc, and hypalon roofing. This product remains permanently flexible, even in below freezing temperatures, affording a watertight membrane that will not chalk or fade with continuous exposure to the elements. The coating is able to withstand hail as well as foot traffic from workmen servicing rooftop installations.

Solving a public health issue

A number of large North American cities, including Philadelphia, have become increasingly concerned about the number of heat-related deaths each summer, especially among the elderly. Two years ago, a Philadelphia non-profit organization, Energy Coordinating Agency Inc. began putting white coating on the roofs of older residents in low-income city neighbourhoods. They found that the coatings reduced the temperature inside these homes, making them safer for elderly residents. Now Philadelphia is considering a new building code mandating that flat or low-slope roofs have high solar reflectance.

In most cases, this means coating them white. Chicago recently adopted a similar building code. Toronto recently introduced an Eco-Roof Incentive Program, designed to promote the use of cool roofs (white reflective) on its commercial, industrial and institutional buildings, and to help its business community take action on climate change.

The Case for White Roofs

Saving the planet one roof at a time

Nearly 17 per cent of all electricity consumed in North America is used to cool buildings. Coating black roofs white can help reduce that number by increasing energy efficiency. During a one-year study, scientists discovered that a building with a white acrylic roof coating consumed nearly 22 per cent less energy for air conditioning in the summer than one with an uncoated black roof. Surprisingly, the white coating also reduced energy demand by almost four per cent in the winter.

White roof coatings also contribute to a healthy environment by extending the lifespan of black roofs. Six per cent of waste material dumped in landfills consists of old roofing waste. White coatings help roofs last longer by reducing the drastic changes in temperature that cause roof wear and tear, and that benefits the environment by reducing the size of our landfills. The cost to coat an existing tar and gravel roof is less than half the cost of re-roofing.

Reflectance of typical Roof Materials

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