Converting the world's roofs from dark colors to reflective white would cancel the warming effect of 1 billion tons of carbon dioxide each year for the lifetime of the roof, according to former California Energy Commissioner Arthur Rosenfeld.
The reduction in warming would be equivalent to taking half the world's cars off the road for 20 years, Rosenfeld wrote in a recent commentary for the International Energy Agency ("White roofs cool the world efficiency" April 3). Rosenfeld's views are influential because he has been among the pioneers of the energy efficiency movement in the United States.
The former physics professor is closely associated with the introduction of new efficiency standards for buildings and appliances such as refrigerators which have headed off the need to build dozens of power plants in California and the rest of the United States since the late 1970s. ("The Art of Energy Efficiency" 1999).
Now Rosenfeld is promoting the Global Cool Cities Alliance, which advocates low cost ways to increase the solar reflectance of buildings and pavements to cool buildings, reduce air conditioning load and mitigate the effects of climate change. The alliance has already signed up Chicago, New York and Singapore among its members.
Since October 2005, California's building code has made it mandatory to install "cool roofs" on most new buildings and when roofs are altered or buildings are extended (California Code of Regulations, Title 24).
New York City has installed cool-roof materials on 3.7 million square feet of roof surface since 2009, according to NYC CoolRoofs, a collaboration between the city's Department of Buildings and voluntary organizations, backed by Mayor Michael Bloomberg.
NYC CoolRoofs claims that replacing roofing or applying reflective coatings can reduce internal building temperatures, making buildings more comfortable during the hot summer months.
It also can cut the urban heat island effect, which makes cities up to 5 degrees Fahrenheit hotter than surrounding areas, and can reduce carbon emissions, cut power consumption and extend the lifespan of roofing materials and air-conditioning equipment.
New York City's building code requires all new buildings with flat or low-sloping roofs to have roofs that either are white or comply with Energy Star requirements published by the federal Department of Energy (NYC Building Code
Since January 2012, cool materials have also been required for all roof replacements of more than 500 square feet under an ordinance approved by the City Council.
The US Department of Energy requires cool roofing materials on all of its buildings during construction or replacement, whenever it's cost effective.
By 2010, the Department's National Nuclear Security Administration (NNSA), which maintains the stockpile of nuclear weapons, had installed cool materials on more than 2 million square feet of roof space. This is still a
small amount, but it continues to roll out the requirement across its estate.
Not just white
The simplest cool roofs are white or another light color. But in many countries, darker roofs are preferred for aesthetic reasons, which is why early cool-roof initiatives focused on flat or low-sloped roofs
not visible from the ground.
Steeply sloping roofs, typically favored for domestic dwellings, were often exempted from early cool-roof
Suppliers of building materials have responded, however, by developing a range of "cool dark" roofs in traditional colors from black, blue and green to terracotta.
While cool dark materials do not perform as well as a white roof, they can still cut the heating effect substantially compared with conventional roofing materials of exactly the same color, according to the US Department of Energy ("Guidelines for Selecting Cool Roofs" July 2010).
In most cases, cool roofing materials cost no more than their conventional counterparts. Advocates say they may actually prove cheaper in the long run because they heat up less, which could extend the lifespan of the materials before they need replacing, though this remains unproven for now.
Standard roofing materials in dark colors such as black and grey absorb 80 per cent or more of the incoming energy from the sun. As a result the surface temperature of a standard black roof can be much hotter than the surrounding air. Some of that heat is lost into the building, raising the internal temperature and requiring additional air-conditioning to compensate.
White roofing materials, by contrast, reflect 90 percent or more of the incoming solar energy, so the roof may be only a few degrees higher than the surrounding air. On a clear day, 80 per cent of the reflected energy will pass into space without warming the atmosphere or returning to Earth.
Cool roofs work in much the same way as the albedo effect, in which polar ice and snow reflect sunlight back into space. About half of the energy from the sun arrives in the invisible near-infrared
part of the spectrum. So roofing manufacturers have been able to design special materials in cool dark colors that reflect most near-infrared energy, while leaving their impact on the visible part of the spectrum and apparent color unchanged.
A roof with a smooth bright white surface can reflect about 85 per cent of the incoming surface and will be 5 degrees Celsius hotter than the outside air on a summer afternoon. A roof with a clean cool red tile can reflect about
35 per cent of incoming sunlight and will be about 20 degrees warmer than the outside air on the same sunny summer afternoon.
But it still does much better than a standard grey roof, which reflects just 20 per cent of incoming light and is 38 degrees hotter than the surrounding air, according to the Lawrence Berkeley National Laboratory ("Cool Roof Questions and Answers" 2009).
Cost and rollout
Cool roofs absorb less sunlight during the winter months, so buildings may need to consume additional energy during the heating season. The so-called "heating penalty" is very small in most cases. Nonetheless, it makes most sense to install cool roofs in areas that have high summer cooling demand and fairly modest winter heating needs.
The case for installing cool roofs is strongest in the southernmost tier of states, stretching from Florida and Texas through to New Mexico, Arizona and California, according to the US Energy Department. Cities further north, such as New York, also see benefits because of their extreme summer cooling problems.
It is rarely economic to replace a mechanically sound roof just to increase its solar reflectance. While cool materials cost about the same as conventional ones, energy savings are rarely enough to cover more than a small fraction of the cost of a new roof. So the Energy Department recommends selecting a cool product for new construction or
when an old roof is scheduled to be retrofitted.
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