What is a white roof?
A white roof is painted with solar reflective white coating and reflects up to 90% of sunlight (as opposed to traditional black roofs which reflect only 20%). A white roofs are a lot more than just white paint! White roofs' benefits are measured partly according to the solar reflectance index, or SRI. A roof's SRI is a measure of a surface's ability to reflect solar heat. SRI measures reflectance (reflecting the sun's rays) and thermal emittance (the roof's ability to radiate absorbed heat). If you've ever worn a black t-shirt in the sun than you already understand that black's reflectance is negligible and its emittance is zero. A white roof's reflectance is as high as 90% and its emittance is 100%. Learn more from our friends at the Cool Roof Rating Council.
Why do we need white roofs?
The world is hot! Read more about this on our Urban Heat Island page.
The Urban Heat Island effect is a measurable increase in ambient urban air temperatures resulting primarily from the replacement of vegetation with buildings, black surfaces like roofs and roads and other heat-absorbing infrastructure. Cities are full of these heat trapping black surfaces.
On a clear night, the temperature in a city with 1 million people or more can be up to 22°F hotter than nearby suburbs due to the heat island effect. Urban Heat Island effects include increased summer energy use, stress on the power grid, increased pollution, and health hazards such as asthma. Urban Heat Islands also contain hotspots (areas that are superheated due to population, environmental injustices or industrialization). White Roof Project targets hot spots on the Urban Heat Island to maximize the benefits of its work. For example, Manhattan's Lower East Side is (home to many of our roof projects) has been identified as a hot spot in a recent NYSERDA report. Hot spots are often high energy users as residents attempt to compensate for summer heat. Con Edison confirms that the Lower East Side is one of the highest per capita energy users in New York.
White roofs keep buildings cool and reduce energy use.
5–10% of summer electricity is used to compensate for the Urban Heat Island effect. Americans spend about $40 billion annually to air condition buildings — one-sixth of all electricity generated in this country! White roofs can reduce summer energy use by 10–40%, saving money and preventing pollution. White roofs can reduce the total Urban Heat Island effect in an impacted city by 1° to 2° F - enough to lower peak energy demand, reducing the risk of brownouts and blackouts. Read more.
White roofs help curb climate change.
White roofs help curb climate change. “...replacing non-reflective, dark roofing materials with white ones ... (every 1000 square feet) would result in an equivalent CO2 offset of 10 metric tons (about $250) annually". Learn more.
The "rate of smog formation depends on temperature, (Los Angeles based models) showed an overall reduction in smog by about 10%, the equivalent of removing three to five million cars from the roads" due to the implementation of white roofs which reduce smog accumulation by lowering heat (ambient air temperature). Read more.
White roofing prevents heat related warping and cracking extending the life of a roof. White roofing can reduce roofing waste added to landfills. Annually "roofing replacement generates 8 to 10 million tons of old roofing waste. Historically, about 95% (or 22 million cubic yards) ... has ended up in landfills". Learn more.
White roofs help builders and building managers meet evolving roofing standards. (Examples: California's Title 24 Energy Efficiency Building Standard, NYC Department of Buildings roof reflectance standards found on our blog at .) Some cities and states now have minimum roof reflectance requirements.
What about white roofs in the winter?
In the northern part of the United States, you might think that a black roof would provide a winter heating benefit to building owners. However, there are several factors that make any potential heat gain relatively immaterial: The laws of physics dictate that hot air will always rise. Thus, any heat that is transferred to the interior of a building structure from the outside will remain at the top of the structure, providing minimal heat savings. In all parts of North America, there are fewer hours of sunlight to affect energy costs. In fact, in some areas, there is a greater than a six-hour difference between peak-summer and peak-winter sunlight, meaning there is less sunlight available to contribute to a building’s potential warming. Plus, the angle of the sun is less direct, which also helps to minimize warming potential.
See www.epa.gov/hiri/strategies/coolroofs.html for charts and calculations illustrating the net benefit of white roofing in major cities. Savings are calculated subtracting any winter penalties. Lawrence Berkeley National Laboratories heat island groups finds that the net benefits of white roofing could save millions in energy costs per year. Read More
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