Greener Roofs Are... White?

On a hot day, a large city like Los Angeles can be up to 5°F hotter than surrounding suburbs. Cities like Los Angeles, New York, and Chicago create what are called “heat islands”. Heat Islands are literally, areas of heat that resemble islands when compared to rural surrounding areas. Within these cities areas develop that are considerably warmer than their surroundings, called “hot spots”. This stems mainly from the fact that buildings, concrete, and roads tend to retain heat better than rural countryside.

If you take a bird’s eye view of most cities, you will find that a large percentage of surface area is dark colored. Between asphalt roads, parking lots, and dark roofs, there is a lot of space that is exposed to the sun, essentially soaking up heat energy all day from the sunlight that hits it. Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored, typically made of a dark composite material consisting of tar and small rocks.

On very hot days, people often joke that it’s so hot that you can fry an egg on the sidewalk. That may not be far from the truth! In the heat of the full sun, a dark surface (like roofs or asphalt) can increase in temperature as much as 120°F, easily reaching temperatures of up to 150-190°F. That heat usually has nowhere to go. Some of the heat will radiate away in the air, but most of it will simply get stored in the “thermal mass” of the material in which it is generated. For roads, this heat can last well into the evening, basically giving off heat during the hottest parts of the day. For roofs, the building absorbs the heat and basically becomes an oven, forcing you to use air conditioning to get back to a comfortable temperature.

Air conditioning is an expensive solution to the heat generated this way. It’s also an expensive solution in general, both in terms of money, environmental impact, and also energy use. What if there could be an easy, low cost solution to this problem? How about if it could not only help solve the problem for you, but also for the people around you? What if it helped the environment without you having to do anything on an ongoing basis? Does that sound too good to be true?

New York City volunteers painting a roof white to save the environment.

The solution, backed by a growing number of environmental advocates and groups, is as simple as painting your roof white. White colored surfaces tend to reflect the majority of light energy (and therefore heat) away. While dark surfaces can absorb enough light/heat to increase their temperatures by 150°F or more, white or reflective roofs only reach around 10-25 °F above the ambient air temperature. White colored roofs can reduce the energy cost of air conditioning for a building by around 20% or more on a hot day.

According to research at Lawrence Berkeley National Laboratory, dark roofs only reflect about 10 to 20 percent of sunlight, absorbing the rest as heat. White colored roofs, on the other hand, reflect around 70 to 80 percent of light (and heat) back into space. White colored roofs also have the added benefit of extending the life of a roof by 5 to 10 years.

This solution offers another potential benefit. Hiring people to paint roofs white could help to stimulate the economy and reduce unemployment. Painting a roof white takes minimal training, can be done by relatively unskilled workers, and would save money in the long term while providing people with jobs in the short term.

In New York City, Mayor Bloomberg started a plant to train unemployed young adults to paint roofs white. Not only did this help generate jobs, but many of the people trained went on to get better jobs in energy-retrofitting companies. This “solution” is vaguely reminiscent of FDR’s plan to repair the struggling US economy after the economic collapse of the Great Depression; get people working while doing worthwhile things for the country.

In New York City, it is estimated that the cost of the paint for most of the roofs was easily offset by energy savings within a couple of weeks. Imagine how much energy could be saved (and jobs created) if every roof in every big city of the US were painted white. In a large, spread out city like Los Angeles, it is estimated that if every roof were painted white, over 600 megawatts of power would be saved per day and over $65 million per year from reduction in air conditioning use alone.

As more roofs in a city are painted white and other dark surfaces are replaced with more reflective surfaces, the cumulative effect increases. If enough cities in the world adopt this plan to paint roofs white, the reduction in energy use is enough to offset the carbon emissions of 300 million automobiles.

According to one of this plan’s most well known proponents, Steven Chu, if you make the asphalt pavement a more concrete type of color than the typical black color, “it's the equivalent of reducing the carbon emissions due to all the cars in the world by 11 years.”

It is, however, important to note that painting roofs white is only a good idea for areas where it does not snow regularly in winter. A dark colored roof is important to help generate enough heat from the sun to melt snow on the roof in winter months.

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Resources:

Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin - Federal study

http://heatisland.lbl.gov/PUBS/PAINTING/ - Information on heat islands, hot spots, and other factors related to city structures and heat.

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Researched and written by Dr. Rebecca Malamed, M.D. with assistance from Mr. Malcolm Potter.