Cool Roofs: Taming the Urban Jungle

Kirsten Nelson-Johnson

Green and cool roofs are proliferating across the tops of buildings all over the world. While rooftop gardens have  been a luxury since perhaps before the 19^th century, their benefit and functionality are now only being realized.  The benefits are two fold - providing carbon capture in urban areas and counteracting rising temperatures cause by the “heat island” effect.

During photosynthesis, plants remove carbon dioxide from the atmosphere and store CO₂ in their leaves, soil and roots, converting sunshine into carbon-based compounds such as carbohydrates and sugar. A green roof strategy would definitely be one way of managing the sequestration of carbon, especially considering  the amount of roof space available in metropolitan areas currently. It is estimated that the city of Detroit has 219 acres of roof space available for green roof projects. If all available black tar roofs were retrofitted in this city alone, 55,000 tons of CO₂ could be removed from the air each year- enough CO₂ to offset the carbon emissions of 10,000 mid-sized SUVs or trucks for an entire year. A study by the Berkeley Lawrence Lab found that if 15 percent of the buildings in Los Angeles installed reflective or green roofs, daytime temperatures would be reduced by 3 degrees Celsius, in turn saving Los Angeles 0.5 to 1 gigawatt of power during peak-use hours. 

“Heat island effect” describes the phenomenon of built up areas being hotter than nearby rural areas due to the use of hot dark materials like brick and asphalt, which absorb more of the sun’s rays and thereby increase the surface temperature. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C). Heat islands affect communities by increasing summertime peak energy demand through air conditioning costs, increasing air pollution and greenhouse gas emissions, exacerbating heat-related illness and mortality, and negatively impacting water quality. Just as one wears light colored clothing in warm climates to keep cool, light colored roofs help to reflect the sunlight and decrease the heat absorption experienced by darker colors. Cool roofs can reduce the roof surface temperature by up to 100 degrees Fahrenheit, thereby reducing the heat transferred into the building below.

Ask an expert to define cool roof, and the explanation will include two very important terms: “reflectivity” and “emissivity.” Strong performance in those two areas - the ability to reflect sunlight off a facility’s rooftop and to minimize roof temperature, is key to any cool roof. From a bottom-line perspective, the major benefit of a cool roof is its potential to deliver huge energy savings. Reducing the absorption of solar heat through the roof decreases the need for air conditioning and lowers a facility’s energy consumption during warmer months.

Benefits of Cool Roofs

• Save on annual electricity bills by reducing summer air conditioning costs.
• Save peak electricity demand costs if you have time-of-use metering.
• Reduce roof maintenance and replacement expenses by extending roof life.
• Increase indoor comfort in summer by reduction of infrared conversion from visible light.
• Reduce the "heat island effect" in cities and suburbs.
• Reduce air pollution and smog formation.
• Reduce roofing waste added to landfills.
• Help builders and building managers meet California's Title 24 Energy Efficiency Building Standards.