Cool Roofs for Cooler Summers

On July 21, 2011, the Environmental and Energy Study Institute (EESI) held a briefing on the huge potential for solar-reflective roofs and other “cool-roofing” techniques to lower the surface temperature of buildings and entire cities. Cool roofs improve comfort on hot summer days and reduce the amount of energy used for air-conditioning – thereby reducing energy costs and improving air quality. Whitening flat roofs is a low-cost solution which, if implemented in certain cities across the globe, has been estimated to have the potential to offset the carbon emissions of 300 million automobiles. At this briefing, renowned physicist and energy efficiency expert Arthur Rosenfeld discussed research at Lawrence Berkeley National Laboratory (LBNL) that for the first time quantifies the reflective power (albedo) of urban surfaces that would be necessary to mitigate the urban heat-island effect and offset carbon dioxide emissions. Panelists also discussed insulated and vegetated (“green”) roofs and how different types of cool roofs may be combined or integrated with solar-roofing systems, photovoltaics (PV) and/or solar thermal technology.

Highlights from Speaker Presentations

• Sen. Chris Coons (D-DE) said that energy efficiency “has positive impacts at all levels of our society and our economy, with both near-term benefits and long-term benefits in cost avoidance and environmental impact,” and that cool roofs are a simple, cost-effective part of an energy efficiency strategy.
• Cities are hotter than surrounding areas due to tree loss, roofs, and asphalt. New York City is about 10°F warmer than surrounding area due to this urban heat island effect.
• In the Chicago heat wave of 1995, 739 people died. The highest risk group was people who lived on the top floor of buildings with black roofs.
• White roofs reflect sunlight, providing a cooling effect in two ways:
• The building stays cooler because it absorbs less heat.
• Less electricity is used for air conditioning, thereby reducing greenhouse gas emissions and mitigating global warming.
• Green, or vegetated, roofs cool buildings and cities comparably with white roofs, but do not reflect as much sunlight and thus have less benefit for global cooling. Green roofs also provide storm water management benefits.
• A typical white roof in August will be only 10°F hotter than the ambient air. Black paint roofs and steel roofs will be 50°F and 90°F hotter, respectively, than the ambient air.
• A 1,000 square foot white roof will offset 10 tons of carbon dioxide (CO2) over its 20 year lifetime. If all eligible urban flat roofs worldwide were whitened, about 24 Gigatonnes of CO2 would be offset. This is equivalent to taking 300 million cars off the road for 20 years.
• The New York City Panel on Climate Change estimates a 3° to 5°F increase in the city's air temperatures by 2050. As part of its strategy to reduce CO2 emissions 30 percent by 2030, NYC requires all new or replacement flat roofs to be either cool or green. This policy is expected to reduce temperatures by 1° to 1.5°F.
• Cool roof coatings cost about $0.40/square foot. New York City’s cool/green roof initiative is expected to save $135 million per year in electrical bills by 2035, translating to a six-year simple payback based on energy savings (not counting healthcare savings).
• In 2005, California’s “Title 24” energy efficiency standards prescribed white surfaces for low-sloped roofs on commercial buildings. Several other states are considering similar standards.
• Cool roof coatings extend the life of a roof by five to 10 years, lowering costs and decreasing solid waste. Roofing accounts for eight percent of solid waste in the United States.
• For maximum energy efficiency benefits, cool roofs should be paired with high levels of insulation.
• Around 100 federal buildings in need of new roofs have been renovated with cool roofs, green roofs and/or solar panels (which are also reflective) using funds from the American Recovery and Reinvestment Act of 2009.

Related Media Coverage

What We're Reading: Keeping cool under a green roof by Senator Chris Coons Blog
On Hottest Day of Summer, Congress Catches Up on Cool Roofs by Spotlight on Polyiso blog

Background

Dr. Rosenfeld and colleagues Hashem Akbari and Surabi Menon presented their findings in the paper Global Cooling: Increasing Worldwide Urban Albedos to Offset CO2 (Climatic Change Vol. 95 Joint Issue 3-4, May-June 2009). As policymakers at all levels of government look for ways to cut costs while maintaining or enhancing infrastructure and services, some see rooftops as valuable but largely underutilized real estate that can address multiple goals.