The Lovin’ Spoonful weren’t kidding — summer in the city is definitely a hot time. In fact, at the peak of the day’s heat, temperatures in urban and suburban neighborhoods can be as much as 10 degrees higher than in nearby rural areas. The phenomenon is known as the urban heat island effect. Essentially, it’s the result of built surfaces absorbing and trapping solar heat.
The heat island effect is a problem because those extra degrees force people to crank up the air conditioning and use even more energy at peak times. According to the Environmental Protection Agency, from 3% to 8% of a communities’ summer energy demand goes to compensating for the effect. The higher temperatures also heat pollutants in the atmosphere, which makes more smog, leading to health problems.
Hot Effect
Roofs, both low-slope commercial and steep-slope residential, play a significant role in creating the heat island effect. Most roof materials, asphalt shingles, for example, are dark and do a poor job of reflecting and emitting heat. To mitigate the heat island effect and reduce a building’s cooling load in summer, experts recommend turning heat-absorbing roofs into “cool roofs” — roofs that have high solar reflectance and high heat emissivity.
According to Energy Star, its program-certified cool roof products can lower a roof’s summer afternoon temperature by as much as 100o F and reduce peak cooling demand by 10% to 15%. A study by the Florida Solar Energy Center, which included both commercial and residential buildings, found that cool roofs could produce an energy savings of 40%.
Steep Slope Limits Options
On commercial buildings or residences with low-slope roofs, it’s easy to create a cool roof by adding a reflective coating or membrane. Such simple fixes aren’t available for most single-family homes, however, because membranes and coatings can’t easily be applied over existing steep-slope roofs. So the choices for single-family houses are generally limited to installing a whole new roof made of reflective material, either metal or reflective clay or cement tiles.
Metal roofs, though highly reflective, absorb heat. But new manufacturer-applied pigments reflect infrared sunlight, which improves reflectivity enough to offset the heat gained by absorption. Clay and cement tiles also have recently benefited from the development of reflective pigments that enable these tiles to perform in the reflectance department and still offer an appealing color.
Of course metal, cement, and clay are all considerably more expensive and more difficult to install than traditional materials. It’s probably a choice that homeowners won’t make solely on the basis of lowering their energy bills. However, though they have their drawbacks, all three reflective roof materials are more durable than traditional asphalt shingles, so there are life-cycle savings to consider, too. If your client has a green streak or is already considering replacing asphalt with a longer-lasting material, explaining the benefits of cool roofs could help make the sale.
—David Zuckerman is a freelance writer based in New York.
