Know-How: Roofing
Mar 1, 2007 12:00 PM, By Mike Kennedy
According to the U.S. Environmental Protection Agency (EPA), more than 90 percent of the roof surfaces in the United States are dark-colored and have low reflectance.
The heat those roofs attract and absorb have several effects, the EPA says, and none of them are positive: increased cooling energy use and more costly utility bills; higher peak electricity demand; reduced comfort in the building; more air pollution because of the intensification of the “heat-island effect;” accelerated deterioration of roofing materials; and higher roof maintenance costs.
So what should education institutions do about it? Get cool.
Cool roofs — roofs made of or coated with materials that have high reflectance and high emittance — will significantly reduce the solar heat gain in a building, according to the U.S. Department of Energy's National Best Practices Manual for Building High Performance Schools.
“In air-conditioned buildings, use a roof surface that is light in color (high reflectance), yet has a non-metallic finish (high emissivity),” the manual recommends.
A roof with high reflectance — a single-ply membrane or liquid-applied coatings — will not absorb solar heat, and the high emissivity will dissipate heat from the roof into the air.
The kinds of white single-ply membranes available include EPDM (ethylene-propylene-diene-terpolymer membrane); PVC (polyvinyl chloride); and TPO (thermoplastic polyolefin). The EPA says costs of single-ply roofing range from $1.50 to $3.00 per square foot, including materials, installation and reasonable preparation work.
For existing roofs, cool roof coatings can be applied. These white liquids have the consistency of thick paint and are most commonly elastomeric, polyurethane or acrylic. The EPA estimates that cool roof coatings cost between $0.75 and $1.50 per square foot for materials and labor, depending on the condition and size of the roof, the number of roof penetrations or obstacles, and the ease of access to the roof.
The benefits of cool roofs can be seen in most climates, but the payback is likely to be less in cooler climates, according to the Best Practices Manual.
Cool roofs tend to lose their reflectance over time, the EPA says, because particles and air pollution can accumulate on the surface. Washing a cool roof periodically or reapplying a coating after a number of years (some manufacturers recommend applying a new coat after 10 years) can help a roof maintain its high reflectance.
Another way to boost a roof's energy efficiency is a green roof system. These roofs have plants that are growing in lightweight soil. They absorb and slow rainwater runoff, add more insulation to a rooftop, and help reduce building heat gain.
The Best Practices Manual cautions that a green roof requires structural steel to support its weight, and the steel may offset some of the environmental benefits of such a roof.
NOTABLE
150 to 190
In degrees Fahrenheit, the temperature of a conventional roof surface at midday in the summer.
100 to 120
In degrees Fahrenheit, the temperature of a cool roof surface at midday in the summer.
Source: U.S. Environmental Protection Agency
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