Know-How: Roofing

Apr 1, 2002 12:00 PM, Mike Kennedy

Common sense tells you not to wear your black turtleneck on a hot, sunny day. For the same reason, if you don't want rays from the sun beating down on your school building and making it uncomfortably hot, you should make sure your roofs are able to stay cool.

How effective a roof is in keeping a building cool is measured in two ways, according to the U.S. Department of Energy's Oak Ridge National Laboratory: solar reflectance and infrared emittance. Solar reflectance is represented as a number between 0 and 1 and measures how much energy from sunlight is reflected away from the roof. Infrared emittance, also represented as a number between 0 and 1, measures how much energy from the solar radiation absorbed by a roof is emitted into the atmosphere. The higher each number is, the better job a roof does of turning heat away from a building.

“On a sunny day in late June, all of July or early August in the Northern Hemisphere, a black roof surface (solar reflectance less than 0.1) may reach peak temperatures exceeding 170 degrees,” says a fact sheet from the Oak Ridge National Laboratory. “At the same time a highly reflective white surface (solar reflectance greater than 0.8) could be less than 110 degrees.”

Facility managers can control solar radiation when they install a roof by choosing a membrane with high solar reflectance and infrared emittance. According to the Oak Ridge lab, traditional built-up roofs that use materials made of asphalt typically have low solar reflectance.

“Single-ply roofing membranes are available with high solar reflectances and high infrared emittances,” says the Oak Ridge fact sheet.

For a facility that already has a traditional black roof, schools can improve the solar reflectance with coatings that can be sprayed, brushed or rolled onto a roof membrane.

“True radiation control coatings are generally white, water-based latex or acrylic products with titanium dioxide added to achieve high solar reflectance,” says the Oak Ridge lab. “The membrane needs to weather several weeks and/or special base coats must be applied to form a good bond between the coating and the roof.”

The lab cautions that weathering can significantly diminish the effectiveness of solar reflectance. “A typical white coating with initial solar reflectance exceeding 0.8 will likely have solar reflectance below 0.55 after a few years of exposure,” the fact sheet says.

NOTABLE

0.06

Solar reflectance for black EPDM (synthetic rubber) roof membrane.

0.06

Solar reflectance for a smooth bitumen roof.

0.12

Solar reflectance for dark gravel on a built-up roof.

0.23

Solar reflectance for gray EPDM roof membrane.

0.34

Solar reflectance for light gravel on a built-up roof.

0.65

Solar reflectance for white-coated gravel on a built-up roof.

0.69

Solar reflectance for white EPDM roof membrane.

Source: Cool Roofing Materials Database, Lawrence Berkeley National Laboratory.