Science of Cool Roofs

 

How do cool roofs work?

To understand how roofs can keep cool in the sun, you need to first understand solar energy. Solar energy is composed of ultraviolet (UV) rays, visible light, and infrared energy. Slightly less than half of sunlight is visible; a small fraction, less than 10%, arrives as invisible ultraviolet light, sunburns, and nearly half of sunlight arrives as invisible near infrared radiation.

Half of the sun’s energy comes in the near-infrared. You can feel it but not see it. Image source: LBNL – Heat Island Group

The solar energy from the sun strikes a rooftop and mostly reflects back into the sky. However, some of that solar energy or radiation is absorbed as heat in the roof. Cool roofs reflect more sunlight and absorb less heat than traditional roofs. Traditional dark-colored roofing materials strongly absorb heat from sunlight, making them warm in the sun and heating the building. On the other hand, cool roofs stay cooler in the sun and transmit less heat into the building.

The "coolness" of a roof is determined by two properties of roofing materials and their combined effects on temperature:

• Solar reflectance – ability to reflect sunlight

• Thermal emittance – ability to cool down by emitting heat that is absorbed

 

Solar reflectance and thermal emittance are the two important properties of roofing materials to determine their “coolness” Image source: Cool Roof Rating Council (http://coolroofs.org)

Both of these properties are measured on a scale of 0 to 1 (0% -100%) – the higher values, the cooler the roof. Together, these properties help roofs to absorb less heat and stay up to 50–60°F (28–33°C) cooler than conventional rooftops during peak summer weather. White does a great job reflecting sunlight while black is a great absorber. Cool roofs began as white roofs, but today there are more cool-colored roof options. These cool-colors also reflect much of the sunlight keeping your house cool.

The standard brown roof (left) reflects 10% of near-infrared sunlight (narrow pink arrow). The cool brown roof (right) looks just like the standard brown roof but reflects about 40% of near-infrared sunlight (wide pink arrow). The building under the cool brown roof requires less air conditioning to maintain comfort. Image source: Patricia McDermond, http://mcdermond.com

The left product sample shows a standard asphalt shingle and the right sample is a cool asphalt shingle. As seen in the thermal image (above), the cool asphalt shingle stays cooler in the sun than the standard asphalt shingle by reflecting the near-infrared sunlight. Image source: LBNL – Heat Island Group