Technology Overview: Cool Roofs

August 27, 2007
 
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Working on a Cool Roof
Working on a Cool Roof

 

Cool roofs have a combination of reflectance and emittance properties that keep them cool on hot days. They reduce the cooling loads of buildings, especially during peak times.

How it works
Almost any roof can be a cool roof. They usually require no special design work or components and can be made from many different materials, including:

  • Light-colored aggregate
  • Applied liquid coatings
  • Shingles, slate, or tile
  • Single-ply membranes, or
  • Vegetation.

How it saves energy
Cool roofs reduce the load on a building's air conditioning system by reducing the amount of heat conducted into the building from the roof. They are especially effective at reducing peak electricity demand.

Cool roofs have a combination of high solar reflectance and high solar emittance. High reflectance means that less infra-red heat is absorbed by the roof, and high emittance means that once the roof has heated up, it re-radiates solar heat back out to the sky more quickly. Cool roofs that use vegetation also increase the thermal mass of the building and reduce water run-off.

Design considerations
Most definitions of "cool roofs" include only their reflectance and emittance properties. It's important to remember, however, that the thermal performance of a roof depends on many factors including roof geometry, substrates, underlayments, thermal breaks, ventilation, and insulation. Even cool roofs benefit from high levels of insulation.

Although most cool roofs are white, some darker colored roofs can also be "cool." This is because visible light makes up less than half the sun's energy, and these darker roofs reflect more infra-red light.

To remain effective, cool roofs must maintain their high solar reflectance. Fungal growth and the build-up of dust are the main causes of degradation and should be counteracted with regular cleaning. The cost-effective frequency of cleaning will depend on the roof material, the pitch of the roof, and the location of the building.

In cooler climates, the high emittance of cool roofs can sometimes be an energy penalty because it increases winter heat loss. So make sure that the emittance of the roof is optimized for the building's climate.

 
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