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Cool Roofs and Roofing

Cool roofs: a roof using reflective roofing material, or an existing roof that has a coating put over it to improve reflective and emitting properties.

"Cool Roofs" are an increasingly popular roofing type primarily used on commercial and industrial buildings. The materials are commonly light colored to reflect heat and light from the building however darker colors are starting to be produced as well. In addition, cool roofs use materials that are reflective and that have a thermal emittance quality to them. Essentially this means that the heat that is absorbed by the roof will more readily be radiated back into the atmosphere than other materials would.

Cool Roof Properties

Cool Roofs are most popular on commercial and industrial buildings which often have flat roofs however cool roofing is not limited to a flat or low slope roofs. White vinyl is the most common material used in the building of a cool roof. It is able to reflect up to 80% of the suns rays, while emitting close to 70% of the heat absorbed by the building.

A roofing material's effectiveness at reflecting sunlight is scored on a scale from 0 to 1. If this number is low and close to 0 it is said to have a low albedo, meaning it has low reflective properties. Low reflectiveness means the building is more likely to absorb heat. The reciprocal, a high albedo, is effective at reflecting sunlight off the roof and will keep the roof cooler. Thermal emittance is scored the same way on a scale from 0 to 1.

While the scale for both reflective and emitting properties are on a scale from 0 to 1, scores are usually combined for roofing materials to give them a Solar Reflectance Index (SRI) score. An SRI score is from 0-100, but includes both reflective and emittance, making it easier to determine if a material makes an effective cool roof material or not. A score of 0 is a poor cool roof score while 100 is the most effective cool roof possible.

Although cool roofs are most effective in hotter climates they can also save money where there are cold winters. A less reflective roof would do better to reduce heating costs in the winter because it will absorb more heat from the sun. However in winter months the sun plays a minimal role due to the shorter days and the position of the sun, which is lower in the sky, therefore it doesn't shine directly on the roof as it would in summer. Also, the cost savings of cooling less in summer usually exceeds any extra heating you would need with a cool roof during winter.

A green roof is considered a type of cool roof.

Pros

  • Reduce annual cooling costs due to less absorption of heat into the building
  • Reduce roof maintenance
  • Extended roof life
  • Reduce the heat-island effect in urban areas, thus reducing air pollution
  • Increase indoor comfort
  • Reduce waste sent to landfills since cool roofs can usually be installed over existing roofs without need for tear off
  • Affordable
  • Homeowners can qualify for a $500 tax credit for meeting the requirements of the Federal Energy Policy Act of 2005
  • Rebates may be offered for a cool roof at the state and local government level

Cons

  • Though minimal, may require extra heating during winter

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