Wednesday, July 25, 2012

The Benefits of Cool Roofs

Cool roofs offer energy savings and global warming mitigation. In its simplest essence, Cool roofs offer high solar reflectance. This is achieved primarily by reflecting the visible, infrared and ultraviolet wavelengths of the sun, which in turn reduces heat transfer to the building. Cool roofs are also are high in thermal emittance (the ability to radiate absorbed, or non-reflected solar energy).  Cool roofs can reportedly save up to 15% off the annual air-conditioning energy use in a single-story building.

Most of the sunlight that falls on a white roof much of is reflected and passes back into space. But when sunlight falls on a dark roof most of it is absorbed and converted into much longer wavelengths which we know as heat. The atmosphere is transparent to sunlight but opaque to heat, which is why white roofs help cool the planet and dark roofs warm the planet.

Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (126 °F), reaching temperatures of 70 to 90 °C (158 to 194 °F). White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all.

A 1,000-square-foot (93 m2) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime. The potential reduction is GHGs is very significant. If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 Gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years.

A 2012 study by researchers at Concordia University estimated that worldwide deployment of cool roofs and pavements in cities would generate a global cooling effect equivalent to offsetting up to 150 Gigatonnes of carbon dioxide emissions - enough to take every car in the world off the road for 50 years.

Cool roofs have a wide range of benefits including:
  • Improved energy efficiency
  • Reduced greenhouse gas emissions
  • Reduction in urban heat island effect and smog
  • Improved occupant comfort (Less heat)
  • Reduced cooling energy load
  • Save peak electricity demand costs if you have time-of-use metering.
  • Extended roof life service life and help
  • Reduce roofing waste added to landfills.
  • Comply with codes and green building programs (like Title 24 Energy Efficiency Building Standards)
Reduced building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day. Research shows that conventional (black) roof membranes degrade from the sun adversely impacting durability. High temperatures and large temperature variations are also detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v and heat degradation.

White or white coated roofing membranes, or white gravel are best but Cool roofs can come in many colors, even some dark colors are EnergyStar rated, but white is the most reflective.

One contested study by researchers at Stanford University suggested that although reflective roofs decrease temperatures in buildings and mitigate the urban heat island effect, they may actually increase global temperature. However, according to a 2008 case study in the Province of Almeria, Southern Spain, cool roofs reduced the ambient temperature by 1.6ºC over a period of 20 years compared to surrounding regions.

© 2012, Richard Matthews. All rights reserved.

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