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.
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)
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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