(John Kemp is a Reuters market analyst. The views expressed are
By John Kemp
LONDON, April 17 Converting the world's roofs
from dark colours to reflective white would cancel the warming
effect of 1 billion tonnes of carbon dioxide each year for the
lifetime of the roof, according to former California Energy
Commissioner Arthur Rosenfeld.
The reduction in warming would be equivalent to taking half
the world's cars off the road for 20 years, Rosenfeld wrote in a
recent commentary for the International Energy Agency ("White
roofs cool the world efficiency" April 3).
Rosenfeld's views are influential because he has been among
the pioneers of the energy efficiency movement in the United
The former physics professor is closely associated with the
introduction of new efficiency standards for buildings and
appliances such as refrigerators which have headed off the need
to build dozens of power plants in California and the rest of
the United States since the late 1970s. ("The Art of Energy
Now Rosenfeld is promoting the Global Cool Cities Alliance,
which advocates low cost ways to increase the solar reflectance
of buildings and pavements to cool buildings, reduce air
conditioning load and mitigate the effects of climate change.
The alliance has already signed up Chicago, New York and
Singapore among its members.
Since October 2005, California's building code has made it
mandatory to install "cool roofs" on most new buildings and when
roofs are altered or buildings are extended (California Code of
Regulations, Title 24).
New York City has installed cool-roof materials on 3.7
million square feet of roof surface since 2009, according to NYC
CoolRoofs, a collaboration between the city's Department of
Buildings and voluntary organisations, backed by Mayor Michael
NYC CoolRoofs claims that replacing roofing or applying
reflective coatings can reduce internal building temperatures,
making buildings more comfortable during the hot summer months.
It also can cut the urban heat island effect, which makes
cities up to 5 degrees Fahrenheit hotter than surrounding areas,
and can reduce carbon emissions, cut power consumption and
extend the lifespan of roofing materials and air-conditioning
New York City's building code requires all new buildings
with flat or low-sloping roofs to have roofs that either are
white or comply with EnergyStar requirements published by the
federal Department of Energy (NYC Building Code Section 1504.8).
Since January 2012, cool materials have also been required
for all roof replacements of more than 500 square feet under an
ordinance approved by the City Council.
The U.S. Department of Energy requires cool roofing
materials on all of its buildings during construction or
replacement, whenever it's cost effective.
By 2010, the Department's National Nuclear Security
Administration (NNSA), which maintains the stockpile of nuclear
weapons, had installed cool materials on more than 2 million
square feet of roof space. This is still a small amount, but it
continues to roll out the requirement across its estate.
NOT JUST WHITE
The simplest cool roofs are white or another light colour.
But in many countries, darker roofs are preferred for aesthetic
reasons, which is why early cool-roof initiatives focused on
flat or low-sloped roofs not visible from the ground.
Steeply sloping roofs, typically favoured for domestic
dwellings, were often exempted from early cool-roof
Suppliers of building materials have responded, however, by
developing a range of "cool dark" roofs in traditional colours
from black, blue and green to terracotta.
While cool dark materials do not perform as well as a white
roof, they can still cut the heating effect substantially
compared with conventional roofing materials of exactly the same
colour, according to the U.S. Department of Energy ("Guidelines
for Selecting Cool Roofs" July 2010).
In most cases, cool roofing materials cost no more than
their conventional counterparts. Advocates say they may actually
prove cheaper in the long run because they heat up less, which
could extend the lifespan of the materials before they need
replacing, though this remains unproven for now.
Standard roofing materials in dark colours such as black and
grey absorb 80 percent or more of the incoming energy from the
sun. As a result the surface temperature of a standard black
roof can be much hotter than the surrounding air. Some of that
heat is lost into the building, raising the internal temperature
and requiring additional air-conditioning to compensate.
White roofing materials, by contrast, reflect 90 percent or
more of the incoming solar energy, so the roof may be only a few
degrees higher than the surrounding air. On a clear day, 80
percent of the reflected energy will pass into space without
warming the atmosphere or returning to Earth.
Cool roofs work in much the same way as the albedo effect,
in which polar ice and snow reflect sunlight back into space.
About half of the energy from the sun arrives in the
invisible near-infrared part of the spectrum. So roofing
manufacturers have been able to design special materials in cool
dark colours that reflect most near-infrared energy, while
leaving their impact on the visible part of the spectrum and
apparent colour unchanged.
A roof with a smooth bright white surface can reflect about
85 percent of the incoming surface and will be 5 degrees Celsius
hotter than the outside air on a summer afternoon.
A roof with a clean cool red tile can reflect about 35
percent of incoming sunlight and will be about 20 degrees warmer
than the outside air on the same sunny summer afternoon.
But it still does much better than a standard grey roof,
which reflects just 20 percent of incoming light and is 38
degrees hotter than the surrounding air, according to the
Lawrence Berkeley National Laboratory ("Cool Roof Questions and
COST AND ROLLOUT
Cool roofs absorb less sunlight during the winter months, so
buildings may need to consume additional energy during the
heating season. The so-called "heating penalty" is very small in
most cases. Nonetheless, it makes most sense to install cool
roofs in areas that have high summer cooling demand and fairly
modest winter heating needs.
The case for installing cool roofs is strongest in the
southernmost tier of states, stretching from Florida and Texas
through to New Mexico, Arizona and California, according to the
U.S. Energy Department. Cities further north, such as New York,
also see benefits because of their extreme summer cooling
It is rarely economic to replace a mechanically sound roof
just to increase its solar reflectance. While cool materials
cost about the same as conventional ones, energy savings are
rarely enough to cover more than a small fraction of the cost of
a new roof. So the Energy Department recommends selecting a cool
product for new construction or when an old roof is scheduled to
The European Parliament's decision on Tuesday to reject
proposals to prop up carbon prices by backloading the auction of
new emissions allowances indicates the politics of climate
change and energy conservation may be changing as policymakers
become more concerned about costs and the impact on
Recent evidence the planet may be warming more slowly than
earlier climate models predicted could also reopen the debate in
the months ahead about the costs and benefits of taking action
on climate change.
But the move towards cooler roofing demonstrates how deeply
entrenched measures to cut energy consumption and emissions have
become in the policymaking process, regardless of the high-level
debate over carbon markets, taxes and global temperatures.
Increased energy efficiency requirements have been written
into building codes and appliance efficiency standards across
the United States and much of Europe for everything from roofs
to light bulbs, refrigerators, water heaters, televisions and
Vehicle efficiency standards and requirements to curb
emissions from power plants have all been enacted or are in the
Even without further intervention, these energy efficiency
and emissions cuts will continue to roll out in the years ahead
as the stock of old appliances and buildings gradually turns
over and is replaced with more efficient versions that comply
with updated codes.
(editing by Jane Baird)