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Cool Roof Technology Could Eliminate Billions Of Tons
Of Carbon Dioxide
By
Steve Hanley
21 June
2023
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In 2010, a study
by Berkeley Lab using data from NASA
estimated that using “cool roof ” techniques on the roofs and pavement
of all cities in the northern hemisphere could have a one-time effect
of removing more than 44 billion tons of carbon dioxide — 24 billion
tons from roofs and 20 billion tons from pavements. That’s more than
the total of all global carbon emissions each year.
“If all eligible urban flat roofs in the tropics and temperate regions
were gradually converted to white (and sloped roofs to cool colors),
they would offset the heating effect of the emission of roughly 24
gigatons of CO2, but one-time only,” said Art Rosenfeld, a physicist
at Berkeley Lab. “However, if we assume that roofs have a service life
of 20 years, we can think of an equivalent annual rate of 1.2 Gt per
year. That offsets the emissions of roughly 300 million cars (about
the cars in the world) for 20 years!”
OK. Are you sitting down? While the world is
frantically pushing the transition to electric vehicles in order to
lower carbon emissions from the transportation sector, and scientists
are pursuing research into geoengineering strategies that could cost
trillions of dollars, here is a technique that is available and quite
inexpensive that could offset the carbon emissions from 300 million
cars and trucks. Why are we just hearing about this now? (Actually, CleanTechnica did
report on this back in 2012.)
The Washington
Post did a feature story on cool roof
technology on June 20th, in which it said that cities, on average, are
now 2 to 5 degrees warmer than rural or natural areas. On hot summer
days, that difference can be as much as 20 degrees. A black roof in
summer in New York City can get hot enough to literally fry an egg —
190º F.
Cities suffer from what is known as the “heat island” effect. They get
so hot during the day that they can’t fully cool down overnight,
meaning they start each new day with excess heat left over from the
previous day. There were more than 10,000 heat related deaths in the
United States between 2004 and 2018, according to the Centers for
Disease Control and Prevention.
Passive Cooling & The Solar Roof
All those charming villages along the shores of the Mediterranean are
not painted white so tourists can take pretty pictures of them. White
reflects more sunlight than darker colors. That’s called solar
reflectance, and it applies to the spectrum of visible light. But
sunlight is also composed of infrared radiation, the energy we feel
from a hot oven but cannot see. A white roof will reflect visible
light, but not infrared radiation. That’s where cool roof research
becomes important.
A “cool” roof should have both high solar
reflectance and high thermal
emittance, says Berkeley
Lab. “We note that since the spectrum of
thermal radiation (4-80 microns) does not overlap with that of solar
radiation (0.3-2.5 microns), thermal emittance is independent of solar
reflectance. Thus, we distinguish between the two when defining a cool
roof.”
Berkeley Lab explains that substituting a cool roof for
a warm roof reduces conduction of
heat into the building, convection of
heat into the outside air, and thermal radiation of heat into the
atmosphere. This benefits our buildings, our cities, and our planet.
-
Cooler outside air. Cool
roofs lower urban air temperatures by reducing the amount of heat
transferred from roofs to the air, mitigating the urban
heat island effect.
-
Fewer power plant emissions. Reflective
roofs reduce cooling energy demand in air conditioned buildings,
decreasing emissions of greenhouse gases and other air pollutants at
power plants that burn fossil fuels.
-
Better air quality. Cool
roofs decrease urban air temperatures and thus slow the formation of
ground level ozone. Ozone, the primary component of smog,
can aggravate respiratory illness and can act as a greenhouse gas.
-
Slowed climate change. Cool
roofs decrease heat absorbed at the Earth’s surface and thus can
lower surface temperatures. This decrease in surface temperatures
reduces the flow of heat into the atmosphere, offsetting warming
caused by greenhouse gases.
-
Energy and cost savings. During
hot summer months, cool roofs reduce the need for cooling in air
conditioned buildings, which saves energy and money.
-
Reduced electrical grid strain. The
diminished demand for cooling energy will also moderate peak energy
demand during heat waves and very hot summer afternoons, thereby
decreasing the risk of power outages.
-
Improved indoor comfort. Cool
roofs lower the indoor air temperature in buildings that do not have
air conditioning, promoting productivity and occupant health.
Cool roof technologies are more effective in hotter regions than
cooler ones, yet studies show that reduced energy costs in the summer
offset higher energy costs in the winter in all but the most northern
portions of the United States.
There are also concerns that a white roof may not
appeal to neighbors who have to look at it. Companies like 3M are
working on cool roof materials that use darker colors and rely on
increased thermal emittance to keep roofs cool. Tim Hebrink, a senior
staff scientist at 3M, tells the Washington
Post his company is now using granules that
can reflect up to 25% of solar energy, even in dark colors — double
the performance of conventional shingles. More advanced materials,
such as polymer films, reflect more than 90% of incoming radiation at
little to no extra cost across many colors. Hebrink says they will
become available within two years.
More variety and falling prices are transforming the market, says Kurt
Shickman, director of extreme heat initiatives with the Adrienne Arsht-Rockefeller
Foundation Resilience Center. For years, advanced passive cooling
roofing materials were priced about 30% higher than conventional ones.
Today, they sell at a modest premium or even price parity. Just like
the cost of other clean technologies, prices fall when large scale
acceptance occurs.
The Atmospheric Window
The effect that cool roof strategies can have on the
Earth results from what scientists call the “atmospheric window,” a
narrow gap in the atmosphere that allows a certain band of
electromagnetic radiation to be reflected back into space. With
passive cooling technologies, a cool roof can be as much 54º F cooler
than a comparable black roof. Mitigating the heat absorption
properties of asphalt focuses more on solar reflectance, but can be of
vital significance on a global scale. There’s a lot of
black asphalt in the world.
Cool roof strategies benefit the entire planet, but
people won’t use them if they offer no cost advantage, just as people
won’t install solar panels just because they are good for the
environment. There are tools that help calculate the savings that a
cool roof will make possible for a homeowner, based on location,
amount of insulation in the roof, type of structure, and energy prices
on the area. Dow offers
a Cool Roof calculator and there is a more
technical calculator available from Oak
Ridge National Laboratory.
Cool Roof Initiatives
The city of Los Angeles was one of the first in the US
to mandate
cool roofs for new buildings in 2013. Today,
Houston, Austin, Toronto, Miami Beach, Atlanta, Denver, Chicago, and
New York all have building codes or ordinances that require cool roof
technologies. Many cities in Europe and India also require their roofs
to have high solar reflectance and thermal emittance standards.
The question is, why is there not a global transition to cool roof
strategies taking place? We have seen recently how policy decisions by
the Biden administration have turbocharged the EV revolution. Similar
policy initiatives could jumpstart a similar revolution in cool roof
adoption.
This is the low hanging
fruit in the fight to limit global heating. Forget spraying
aerosols into the stratosphere or trying to capture
carbon dioxide directly from the air. This
would cost far less than either of those suggestions and be more
effective as well. As the Washington
Post says in its article on cool roof
technology, “In the end, a cool roof is for everyone.”
Green Play Ammonia™, Yielder® NFuel Energy.
Spokane, Washington. 99212
www.exactrix.com
509 995 1879 cell, Pacific.
exactrix@exactrix.com
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