Friday, July 27, 2012

The Effectiveness of Cool Roofs in Hot and Cold Climates

A relatively simple idea like light reflective roofing material can help reduce global warming. Research shows that cool roofs can provide significant benefits in hot climates, but there is some controversy as to whether cool roofs make sense in colder climates. In places where there are more hot days than cold days, cool roofs may not provide a net benefit in terms of cash savings or emissions. In northern countries like Canada this may seem intuitively obvious, but in the US the situation is not so straightforward. According to the US Energy Information Administration's, 2003 Commercial Buildings Energy Consumption Survey, heating accounts for 36% of commercial buildings' annual energy consumption, while air conditioning only accounts for only 8%. However, according to the Cool Roofs Rating Council (CRRC), "The roof is an insignificant source for heat gain in winter. While cool roof owners may pay slightly more to heat their homes, this amount is usually insignificant compared to the cooling energy savings during the summer".

Energy calculators often show a yearly net savings for dark-colored roof systems in cool climates.  Additionally, higher R values for insulating materials in the roof assembly and snow covering on roofs can lessen the impact of roof surface color.

In a 2001 federal study, the Lawrence Berkeley National Laboratory (LBNL) measured and calculated the reduction in peak energy demand associated with a cool roof’s surface reflectance. LBNL found that, compared to the original black rubber roofing membrane on the Texas retail building studied, a retrofitted vinyl membrane delivered an average decrease of 24 °C (43 °F) in surface temperature, an 11% decrease in aggregate air conditioning energy consumption, and a corresponding 14% drop in peak hour demand.

The average daily summertime temperature of the black roof surface was 75 °C (167 °F), but once retrofitted with a white reflective surface, it measured 52 °C (126 °F). Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or $0.07/square feet.

Concordia Univeristy's Hashem Akbari is an engineering professor whose research indicates that in addition to driving down cooling costs, cool roofs can potentially delay the effects of global warming.  Before joining Concordia, Akbari spent 26 years at the Environmental Energy Technologies Division at Lawrence Berkeley National Laboratory (LBNL) at the University of California, Berkeley. Akbari is also a founding member of the board of the Oakland-based Cool Roof Rating Council (CRRC).

Akbari reports that dark roofs absorb 80 to 90% of sunlight trapping heat in the city. Those same surfaces covered with his reflective materials (white for roofs, lighter colours for pavements) will absorb only 30 to 65%. By decreasing absorption of roofs by 25% and pavements by 15%, the overall temperature of a city can be reduced by two to three degrees Celsius.

Together, pavements and roofs comprise over 60% of urban surfaces. Akbari states 10 sq m of white roof replacing a dark roof can offset one tonne of CO2. In other words, the temperature reduction due to radiation not being absorbed by the earth is equal to the increase in temperature caused by one tonne of CO2 in the atmosphere, effectively balancing any change.

Akbari's research indicates that if a city like Montreal resurfaced 60 percent of the 500 sq km of pavements and roofs with reflective materials, the city alone could offset over 12 million tonnes of CO2.

However, Akbari admits cool roofs may be more useful in hot regions, but he insists that even the coldest climates could positively contribute to the overall effect. “If major cities around the world adopt the technology, we’d be well on our way to making a very significant dent in rising temperatures,” he says.

Akbari estimates that permanently retrofitting urban roofs and pavements in the tropical and temperate regions of the world would create a one-time offset of 44 billion tonnes of emitted CO2.

US Secretary of Energy and former LBNL director Steven Chu, strongly supports cool roof technology.

For more information on Akbari's research click here.

© 2012, Richard Matthews. All rights reserved.

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