August 3, 2010
Column | Korky Koroluk
Cool roofs take off as things heat up
When we first began to hear about “urban heat islands” a couple of decades ago, the phenomenon fell into the category of things we know but don’t do much about — sort of like the hall closet we know is filled with junk, but which we never clean out.
We thought about it in the summer, especially during heat waves, because we wanted to head for the countryside, which was always cooler.
From time to time we would hear about experiments with white (or at least light-coloured) pavements, but the idea never seemed to gain much traction.
Of late we have heard more about white roofs as a means of reducing the amount of energy used for cooling. And, it seems, we’re likely to start hearing more.
A new study by U.S. government researchers tells us that implementing cool roofs and cool pavements in cities around the world can not only help cities stay cooler, but also lead to reduced carbon dioxide emissions because of less energy use.
They’re being called cool roofs and cool pavements, because they don’t have to be white. Simply making them lighter in colour produces greatly increased reflectivity and the energy savings that go along with it.
In fact, the study cites experiments showing that cool roofs reduced daily peak roof surface temperature of each building and could reduce cooling costs by up to 52 per cent.
That led U.S. Energy Secretary Steven Chu to comment that cool roofs are “one of the quickest and lowest-cost ways we can reduce our global carbon emissions,” a statement he has made on several occasions.
Now he has ordered that cool roofs be used on all of his department’s buildings whenever they would be cost-effective over the lifetime of the roof. He has also written to heads of other U.S. federal agencies, encouraging them to do the same.
The latest study used a detailed global land-surface model containing regional information of such surface variables as topography, evaporation, radiation and temperature and cloud cover.
For the northern hemisphere summer, they found that increasing the reflectivity of roof and pavement materials in cities with population of one million or more, would achieve a one-time offset of 57 billion tonnes of carbon dioxide emissions, with 31 billion tonnes from roofs and 26 billion from pavements. That’s double the worldwide CO2 emissions of 28 billion tonnes in 2006.
Some observers have pointed out that cool roofs don’t make much sense in cooler climates because of what has been called “winter penalties.” That means that buildings cooled by white roofs would need more energy to heat them in the winter. But the study found that energy savings from cooler buildings usually outweigh any increase in heating costs.
It also points out that in the winter there tends to be more cloud cover. Also, the sun is lower in the sky and the days are shorter, so that a flat roof’s exposure to the sun is significantly reduced.
Although white roofs are built occasionally in Canada, many owners and facility managers opt for more conventional roofs, citing Canadian winters as the reason.
But a co-author of the paper, Hashem Akbari, is well aware of Canadian winters. He now is a research professor at Concordia University in Montreal.
I find the mentions of white pavements to be particularly intriguing. The portland cement industry has been pitching the idea of concrete paving for decades, and more recently white pavements have been installed as demonstrations in a few places. But the idea hasn’t had much traction so far.
That might change. It’s possible this latest study will lend impetus to the idea, especially as the cost of asphalt paving increases.
Korky Koroluk is an Ottawa-based freelance writer. Send comments to firstname.lastname@example.org
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