October 3, 2012

How to beat the heat island effect

Construction Corner | Korky Koroluk

We had a number of days during this summer when you might have been able to fry an egg on the sidewalk.

It's an old cliché, but apt, as the sun beat down on the mostly dark surfaces with which we cover so much of our cities.

We are told that in a typical city, pavements make up between 35 and 50 per cent of surface area. About half of that is made up of streets and only a little less is exposed parking lots.

Most of those streets and lots are covered with dark materials.

Because those dark surfaces absorb almost all of the sun’s energy, the surfaces heat up.

That warms the air above them and aggravates the effect we have come to know as urban heat islands.

These islands have been the focus of concern for many scientists, since cooling our cities will not only make them more livable, it will help reduce the cooling load during hot summer weather.

That translates into energy savings, which means reduced building operating costs.

Much of the publicity about creating cooler cities has been about roofs — green and white — in the hope of reducing reflectivity. Green roofs have other benefits, of course, including improved stormwater management. And there has been skepticism about the value of reflective roofs during cold Canadian winters.

But the concept of cooling our cities is gaining ground, both among ordinary folk who have to cope with reflected heat as they go about their summertime business, and among scientists, who see cooler cities as one small part of the greater fight against climate change.

That’s why the Lawrence Berkeley National Laboratory, in California, has established a heat island group. As well as its work on white roofs, the group is concerned about reducing reflectivity of streets and parking lots.

Cool pavements, said scientists in the group, can reflect as much as 30 to 50 per cent of the sun’s energy.

New asphalt surfaces reflect only five per cent of that energy, and aged asphalt reflects between 10 and 20 per cent.

So, the group has paved patches of a parking lot with cool pavement coatings applied directly to the existing paved surface. At present, they are testing six coatings donated by two manufacturers—Emerald Cities Cool Pavement and StreetBond. They expect to test more products from other manufacturers next summer.

Cool pavements can be traditional pavements, like those based on portland cement concrete, which have a higher solar reflectance because of the concrete’s lighter colour. Or, they can take the form of cool-coloured coatings or surface treatments for asphalt pavements.

That’s why the group is working with both the asphalt and cement industries.

The hope is that the data derived will help sell the concept of cool pavements.

Cost has been one hurdle encountered so far because the cool pavement coatings tend to be more expensive than traditional sealants. Another hurdle the group found is that the benefits of cool pavements help the public at large, rather than the building owner.

But, said researcher Benjamin Mandel, cool pavements can eventually pay for themselves.

“The benefits are less immediately tangible than for cool roofs,” he said, “but the initial cost premium can potentially be offset over the lifespan of the product with increased durability and less need for ongoing maintenance, which are factors we are working with manufacturers to investigate further.”

Since cool pavements benefit everyone, the group is also talking with local governments, in the hope that they will start requiring cool pavements in places like parking lots.

The same group is doing a lot of work in the different aspects of the problem of cooling our cities.

Go to Lawrence Berkeley Lab website and search for Heat Island Group.

Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments or questions to editor@journalofcommerce.com.

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