It's easy to find disagreement in the construction industry about the comparative advantages of green - or vegetated - roofs and white roofs. As often happens, though, the argument is among those who have vested interests in one or the other, plus a tendency to believe that even in the world of roofing, one size fits all.
It doesn’t, of course. White (or light coloured) roofs provide reflectivity that bounces the sun’s rays back into the atmosphere, leaving the buildings under those roofs a bit cooler. Green roofs provide insulation in cold weather, and help in the management of stormwater runoff.
Whether your roof reflects the sun’s energy or absorbs it is an important question, though, to a group of researchers at the Lawrence Berkeley National Laboratory in California. They are members of the lab’s new Heat Island Group, which is concerned about the amount of heat our cities absorb from the sun, creating islands in which the air is measurably warmer than the air in the countryside.
The group has just finished a study of white, green and black roofs, and found that white roofs are most cost-effective over a span of 50 years. Green roofs, they found, are more expensive, but their environmental benefits might make up for their higher costs. And black roofs don’t show up well in the study at all.
The researchers are careful to note that the relative costs and benefits of white and green roofs vary depending upon circumstances.
But, they say, unlike white roofs, green roofs do not offset climate change. White roofs reflect roughly three times more sunlight back into the atmosphere. By absorbing less sunlight, they offset a portion of the warming effect from greenhouse gas emissions. Both, however, do a good job of cooling the building and the surrounding air.
A paper describing the results of their study will be published later this month.
But there’s more to cooling the urban heat island than just roofs. The group is also examining reflective paving for city roadways.
To do that, they’ve treated patches of a parking lot paved with asphalt with solar-reflective coatings in a variety of pastel colours. That’s why they refer to “cool” pavements, rather than “white” pavements, although one of the test patches is, indeed, white.
They’ve found that cool pavements reflect as much as 30 to 50 per cent of the sun’s energy, compared to only five per cent for new asphalt and 10 to 20 per cent of aged asphalt.
Their results so far indicate that using traditional concrete pavement materials of light colour might be a good idea for new installations. But if you have an old asphalt surface, you might want to think of cool-coloured coatings or surface treatments.
An ideal design goal would be a pavement with solar reflectance of at least 35 per cent, one researcher says, but how you get to that figure will vary depending on the product you use.
“Sealcoats are a common maintenance practice for parking lots and schoolyards,” he says, since the asphalt pavement degrades over time.
But if you’re thinking about doing an eco-makeover of urban surfaces, don’t forget things like sidewalks.
A research group in The Netherlands has come up with a way to treat conventional concrete paving so that it absorbs the air pollutant that causes smog.
It’s really quite simple. They’ve used conventional concrete treated with titanium oxide to create what they call photocatalytic concrete, making it able to absorb as much as 45 per cent of smog-producing oxides of nitrogen.
Italian scientists have been working on similar concrete for a while now, and an American version has been installed along some bicycle lanes in downtown Chicago.
Titanium dioxide is expensive, though, so widespread acceptance of the technology will likely have to wait until the cost comes down.
Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments or questions to firstname.lastname@example.org.