July 13, 2010

ALGONQUIN COLLEGE

A green roof planned for the Algonquin Centre for Construction Excellence will help manage stormwater runoff.

Debating the merits of green, white roofs

KORKY KOROLUK

correspondent

OTTAWA

Just days after being named U.S. President Barack Obama’s energy secretary, Stephen Chu gave a speech praising reflective white roofs for their energy efficiency, and expressing hopes that white roofs would multiply.

Some people took the advice to heart. After all, Chu occupies an important position in government, and he is no politician. He is a Nobel prize-winning scientist with extensive experience in the energy sector.

His advice may have been good, but whether saving energy is as simple as installing a reflective roof is open to debate. In hot southern climates, they do reduce cooling costs. But in northern climates — most of Canada, for example — there may be a winter-time heating penalty, says Peter Kalinger, technical director of the Canadian Roofing Contractors’ Association.

The popularity of both white and green roofs is undeniable, and Kalinger cites a couple of reasons.

“One is that if you install a reflective or a vegetative (green) roof you can earn LEED points,” he said in an interview. “The other is that there has been a lot of advocacy for vegetative roofs. That’s led to some regulations coming into effect, particularly in Toronto, where they have a bylaw that stipulates that buildings over a certain size in all sectors shall have green roofs or white roofs.”

Quite apart from energy considerations, green roofs have an attribute missing from white roofs: They help control storm-water runoff.

A vegetative roof with 10 centimetres of growing medium can hold about 40 litres of water per square metre. So a big roof of, say 1,000 square metres could hold 40,000 litres, or about 245 barrels of water, releasing it slowly so that storm sewers aren’t overloaded.

New York City is serious enough about green roofs to sponsor scientific studies of the way they perform, turning five large roofs in the city into “research stations.” The city wanted to know if measured results matched predictions of the roofs’ performance.

They do. A study published late last year used the data collected from the five stations to show that green roofs really do mitigate urban heat-island effect; they really do absorb stormwater; they really do help keep the building below them cool in the summer and warm in the winter.

But green roofs can be costly, and retrofitting an old building to accommodate a green roof can be complex. So there will always be room for conventional black (or dark) roofs, as well as for reflective white roofs.

Despite a surge of popularity for white roofs following Chu’s ringing endorsement, Kalinger said there is “some sober second thought with regard to these systems,” he said.

Both white and green roof systems have shown they can be beneficial in reducing a building’s carbon footprint, or in reducing the urban heat-island effect, “but there is some debate as to how to quantify both the benefits and the costs of these systems.”

A result, he said, is that many organizations have studied ways to measure the impact of the systems, and most have concluded that benefits derived depend on the climate. So careful inclusion of climate while designing the roof will help the owner determine whether he’s really going to realize all the benefits that have been talked about.

Kalinger cited, especially, a paper written by Samir Ibrahim, who is director of design for Carlisle SynTec, Inc. The firm manufactures both light and dark roofing membrane systems, “so they don’t really have an axe to grind,” Kalinger said.

It was in that paper that Ibrahim pinpointed the possible heating penalty that might go with white roofs in cold climates.

It also pinpointed a similar problem with extensive vegetative roofs — those with a shallow overburden. Such roofs, Kalinger said, “are very good at mitigating the flow of heat into the building on a hot summer day, but they’re not so good at preventing the heat from flowing outward from the building in the middle of winter.”

The roofing industry, Kalinger said, had been “basically looking at one size that fits all, and we were hoping that these things were the magic bullets.”

“We’re finding out that is not the case, so a building owner or designer should give careful consideration to the specific requirements of the project <0x2026> before selecting any type of roof system.

Because roofing systems that save energy appear to offer such large paybacks, researchers have been working on a variety of technologies, including some involving roofing materials that change colour with the temperature — turning a light color in hot weather or darkening in cold weather.

Kalinger said he hasn’t seen any of them yet, but has talked to people about them.

“My sense is that this technology is still very far away,” he said.

“I know something like that has been used in windshields in the automotive industry for a number of years, but to incorporate that into a roofing system — I don’t understand how the technology could be inexpensive enough to do that.”

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