Reflective roofs

Can this Green building technology work in Alberta?

Reflective roof systems are touted around much of North America as the latest technology in Green building construction to combat global warming.

When the sun's radiation hits a roof's surface, some heat is absorbed by the roof and transferred to the building. Solar reflectance is the fraction of solar energy that is reflected by the roof. Thermal emittance is the relative ability of the roof's surface to radiate absorbed heat.

As the name implies, a reflective roof system reflects and emits the sun's heat back to the sky instead of transferring it to the building. Reflective roofs have high solar reflectivity and thermal emissivity. The roof can either have a coating applied over an existing roof system or a new single-ply waterproofing membrane. Reflective roof systems are credited with reducing energy consumption, reducing consumers' power bills, reducing greenhouse gas emissions, and reducing what's known as the urban heat island effect (why cities are generally hotter than rural areas).

But can reflective roofs keep buildings warm in cold climates such as Alberta's?

Jon Peat, general manager at IRC Building Sciences Group Alberta Inc., says a reflective roof system has a low slope and satisfies the requirements for solar reflectance of 0.80, thermal emittance of 0.85, or a solar reflectance index of 78. The Cool Roof Rating Council, a U.S.-based authority on reflective roofs, typically sets these values.

In Alberta, Peat says reflective roof systems might not perform as well in cold climates as they do in warm climates.

"While reflective roofs can reduce energy consumption in the summer months, they can actually increase heating costs during the winter months," he says. "A black roof, or a non-reflective roof, will transmit solar radiation into a building, which reduces energy consumption in cold temperatures. Energy calculators show that, in cold climates, reflective roofs have a heating penalty in terms of costs and energy consumption."

Peat further explains, "For example, Calgary has approximately 9,885 heating degree days and 1,167 cooling degree days. Aside from winter days, where roofs are snow-covered and perform similar to a reflective roof, the remainder of the heating degree days use energy to provide heat.

"A typical building can experience a reduction in energy usage if a roof transfers solar radiation instead of reflecting it. It might be more beneficial to increase the thermal value of the roof to maximize energy savings rather than install a reflective roof."

The Cool Roof Rating Council says it's important to understand your climate zone and the actual energy your building will save when selecting your roof.

In the United States, the Department of Energy mandated last year that all its offices install reflective roofs when constructing new roofs or replacing old roofs. Power companies in 14 U.S. states offer rebate programs for installing reflective roof systems. While each program is unique, the main purpose of each is to reduce the amount of energy houses and buildings consume and the amount of emissions they emit.

The only city in Canada to offer a rebate program for reflective roof systems is Toronto. Toronto's Eco-Roof Incentive Program promotes the use of reflective roofs on existing commercial, industrial and institutional buildings. Since the program came into effect in 2009, the city has approved over 50 applications for funding.

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