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Cool Roofs
July, 2010
By Henry Jansen
No, this is not the name of a new band, an interesting coffee shop or a local pub, rather “Cool roofs” is a term being used to describe the hot, new green technology for flat (low slope) roof designs. Since the beginning of high and low rise building construction, the maintenance requirements of roof systems have always been a major issue. Since the roof system of a building is one of the primary components of the building envelope, the systems that keep the interiors dry and the temperatures consistent, a good roof design is of key importance. Ask any condominium board member or property manager: users of urban mid and high-rise buildings often face flat roof maintenance issues. Now more then ever, a new wrinkle in the roofing decision process has arisen: black, white or green?
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In the past, most flat roofs were made with a rubber-like elastomeric membrane called EPDM. Though black EPDM still accounts for the majority of flat membrane roofs for condominiums, commercial and industrial buildings in Canada and the northern United States, things are rapidly changing.
From a high level perspective, there is evidence developing that shows the heat island’s effect of black surfaces, including not just roofs but also parking lots, paved roads and building facades, can impact local weather characteristics.
Cool roofs are designed to reduce energy consumption as well as what is commonly called the urban heat island effect. Cool roofs are categorized into three basic types: white, reflective-coated or green (vegetated) roofs. White roofs have been the most common cool roof, with TPO (thermoplastic polyolefin) and PVC (polyvinyl chloride) being the typical choices. PVC and TPO roofs have the same wear and cost factors. However, PVC materials have some negative characteristics, such as high toxicity and unrecyclability. So, for ease of comparing black to white, we will consider EPDM vs. TPO in our comparison discussions.
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TPO roof membranes have longer lives, on average lasting 25 years compared to EPDM lasting 20 years. This is partially due to TPO’s resistance to UV and thermal expansion damage. Some TPO products developed bad reputations due to their inability to handle severe cold, which in some cases caused the membrane to shatter. These problems are reported to have been fixed with today’s TPO roofing materials.
The initial cost favours EPDM roofs. However, when lifecycle and energy cost issues are considered, the black and white roofs become competitive. Installation methods differ in that EPDM seams are taped or adhesively sealed while TPO seams are welded by a thermal process. In addition, as EPDM roofs have been the industry standard for so many years, it is sometimes difficult to find a roofing installer with good experience installing TPO roofs. Installation is a major factor in the performance of a roof system. The attention to detail during the installation process can be the difference between a roof that performs well and a roof that leaks like a sieve.
There are strong forces pushing the general acceptance of white roofs. States, such as California, passed laws in 2005 requiring the use of reflective roofing materials. Plus, individual cities, such as Chicago, have established building codes to favour its use.
What about Canadian cities adopting white roof systems? With all of this horsepower pushing for cool roofs, it seems like and easy “black or white” decision for the condo board facing a roof replacement project. Maybe not, as for northern condos the problem is a little more complicated. Building modelling data reveals that, in northern areas, heating is a much more significant factor in energy use than cooling. In fact, heating accounts for 29 per cent of energy used compared to only six per cent for cooling. In most Canadian cities, the benefits of a white roof don’t always add up as the potential heat loss during the winter months can outweigh the energy savings during the summer periods.
This surely complicated the acceptance of white roofs in Canada. If there is no simple answer on whether to build a convention black roof or an innovative white roof, what other options are there? This is where green roofs come into the picture.
Green building advocates all over North America are influencing architects and building developers to create “green” building by offering favourable grants and energy rating with cool roofs. For instance, to receive an Energy Star label under its Roof Product Program, it must have a solar reflectivity of at least .65 and weathered reflectance of at least .50, according to specific testing procedures.
In addition, the Cool Roof Rating Council (CRRC) crated a rating system for measuring the solar reflectance and thermal emittance of 8950 roofing products. The Green Building Initiative has instituted its Green Globe system in the United States and Canada to develop benchmark criteria for a building’s likely energy consumption as a result of roofing material’s solar reflectance and thermal emittance.
Leadership in Energy and Environmental Design (LEED) rating system is now widely used for most publicly funded building projects and many high-profile, non-government buildings. Architects seeking a LEED certification for their project will receive credit for white, cool roofing meeting LEED solar reflective index guidelines.
Of course, green roofs also fit under these guidelines.
A green roof is actually a layered roof system built out of membranes of various materials, soil and vegetation. These are also commonly known as “living roofs” and can have everything from grass and herbs to shrubs and small trees growing on the substrate.
Some of the most prevalent benefits of a green roof are how well it insulates during the summer months, keeping the building cool during especially hot periods. It also helps to positively manage roof water and drainage. There have been numerous studies that show green roofs conserve energy in the winter months as well. Roof plants can help reduce wind speed over the roof area, thus reducing heat flow from the building.
For decades, Germany has been in the leader in green roof implementation, starting to build buildings with green roofs in the 1960s. However, Canada has been getting quite involved over the past number of years. In Toronto and Vancouver alone there are over 600 green roofs. In the City of Toronto, there is an estimated 36,500 m2 or 120,000 ft2 of green roof space. In fact, the City of Toronto was the first municipality in North America with a mandatory green roof by-law. The by-law requires green roofs to be built on new commercial, instutional and residential developments with a minimum Gross Floor Area of 2,000m2. Although this can increase construction costs by $18 - $28 a square foot, developers know that building green has become a key element to successful marketing and unit sales so the benefit out weights the costs.
The green roof system has positive aspects from the top down. Not only is the end product an environmentally sound solution, but the entire concept of green roof construction has created a new spark of interest and excitement in young building designers and architects. The “green” mentality can now be implemented from the early stages of conceptual design. This is exciting and can help developers to continue to push the envelope of sustainable designs and innovative construction projects. With the rapid pace at which our cities are growing and developing, there is no doubt that green roofs will play an important role in healthy condominium buildings and communities.
Henry J. Jansen, P.Eng., is president of Criterium - Jansen Engineers, an engineering firm that provides building inspections, reserve studies, and related services. Criterium – Jansen Engineers is affiliated with Criterium Engineers which has been serving North America since 1957.www.criterium-jansen.com. |
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