Are green roofs always green?

Green or vegetated roofs are all the rage, and for good reason. They are a beautiful, tangible way to identify a building as sustainably designed. Green roofs have also been shown to decrease the urban heat island effect, caused by flat dark surfaces such as parking lots and dark roofs that can raise a city’s temperatures by five degrees compared to rural areas. Similarly, buildings with dark roofs which absorb the sun’s energy rather than reflecting it will heat up faster and require more air conditioning than their lighter-coloured or green counterparts. Also, the vegetation on green roofs mitigates stormwater runoff by absorbing rainfall. It is for these reasons that the City of Toronto recently mandated that all new buildings must have green roofs and many architects feel that a green building must have a green roof.

Why is it then that only about 10 per cent of Enermodal’s 57 LEED-certified projects have green roofs? We consider four factors in examining the case for green roofs on a building. Let me explain how we applied these factors on our new LEED Platinum candidate office building, A Grander View — Canada’s most energy-efficient office that doesn’t have a green roof.

1) Is stormwater a liability or an asset?

Instead of thinking of stormwater as a liability that has to be mitigated, it should be viewed as free clean water that can be used to reduce the demand for municipal water. Installing a rainwater cistern is effective at controlling stormwater and provides free water for non-potable needs. At A Grander View, our 700 square metre roof drains into an 18 cubic metre cistern that provides almost all of the water required for flushing toilets and urinals and for outdoor hose bibs for spot irrigation and employee vegetable garden plots. The combination of the cistern and low-flow fixtures reduces city water demand by 80 per cent.

2) Does the site have green space?

In dense urban areas, an accessible vegetated roof may be the only green space available for building occupants to enjoy. However, most building projects have some green space that, if properly landscaped, can provide equal or superior benefit at much less cost. At approximately $150 to $250 per square metre, green roofs are expensive. At A Grander View, even though located in an urban area, we have approximately four times more green space than roof area. Instead of spending $150,000 on a green roof, we have outdoor landscaping comprised of native trees, shrubs and ground cover (with no sod). Amenity space consists of a gazebo, patio areas, walkways, and a roof deck. The total cost of the hard and soft landscaping package is under $100,000. The extensive plantings of trees and shrubs will do far more to mitigate the urban heat island effect than the low intensity sedum or grasses used in most green roofs.

3) What is the best way to reduce the energy impact of the roof?

Studies by the National Research Council show that a vegetated roof can effectively eliminate the cooling load associated with the roof. For low-rise buildings in cooling-dominated climates, this can be a significant savings. However, for most Canadian buildings, the energy benefit from green roofs is much less, and in cold climates such as Winnipeg, the impact may be negative. At A Gander View, we used a reflective white roof and upgraded roof insulation to minimize the energy impact of the roof. The additional cost was less than 10 per cent that of a green roof.

4) Is renewable energy an option for the project?

The past few years have seen a dramatic improvement in the cost effectiveness of photovoltaics. The increased worldwide supply of PV modules has cut costs in half to under $3,000 per kilowatt, and incentives for installing PV are available in several jurisdictions. In Ontario, power from roof-mounted PV can be sold for upwards of 80 cents per kWh and the payback on the investment is in the range of ten years. Several companies will finance the installation and pay the building owner to lease their roof space. Installation of a green roof limits the potential for roof-mounted PV as both the PV and the plantings need access to the sun. At A Grander View, 24 PV modules are mounted on the roof and provide close to 10 per cent of the building energy. The white roof provides the additional benefit of reflecting more light onto the panels.

Green roofs are a neat idea and can provide interesting outdoor space for building occupants in urban areas where such space is hard to find. They also serve a variety of purposes, particularly in downtown locales. However, in many applications they are a flashy — and expensive — aesthetic feature that may limit other green options such as rainwater harvesting and renewable energy. Every project team needs to identify the best use of their financial resources in order to truly minimize the building’s environmental impact. While efforts to promote green roofs are a step in the right direction — they may be mandating that design teams consider sustainable design features — green roofs are not necessarily the right solution for many green buildings.

Stephen Carpenter is president of Enermodal Engineering and was Canada’s first LEED Accredited Professional as well as serving as the current chair of the Technical Advisory Group for the Canada Green Building Council. Enermodal has been Canada’s largest firm exclusively dedicated to creating energy and resource efficient buildings since 1980, certifying more LEED buildings than any other firm.

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