Replacing a building’s unsightly asphalt roof with lush green vegetation has long been an option for real estate owners and city planners, but while past research on green roofs focused mostly on reducing air conditioning demand on scorching summer days, Dr. Brad Bass of the centre for environment has taken the idea a step further by designing a green roof that lowers heat loss in winter.
Installation of a winter green roof could lead to “an estimated 8.5 per cent decrease in the heating load in an energy-efficient winterized house”, said Bass.
His green roof prototype consists of three layers: an egg-carton-like plastic membrane layer that protects the roof and stores water, some light-weight soil, and a layer of light, shallow-rooted vegetation.
Conventional green roofs have only used seasonal plants, such as sedum and wildflowers, which die or turn brown as the weather cools. Bass’s previous studies showed that during typical Canadian winters, the membrane layer of the green roof experience vast temperature fluctuations. “This leads to the expansion and contraction of the membrane layer which eventually cracks and causes problems,” he said.
Instead, Bass designed a green roof with evergreen juniper shrubs and a thicker layer of soil to create a “winter canopy” that can survive the harsh Canadian winters. A test roof built by Bass’s collaborators at the Institute for Research in Construction, in Ottawa, showed that the winter canopy evens out temperature fluctuations on the roof, helping to retain heat in the building.
The beneficial effect of the winter canopy, Bass explained, is mostly due to “thermal insulation added to the roof by the soil”. Also, the shrubby plants provide a “roughness factor” that lowers wind speeds on top of buildings. This decreases the wind chill over the roof.
While Bass agreed that few existing downtown roofs could support the weight of the winter roof, he believes his winter green roof concept could work for small office buildings or warehouses in business parks. These have flat roofs and are sound enough to support the weight of a winter green roof. Bass envisions a lighter weight version of the winter green roof, which make it an option for more types of buildings.
Green roofs may become a more common sight, said Beth Anne Currie, Bass’s research assistant on the project. She was one of several authors of an environmental analysis on green roofs presented to the city’s Roundtable on Environment last November. Their recommendations will be discussed at a city committee meeting later this month, and could be forwarded to the City Council soon thereafter.
The report argues that green roofs would provide a myriad of benefits to the city, including collecting stormwater, which would otherwise go into the sewer system, saving energy, reducing the urban heat island effect, recreating biodiversity and adding green space. In a survey last year, the city counted 59 private and public green roofs, with another 17 under construction.
Reroofing costs are one barrier to the implementation of green roofs. That costs $75 to $90 per square metre-and a green roof should cover at least 350 square metres to be considered as such-according to the report. This would cost between $26,000 and $32,000. The city is considering financial incentive programs to help offset the cost.
