Toronto made headlines back in 2009 when the City adopted a bylaw requiring the inclusion of green roofs on all new commercial, residential or institutional developments or additions with gross floor areas exceeding 2000 m2. Exceptions or green roof area reductions are permitted with a fee of $200/m2. Other cities — Denver, San Francisco, and Portland, Oregon — have followed suit. New York City is thinking about it.
Regulations notwithstanding, green roofs offer compelling, passive solutions to several environmental concerns. The insulation provided by the soil delivers year-round energy savings and interior noise reduction. The vegetation absorbs carbon dioxides in the atmosphere while offering wildlife biodiversity and habitat. The soil’s water absorption reduces downstream loads placed on storm water systems. Supporters also point to green roofs as attractive outdoor relaxation spaces for occupants.
On the other hand, buildings owners in some jurisdictions are hesitant due to concerns over the higher costs for installation and maintenance associated with green roofs. They prefer PV (photovoltaics) solar arrays on their rooftops instead. Arrays are a one-time investment with a 25-year lifespan that requires minimal maintenance while contributing positively to the property’s bottom line by creating electricity. Proponents also point to the benefits of the shade provided by the arrays to the rooftop surface in summer, claiming a reduction of the sun’s heat up to 20 per cent, thus reducing the demand for air conditioning.
However, the choice between a green roof or a PV solar array system does not have to be an either-or decision. There is the BioSolar solution.
Sam C M Hiu of the Technological and Higher Education Institute of Hong Kong has studied integrated green and solar rooftop systems. He says that overall building energy demands are reduced when the two are installed in tandem rather than as stand-alone systems. Furthermore, Hui says that, when installed as a duo, green roofs and PV arrays, “can enhance their functions and effectiveness by cooling and shading effects,” to the point that PV array systems can deliver more electricity in conjunction with a green roof than when installed singly.
How is this possible? First, the protective shading provided by the arrays reduces soil dryness, which in turn can reduce irrigation requirements for the vegetation. In cold climates like Canada, protection from the arrays significantly reduces plant loss over the winter.
In turn, green roofs add to the installation integrity of the solar arrays. Jelle Vonk, Business Manager for green roof manufacturer ZinCo Canada Inc., says, “the green roof can be used to ballast the solar array against wind uplift. Traditional systems are ballasted with heavy concrete blocks (resulting in high point loads) or are fastened to the roof deck (resulting in penetrations through the waterproofing membrane with a higher risk of leakage).”
From a performance standpoint, Vonk points to a study comparing surface temperature differences measured on solar panels installed on an exposed bitumen waterproof membrane versus those installed on a green roof. “The efficiency of photovoltaic panels depends on their temperature. The greater the temperature of the panel, the lower the level of efficiency,” he says. “The three-year research test has shown that the temperature of solar PV panels installed on the green roof remained closest to the air temperature, while the solar PV panels on the bitumen membrane were considerably warmer. The result is a difference of four per cent higher efficiency annually.”
There’s another matter. As forward-thinking as Toronto’s green roof bylaw may be, the growing popularity of mass timber construction (MTC) pits the City against Ontario Building Code requirements for non-combustible rooftops that protect roof decks from fire exposure. Green roof vegetation is, after all, flammable.
Practical experience has demonstrated, however, that project-specific design accommodations for MTC are possible, making good use of the otherwise wasted space on roof tops across major cities. Nevertheless, cities with green roof bylaws that also wish to embrace mass timber construction may have to consider reviewing their current regulations.
John Bleasby is a Coldwater, Ont. based freelance writer. Send comments and Inside Innovation column ideas to email@example.com.