Download the entire NRCC-45345 PDF “Energy efficiency and environmental benefits of rooftop gardens” by Dr. Karen Liu from The National Research Council of Canada here, which has figures, graphs, charts and photos along with detailed research descriptions.
The following text is taken from the study by Dr. Karen Liu, of The National Research Council of Canada (see link below): “The National Research Council of Canada (NRC), in collaboration with Environment Canada, the Climate Change Action Fund and members of the Canadian roofing industry, is leading a research project to study the various benefits of this technology. The objectives of this project are to identify sensitivities to climate variability and to quantify the benefits of the technology under Canadian climatic conditions.”The NRC Field Roofing Facility at its Ottawa campus “has an experimental roof area of about 70 m2 (800 ft.2) and can represent a low-slope industrial roof with a high roof-to wall ratio. The roof is divided into two equal areas separated by a median parapet: a generic extensive rooftop garden was installed on one side and a conventional roofing assembly was installed as a reference on the other. While the reference roof and the rooftop garden have the same basic components up to the membrane level, the rooftop garden incorporates additional garden components (root repellent on the membrane, drainage layer, filter membrane and growing medium) to support plant growth. The garden has a wildflower meadow growing in 150 mm (6 in.) of lightweight growing medium.
“The roof is surrounded by a 1-m (3-ft.) parapet and each section is structurally sloped at two per cent toward a central drain. Any run-off from one section flows through the central drain in that section into an individual flow meter in the building. There the run-off from each roof can be measured and compared.”The field roofing facility has been monitored continuously since its commissioning in November 2000. The data collected from the rooftop garden and the reference roof have been analysed and compared to assess the thermal performance, energy efficiency and storm water retention.”Example of some of the findings: “the growing medium and the plants enhanced the thermal performance of the rooftop garden by providing shading, insulation and evaporative cooling. It acted as a thermal mass, which effectively damped the thermal fluctuations going through the roofing system. The average daily energy demand for space conditioning in the case of the reference roof was 6.0-7.5 kWh (20,500-25,600 BTU). However, the growing medium and the plants modified the heat flow and reduced the average daily energy demand to less than 1.5 kWh (5,100 BTU)?a reduction of more than 75 percent.”