A step towards reaching carbon neutrality: Estimating the carbon offset potential of green roofs in British Columbia

Background information
What is the objective of the project?
What type of plants are used on the roofs?
What type of measurements are made?
How will these measurements be used?

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Background information

Green roofs are engineered roof systems incorporating the use of vegetation and soil on conventional roof structures. As stated on greenroofs.org: "
a green roof system is an extension of the existing roof which involves a high quality water proofing and root repellant system, a drainage system, filter cloth, a lightweight growing medium and plants". Two types of green roofs are recognized in the literature, intensive and extensive, depending on the depth of the planting medium and the amount of maintenance required. Intensive green roofs are usually seen as "traditional gardens" and require a substantial amount of maintenance for irrigation, fertilisation etc. They also have a deeper soil layer than extensive roof systems. Extensive green roofs are designed to be self-sustaining, have a limited soil depth (usually less than 6 inches) and require maintenance only once or twice a year for weeding and fertilisation.

Although much work needs to be done to investigate the environmental, economical and health benefits associated with green roofs in North America, much knowledge has been gained from studies in European countries. In terms of the their microclimate benefits, green roofs are known to reduce energy consumption for cooling and heating through improved thermal performance, reduce temperature extremes of conventional roof membranes therefore increasing their life time, and alter the roof microclimate due to the presence of vegetation and soil which directly contribute to a reduction in the heat island effect. In terms of their water management benefits, vegetation and soil associated with green roofs are known to retain a significant portion of storm and rain water and lower discharges into municipal systems. Green roofs have also been shown to improve water and air quality, allow the development of local agricultural systems and increase biodiversity of urban habitats. For more information regarding benefits associated with green roofs, please visit greenroofs.org

One of the remaining unknowns with respect to the use of green roofs in urban environments relates to their carbon sequestration benefits. A survey of available literature has revealed that this type of research is currently lacking. As for any natural ecosystem, the carbon sequestration of a green roof represents a delicate balance between three important physiological processes, carbon assimilation by plant photosynthesis and carbon release by plant respiration and microbial decomposition in the soil. Carbon can also be lost through dissolved organic and inorganic constituents in the water discharge. A better understanding of the complete carbon balance of green roofs systems will allow determining if they can be used to offset carbon emissions from building operations. This has immense potential to offer mitigation strategies to climate change from residential, commercial and industrial perspectives.

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What is the objective of the project?

The main objective of the project is to quantify and model the performance of extensive green roofs on Vancouver Island, British Columbia, with respect to their carbon sequestration properties. The study is conducted at four independent locations in the Nanaimo and Duncan regions: (1) at the Gathering Place of Vancouver Island University (VIU-GP) (Nanaimo campus), (2) at the VIU Cowichan campus (VIU-COW) in Duncan, (3) at the new office building of Island West Coast Developments (IWCD), our industrial partner, and (4) at the transit building of the Regional District of Nanaimo (RDN), our municipal partner.

Click on the following images to get a description of each roof:

VIU-GP

VIU-COW

   


IWCD


RDN

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What type of plants are used on the roofs?


A mix of endemic and ornamental plant species have been planted on the roofs (Table 1). These plants have been chosen because they are either representative of local habitats, are low maintenance and/or resist well to temperature and water extremes.

Table 1. Example of the most important plant species used on the green roofs (if species is endemic, click on the species name to access information on E-flora British Columbia)

Sedum oreganum Oregon stonecrop  
Sedum spathulifolium Broad-leaved stonecrop  
Sedum divergens Spreading stonecrop  
Gaultheria shallon Salal  
Festuca sea urchin Greenleaf fescue
Festuca ovina viridula Rocky Mountain fescue

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What type of measurements are made?

VIU-GP is equipped with a permanent and automated chamber system to monitor the net carbon dioxide exchange between the roof and the atmosphere (click here to get a full description of the system used on this roof) while VIU-COW, IWCD and RDN are visited bi-weekly in summer and monthly in winter with a compatible portable system. Each roof is also equipped with an automated climate station to monitor specific climate variables related to radiation, temperature and moisture (click on pictures of the sites above to get a description of the climate stations).

VIU-GP is also equipped with a digital camera to monitor the phenological development of the roof, i.e. the vegetation establishment and flowering patterns over time. Click here to see a cool video from May 2011 to May 2012!

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How will these measurements be used?


The data gathered in this study will first be used to calculate the carbon offset potential of the green roofs with respect to the building carbon emissions. Second, the data related to the roof microclimate and carbon dioxide exchange properties will be used to develop and validate a process-based model of carbon dioxide exchange for the green roofs. This model will be run with future climate data to evaluate the long-term carbon sequestration potential of the infrastructure. Eventually, the model will be tested for other green roofs in British Columbia and around the world.

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