Green Roofs - Living Architecture Monitor - Fall 2011 - (Page 10)
THE BUSINESS CASE
GREEN ROOF LCA
INSIGHTS FROM COST-BENEFIT ANALYSIS OF GREEN ROOFS VERSUS CONVENTIONAL ROOFS, PART OF A LARGER STUDY FOR THE GSA
By Adam Friedberg & Jordan O’Brien
stormwater at the sewage treatment plant will also be borne by the public, either through taxes or fees – a point that should be accounted for in any such analysis. Additionally, if future legislation regulates the quality of stormwater discharge, a green roof may improve or degrade the stormwater (depending on limited pollutant) and so, this may also need to be taken into account.
he financial decision whether to install a green roof should be considered on a case-by-case basis as the variability in structure, municipality, ownership, tenant, investment, technology, climate, installer experience, etc., requires specific attention. In some cases, a life cycle assessment will help decision-makers to better understand the impacts of a green roof over the long-term. With this in mind, a cost-benefit model was developed by the authors to compare the costs of green and conventional roofs – the installation, maintenance and replacement of the roof – with the benefits, savings and value produced by green and conventional roofs, as part of a larger study for the United States General Services Administration (GSA). A green roof is a holistic sustainability strategy that has direct and indirect benefits for multiple stakeholders including: developers and owners, through installation costs, rental impacts, operational costs, to name a few; municipalities, through reduced infrastructure maintenance or replacement costs; the community, through improved aesthetics, biodiversity, and/or green job creation; and building occupants through productivity gains and/or improved psychological and physical well-being. To account for these numerous costs and benefits, the analysis reviewed various methodologies and studies of green roof cost-benefit analyses. The most frequent benefits found and studied in the cost-benefit analysis can be found below. While some benefits are quantifiable, others are more qualitative and have limited quantitative evidence.
The energy cost-savings potential of a green roof is dependent on climate, energy costs, location, and building and mechanical design. It is also dependent on the building’s shape (i.e. the impact of a 10,000-squarefoot green roof on a 10-storey building compared to a two-storey building). The amount of energy to cool or heat the building can be affected by the temperature of the air intake, specifically rooftop equipment. The green roof can provide a more stable environment on the roof; one that is cooler when it is hot and warmer when it is cold, which can reduce cooling and heating requirements. In addition, the potential energy savings from the green roof may help achieve carbon credits if available.
Biodiversity is very difficult to measure economically but its value is generally created by adding biodiversity back into dense urban regions. Organizations and governments have designed schemes to value biodiversity financially by creating credit systems, or banking systems – the BushBroker scheme and the biodiversity banking system (BioBanking) for example. In addition, there is potential economic value to bee farms and nearby agriculture with a green roof having the potential to attract bees to pollinate plants.
HEAT ISLAND BENEFITS
The indirect effect of the ability for green roofs to mitigate the heat island effect is substantial. Reducing heat island has a societal affect that can reduce average and peak temperatures, air pollution, and heat/pollution related illness. The addition of green roofs, specifically at a city-wide level can have an economic benefit – the reduction in daily and peak temperatures as a result of reducing the temperatures of the surroundings.
Many municipalities require stormwater management onsite. Compared to a conventional roofing system, which requires additional stormwater management practices such as cisterns, retention ponds, etc., green roofs may be credited towards achieving the regulatory requirements. These best management practice (BMP) stormwater measures also require capital and maintenance costs and, therefore, should be included in an analysis. Municipalities may impose runoff fees to help replace the cost for future city-wide stormwater infrastructure required by a Long Term Control Plan, and/or incentivizing future runoff rates such as is the case with the Washington D.C. utility fee. Such fees may be mitigated through a green roof, depending on the project type. The green roof might also affect public infrastructure, whether the stormwater in a particular city is discharged into a Combined Sewer Overflow (CSO) system, and/or captured and filtered in other ways. Maintenance such as cleaning and replacement of filters used in the capture/filtration process must also be accounted for. Of course, in an area reliant on a CSO system, the cost of energy and materials required to treat
LIVING ARCHITECTURE MONITOR
Agricultural services like food, biofuel and nursery growth could generate benefits from green roofs, though they are typically not observed on roofs with three- or six-inch growth medium
Buildings below highways or high air traffic areas can receive benefit from a green roof through noise mitigation. However, the benefit may be limited to green roofs under these conditions.
CLEAN AIR BENEFITS
Green roofs may help to mitigate air pollution. The absorption, temperature cooling and sequestration ability of the green roof can potentially offset air treatment systems for NOx, SOx, PM and CO2. In addition, at the city-scale, the green roofs cooling ability (see Heat Island Benefits above) potentially even further offset NOx and Ozone.
AESTHETICS & QUALITY-OF-LIFE BENEFITS
Productivity increase and a reduction in absenteeism due to green roofs is
Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Fall 2011
Green Roofs - Living Architecture Monitor - Fall 2011
From the Founder
On the Roof With…
Policy: The Game Changer
Policy: Toronto’s Green Roof by-Law
Policy: Energy Code Break-Through
GRHC: Welcome New Members
Green Roofs - Living Architecture Monitor - Fall 2011