Life-cycle cost–benefit analysis of extensive vegetated roof systems

https://doi.org/10.1016/j.jenvman.2007.01.024Get rights and content

Abstract

The built environment has been a significant cause of environmental degradation in the previously undeveloped landscape. As public and private interest in restoring the environmental integrity of urban areas continues to increase, new construction practices are being developed that explicitly value beneficial environmental characteristics. The use of vegetation on a rooftop—commonly called a green roof—as an alternative to traditional roofing materials is an increasingly utilized example of such practices. The vegetation and growing media perform a number of functions that improve environmental performance, including: absorption of rainfall, reduction of roof temperatures, improvement in ambient air quality, and provision of urban habitat. A better accounting of the green roof's total costs and benefits to society and to the private sector will aid in the design of policy instruments and educational materials that affect individual decisions about green roof construction. This study uses data collected from an experimental green roof plot to develop a benefit cost analysis (BCA) for the life cycle of extensive (thin layer) green roof systems in an urban watershed. The results from this analysis are compared with a traditional roofing scenario. The net present value (NPV) of this type of green roof currently ranges from 10% to 14% more expensive than its conventional counterpart. A reduction of 20% in green roof construction cost would make the social NPV of the practice less than traditional roof NPV. Considering the positive social benefits and relatively novel nature of the practice, incentives encouraging the use of this practice in highly urbanized watersheds are strongly recommended.

Introduction

The relationship between the built and natural environment has traditionally been one of complete opposition. Both terrestrial and aquatic ecosystems are drastically, and often times irrevocably, altered during the process of urbanization (Pickett et al., 2001; Paul and Meyer, 2001). Water regulation and supply, erosion control and sediment retention, nutrient cycling, climate regulation, and waste treatment changes are all ecosystem services either eliminated or significantly degraded in highly developed landscapes (Costanza et al., 1997). The construction of man-made structures and impervious surfaces that are a defining feature of highly developed areas are an important causal element behind environmental decline in urban areas (Arnold and Gibbons, 1996).

One reason why construction practices lead to environmental problems is that the costs of environmental degradation are not fully realized by the party who caused the damage. Thus, when evaluating construction costs, developers have historically viewed environmental damage as exogenous to the development process. Federal and state environmental laws have altered this situation to some extent in the last several decades. Developers have been limited by laws and regulations concerning erosion and sedimentation control, post-construction stormwater control and urban tree preservation. Nonetheless, developers still make land use decisions without considering the full cost of the environmental damage that their activities create.

Positive incentives have been developed for more ecologically sensitive development, particularly for buildings. A rating system called leadership in energy and environmental design (LEED) has been created by the United States Green Building Council for certification of commercial buildings that have a reduced environmental impact. As of 2005, 393 projects had received LEED certification and many municipalities require buildings built with public funds to receive LEED certification (Cassidy, 2003). Other organizations such as the National Association of Home Builders have recently developed green building guidelines based on similar standards (NAHB, 2004).

Specific building construction practices are being refined to create structures which have a much smaller impact on the surrounding landscape than previously thought possible. At the broadest scale, sites are selected for their proximity to public transportation, their ability to maximize open space and protect habitat, effectively manage stormwater runoff, address the heat island effect found in urban areas, and reduce light pollution (www.usgbc.org). Sustainable water use for a building may involve xeriscaping, graywater reuse for irrigation, and the use of low-flow or composting toilets and non-water urinals, which are becoming increasingly cost effective (Gleick, 2003). A building's energy use is also an extremely important component of sustainable design. From simply designing smaller structures to installing active solar panels or other on-site sources of self-supplied energy, there are a wide range of practices available to reduce a building's reliance upon fossil fuel energy sources. Increasingly, building materials contain recycled material content in new construction and attempt to reuse as much of the existing structure in renovations as possible (Horvath, 2004). Indoor environmental quality is also an important feature of green buildings. Paints and adhesives designated “Low-VOC” or “No VOC” (volatile organic compounds) reduces the low level toxic emissions found in older materials and improves indoor air quality for building occupants. Day-lighting larger portions of the structure improve the working environment in commercial buildings as well as reducing energy costs when high performance windows are used.

Of these many ways that buildings can be designed and constructed in a more sustainable manner, the roof surface can easily be overlooked as space that can be designed into an environmental amenity for the building, not simply contributing to environmental problems. The rooftop is typically the same size as the building's footprint and is the structure's prime barrier against precipitation and solar radiation. To the extent that the roof surface can be transformed into useful space, the building becomes economically and functionally more efficient and can have a more benign effect on the surrounding landscape.

Published research has focused largely on the energy savings associated with different types of roofing systems. Akbari et al. (2001) found that changing a roof from one with low albedo to high albedo in Sacramento, CA would decrease cooling energy use by 80%. Other studies have documented the affect of insulation on the heat flux at the roof surface (Al-Sanea, 2002), how to incorporate active and passive solar designs into rooftop systems (Heras et al., 2005; Maneewan et al., 2005), and the energy benefits associated with ventilated roof systems (Ciampi et al., 2005). These alternatives to traditional roofing systems are beginning to gain more of a market share and EPA has established an Energy Star rating system for roofing products, primarily identifying roofing membranes which have high albedos and the potential to significantly reduce building energy costs (www.energystar.gov).

While energy savings are an important function of alternative roof systems, other benefits may also be realized. In a traditional roofing system, rainfall hits the rooftop and is quickly channeled into the nearest gutter or storm sewer system with the goal being to have the roof shed water as quickly as possible. As regulations have mandated stormwater management plans for municipalities, rooftop runoff control has become an important management practice for minimizing degradation of aquatic ecosystems. One solution is to create rainwater storage tanks which can capture rainfall from the roof surface and store it for a time before it is reused or slowly discharged (Vaes and Berlamont, 2001).

The application of vegetation and growing media to the roof surface is an increasingly popular practice which produces improvements in both energy conservation and stormwater management. These green roofs are multi-functional in that they provide numerous environmental benefits simultaneously. These benefits include: decreasing the surface temperature of the roof membrane and energy use in the building (Kumar and Kaushik, 2005), retaining stormwater for small storm events (Carter and Rasmussen, 2006), increasing biodiversity and habitat in urban areas largely devoid of such space (Kim, 2004; Brenneisen, 2005), and improving ambient air quality (Clark et al., 2005). While these benefits are inherent in all green roof systems to some degree, depending on the design of the roof there is potential for other amenities as well. Accessibility and esthetic appeal for the building occupants, sound insulation and the potential for urban agriculture are all realistic benefits provided by green roof applications (Peck et al., 1999).

There are two general types of modern green roof systems: intensive and extensive. Intensive systems are characterized by deep (>6 in) growing media, opportunities for a diverse plant palate on the rooftop and high cost and maintenance requirements. Extensive systems are designed to be lightweight and easily retrofitted on existing roof surfaces. They contain thin growing media depths (2–6 in) and can support a limited number of drought-tolerant plants that thrive in the limited water and nutrient conditions. Over 80% of green roofs in Germany are extensive systems and these types of green roofs are expected to offer the most cost-effective approach for roof greening (Harzmann, 2002).

While green roof projects have recently generated significant interest in design fields such as landscape architecture, little research has been done to evaluate the costs and benefits of green roof systems for urban applications. Much of the peer-reviewed literature on the economics of green roofing systems is found in conference proceedings and evaluate the private benefits at a single roof scale. Lee (2004) compared green roof and traditional roof life-cycle costs over 60 years for a single roof in Oregon. They found the green roof to be 7% more expensive than the conventional roof over this time. This analysis included extended roof life, energy savings, and stormwater fee reduction in the economic benefits that the green roof provided. Clark et al. (2006) demonstrated a return on investment of 11 years on a single green roof in Michigan when low green roof installation costs and high environmental benefits were considered. Alternative metrics to monetary values such as Eco-indicator values and energy analysis have been used to compare green roofs to conventional roofs in a sustainability context. These studies find green roofs provide significant environmental benefit over a traditional roof relative to the life cycle and embodied energy of its materials (Alcazar and Bass, 2006; Coffman and Martin, 2004; Kosareo and Ries, 2006). Other published reports typically focus on a single green roof benefit (Wong et al., 2003) or qualitatively describe a series of benefits derived from different types of green roofs (Peck et al., 1999; Banting et al., 2005).

Benefit cost analysis has been widely recognized as a useful framework for assessing the positive and negative aspects of prospective actions and policies, and for making the economic implications alternatives an explicit part of the decision-making process (Arrow et al., 1996). Benefit–cost analysis compares alternatives over time as well as space, and uses discounting to summarize its findings into a measure of net present value (NPV) (Hanley and Spash, 1993) The test of NPV is a standard method for assessing present value of competing projects over time. In the case of this study, the roofing scenario with the lowest NPV is the preferred option as the low value indicates the least costly alternative.

This study quantifies the costs and benefits of thin-layer, or extensive, green roof systems as they compare to typical flat roofs in an urban watershed. The authors combine local construction costs for an established green roof test site with experimentally collected stormwater retention data and building energy analysis data into a single metric using conventional cost–benefit analytical techniques applied over the life cycle of a typical green roof. In order to carry out this analysis we rely in part on published data from other green roof research and practice for estimating these effects. This may introduce some bias, and indicates that this work is subject to revision as increasing experience with green roofs produces more and better data. We then use this information to evaluate an entire local watershed, using a variety of spatial scales as a case study for application of widespread green roofs. As green roof popularity continues to grow, it is important for accurate life-cycle benefit–cost analyses (BCA) of green roof systems to be performed to inform both policy makers who may allocate public funds for projects with public benefits, and private building owners who may see a future financial incentive to invest in new and relatively unproven technology.

Section snippets

Project site and test plot

The project examines the feasibility of replacing all the flat roofs in an urban watershed with green roof systems. The Tanyard Branch watershed was selected as a study site. This highly urbanized watershed contains a second-order stream system and is located in Athens, GA approximately 60 miles east of Atlanta, GA. The watershed contains significant portions of the downtown commercial district of Athens, the University of Georgia, and both single and multi-family residential areas. Using 2003

Theory and calculation

The economically relevant impacts of widespread roof greening were established and physical quantification of these impacts were performed using the green roof test plot as a template for all new green roofs in the watershed. The benefits were divided into categories found in Table 1 with the conceptual framework outlined in Fig. 4. Analysis for the social BCA was performed at the watershed scale while a private BCA was performed using a typical one-story 929 m2 roof. Details of each category

Green roof private and social benefits

Green roof benefits were estimated for both private and social institutions. Results from these runs are shown in Table 7, Table 8. The benefits are considered conservative estimates where current pricing conditions are assumed and values base on the campus test plot are used.

Applying the NPV test

Compiling all the discounted costs and benefits associated with these two roofing systems allows for an NPV test to be performed. Using a 4% discount rate over 40 years, the total costs of installing thin-layer green roof

Discussion

BCA of widespread extensive roof greening in the Tanyard Branch watershed reveals a number of important considerations for both the private and public sectors when considering green roof installation. The most significant economic benefits are the increase in roof life, stormwater BMP cost avoidance, and energy savings. The main construction benefit, and best overall benefit in economic terms, of the extensive green roof is that it extends the life of the waterproofing membrane and eliminates

Conclusions

Expansion of urban areas and the built environment, combined with greater public interest in maintaining the integrity of ecological systems in these areas, has caused the construction industry to begin developing practices that have less environmental impact. Innovative new materials and techniques will be largely governed by economic returns on this investment. Since many of the environmental goods affected by development are public in nature and rarely internalized by private firms, it is

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