Green Roofs
Project Drawdown defines green roofs as: building roofs that use light reflection and/or absorption to reduce space heating and cooling needs. This replaces the conventional practice of building traditional, dark-colored roofs not covered with vegetation.
“Green roofs” impact global warming by providing an insulating layer of vegetation on top of residences and commercial properties that both reduces the energy load and emissions related to heating and cooling buildings, and lowers the ambient air temperature to make HVAC systems more effective. “Cool roofs” reflect more incoming sunlight than traditional darker roofs, which in turn reduces the heat of the roof surface and the cooling load of a building. By reducing overall energy load and lowering the surrounding air temperature, cool roofs effectively mitigate greenhouse gas emissions related to air conditioning.
NOTE: Drawdown has combined the adoption of green roofs and cool roofs into the solution green roofs.
Methodology
To model the adoption of green roofs, two models were created—one for green roofs and one for cool roofs—and the results were added together. Both models share the same functional unit, square meter of roof, which also doubles as the unit of implementation.
Total Addressable Market [1]
The total addressable market for green roofs was first calculated for each type of roof. Three data sources were used to estimate total roof space: Global Buildings Performance Network (GBPN), McKinsey, and Navigant. From a total possible global roof area estimate, we used a “climate appropriate” discounting method to limit the addressable market of green roofs to those areas of the world with climate characteristics conducive to green roofs. The same was done for cool roofs. The global market for green roofs in 2014 was 25.8 billion square meters of roof area, growing to 48 billion by 2050. The global market for cool roofs in 2014 was 16.7 billion square meters of roof area, growing to 37.7 billion by 2050.
Current adoption [2] of each roof type was estimated for 2014. Green roofs were estimated at current adoption of 165 million square meters, and cool roofs were estimated at 1.5 billion. This current adoption as a percentage of the market was used to calculate the Reference Scenario from 2020-2050. In the Reference Scenario, any roof that is not adopted as a green or cool roof is assumed to be a conventional asphalt shingle roof.
For forecasting adoption of green roofs, a default sigmoid curve was used to grow adoption as a percentage of the market to the year 2050 (reaching 30 percent for green roofs and 60 percent for cool roofs). These percentage targets are justified by current adoption trend characteristics, current policy and incentive trends, and likely barriers to adoption (first cost and operating cost considerations). With this adoption scenario established, key financial and climate variables were applied to the roof area adopted to determine greenhouse gas mitigation potential and cost/savings results for green roofs.
Adoption Scenarios [3]
Impacts of increased adoption of green roofs from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.
- Plausible Scenario: A global adoption of 37 billion square meters of green roofs was calculated over the period 2020-2050, resulting in a mitigation of 775 megatons of greenhouse gas emissions compared to the Reference Scenario.
- Drawdown Scenario: An optimization of new building policy was assumed, resulting in a greater adoption by 2050 (50 percent for green roofs, 90 percent for cool roofs). Cool roofs are favored for adoption because of lower costs (nearly equivalent to the cost of conventional roofing).
- Optimum Scenario: This scenario considers the maximum potential of green roofs, assuming adoption by 2050 of 75 percent for green roofs and 90 percent for cool roofs.
Financial Model
Financial variables were statistically assessed by comparing multiple sources of both first cost (cost of solution acquisition and implementation) and full operating cost (operation and maintenance of the roof, and the energy load costs of the building associated with the roof type). A conventional operating cost was constructed in both cases, and the reduction data was applied to that conventional operating cost value.
Integration [4]
Integrating the Drawdown solutions reduced the impact of green roofs by incorporating the energy savings created by increased insulation of the building envelope. The emissions factors associated with on-site fuel and electric grid use were discounted for green and cool roofs based on the reduction in heating and cooling energy use resulting from the adoption of insulation.
Results
Drawdown found a potential for 0.78 gigatons of carbon dioxide-equivalent greenhouse gas reductions in the Plausible Scenario over 2020-2050, with a net implementation cost of US$1,393 billion and a net operational savings of US$3,008 billion. [5] For the Drawdown Scenario, the emissions avoided amount to 1.31 gigatons, and the Optimum Scenario results in 1.69 gigatons of reductions.
Discussion
Green roofs show promise in the appropriate climate conditions to have significant emissions mitigation impact, but the likely driver of adoption will their impact on urban stormwater retention and wildlife habitats. Cool roofs are an excellent tool to assist with the localized urban heat island effect and impact energy savings. However, it is clear that there needs to be a more nuanced approach to implementation, particularly with local climatic conditions, as there can be a net negative effect with implementation. Some reviewers and readers might be surprised by the modest mitigation potential results modeled for cool roofs.
Many researchers have modeled and/or reported on the potential impact of increased albedo through cool roofs on global temperatures. The results of some of these models show significant global cooling. However, other studies assert that cool roofs which increase urban surface albedo are limited in their potential. [6] The significant difference in results has prompted at least two papers which assess the methodologies involved and discuss the different approaches, assumptions, and limitations. [7] Differences in models and how they account for oceans, aerosols, moisture balance, cloud formation, and spatial resolution account for the controversy. Here, the conservative approach has been taken and potential global cooling because of increased urban albedo has been discounted for all Drawdown scenarios. It should be noted that researchers agree that cool roofs have an effect of reducing overall building energy use, which is the mitigation effect modeled in all Drawdown scenarios.
[1] For more on the Total Addressable Market for the Buildings and Cities Sector, click the Sector Summary: Buildings and Cities link below.
[2] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
[3] To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Buildings and Cities Sector-specific scenarios, click the Sector Summary: Buildings and Cities link.
[4] For more on Project Drawdown’s Buildings and Cities Sector integration model, click the Sector Summary: Buildings and Cities link below.
[5] All monetary values are presented in US2014$.
[6] Including Irvine, Ridgwell, and Lunt (2011), Jacobson and Ten Hoeve (2012), and Zhang et al. (2016).
[7] Menon, Surabi, and Ronnen M. Levinson; Cool roofs and global cooling: a response to Jacobson & Ten Hoeve (2011) and Jiachen Zhang, Kai Zhang, Junfeng Liu and George Ban-Weiss (2016).
