Green roof at Arlington's Walter Reed Community Center.
Technical Summary

Green and Cool Roofs

Project Drawdown defines cool roofs as building roofs that use light reflecting materials or paints. We also define green roofs as building roofs with natural vegetation. 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 reduces the energy load and emissions related to heating and cooling buildings, lowers the ambient air temperature to make HVAC systems more effective and captures or slows water runoff. 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.

Methodology

To model the adoption of cool roofs and 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 markets (TAM) for cool roofs and green roofs were calculated for each type of roof solution. These are based on the Project Drawdown integrated buildings TAM model which collectively calculates the TAMs of building floor area, roof area, space heating and cooling, and all other floor-area driven TAM’s used in the building sector. The estimated areas are also subdivided by building type (residential and commercial), and by building climate zone[2]. This model used numerous sources. From a total possible global roof area estimate, we apply two filters. First, we assume a specified percent of the total roof area in each climate zone is appropriate for each roof type, and for cool roofs, this varies from 0 percent in “cold” and “arctic” climate zones 7 and 8 to 100 percent in “extremely hot” climate zone 0. Secondly, we apply a filter to represent the appropriateness of roofs in the filtered zones to have the specified solution. For instance, due to soil weight, green roofs require structural support and minimal slopes, so we assumed that only 15 percent of existing buildings fulfil these criteria, but 50 percent of new buildings can fulfil it (with sufficient global action to promote green roofs). The global market for green roofs in 2018 was 17 billion square meters of roof area, growing to 45 billion by 2050. The global market for cool roofs in 2018 was 95 billion square meters of roof area, growing to 146 billion by 2050.

Current adoption[3] of each roof type was estimated for 2018. Green roofs were estimated at current adoption of 1 percent, and cool roofs were estimated at 5 percent. This current adoption was used to calculate the Reference Scenario from 2020 to 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. Percentage targets were used and are justified by current adoption trend characteristics, current policy and incentive trends, and likely barriers to adoption (first cost and operating cost considerations). With adoption scenarios established, key financial and climate variables were applied to the roof area adopted to determine greenhouse gas mitigation potential and cost/savings results for cool roofs and green roofs.

Adoption Scenarios[4]

Impacts of increased adoption of green roofs from 2020 to 2050 were generated based on two growth scenarios. These were assessed in comparison to a Reference Scenario, in which the solution’s market share was fixed at the current levels.

  • Scenario 1: Cool roofs were assumed to grow at a 7 percent equivalent rate annually following an s-curve, which would indicate a 30 percent adoption in 2050. Green roofs were assumed to grow at a 9 percent equivalent rate annually following an s-curve, which would indicate a 9 percent adoption in 2050.
  • Scenario 2: Cool roofs were assumed to grow at a 9 percent equivalent rate annually following an s-curve, which would indicate a 48 percent adoption in 2050. Green roofs were assumed to grow at an 11 percent equivalent rate annually following an s-curve, which would indicate a 13 percent adoption in 2050.

Emissions Model

Emissions included in this analysis are electricity and fuel for cooling and heating (including a heating penalty for cool roofs). These variables use data from a range of sources in the peer-reviewed literature, and most of them were weighted by climate zone. Emissions factors were obtained from the Intergovernmental Panel on Climate Change (IPCC) guidelines.

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 (maintenance of the roof, and the energy load costs of the building associated with the roof type). A conventional operating cost was constructed, and the reduction data was applied to that conventional operating cost value. For green roofs, a particular benefit is managing storm water which is easier and cheaper with green roofs. For this reason, storm water related costs have been included in this model.

Integration[5]

Drawdown’s roofs solutions were integrated with others in the Buildings Sector by first prioritizing all solutions according to the point of impact on building energy usage. This meant that building envelope solutions like Insulation were first, building systems like BAS were second, and building applications like Heat Pumps were last. Thus, the cool roofs and green roofs energy saving potential was reduced to represent the prior energy savings of the higher-priority solution: Insulation.

Results

Drawdown found a potential for 0.6 gigatons of carbon dioxide-equivalent greenhouse gas reductions in the Scenario 1 over 2020–2050, with a net implementation cost of US$624 billion and lifetime operational savings of US$330 billion.[6] For Scenario 2, the emissions avoided amount to 1.1 gigatons, the lifetime savings, $593 billion, and the net cost, $954 billion. However, most of the cost ($815 billion in Scenario 2) comes from the green roof solution, yet most of the emissions reduction and operational savings (1 gigaton and $735 billion, respectively, in Scenario 2) come from the cool roofs solution.

Discussion

Green roofs show promise in the appropriate climate conditions to have significant emissions mitigation impact, but the likely driver of adoption will be their impact on urban stormwater retention and wildlife habitats supported by urban policies. While we have included mostly costs for extensive  green roofs which are characterized by under 15 cm of soil cover and only grasses, intensive green roofs are far more expensive as they require even stronger roofs to support more soil, more captured water and larger plants. The additional aesthetic and social benefits have not been captured, but they can be significant in urban environments.

Cool roofs are an excellent tool to assist with the localized urban heat island effect and impact energy savings. There are numerous methods of implementation including special roof tiles and paints, and it could be a low cost option to improve building temperatures in some hot developing regions. 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 making a heating penalty a reality. Cooling in the warmer seasons is good, but cooling in the cooler seasons is not, and cool roofs are unchangeable across seasons in temperate regions.

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.[7]  The significant difference in results has prompted at least two papers which assess the methodologies involved and discuss the different approaches, assumptions, and limitations.[8] 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 Scenario 2s. 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 Scenario 2s.

Note: August 2021 corrections appear in boldface.

[1] For more on the Total Addressable Market for the Buildings Sector, click the Sector Summary: Buildings link below.

[2] We were guided by the ASHRAE 169 building climate zone standards

[3] Current adoption is defined as the amount of functional demand supplied by the solution in 2018. This study uses 2014 as the base year.

[4] To learn more about Project Drawdown’s growth scenarios, click the Scenarios link below. For information on Buildings Sector-specific scenarios, click the Sector Summary: Buildings link.

[5] For more on Project Drawdown’s Buildings Sector integration model, click the Sector Summary: Buildings link below.

[6] All monetary values are presented in US2014$.

[7] Including Irvine, Ridgwell, and Lunt (2011), Jacobson and Ten Hoeve (2012), and Zhang et al. (2016).

[8] 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).