Cool vs. Green: Rooftop Solutions for Urban Energy Efficiency

A Study on the Energy-saving Potential of Cool Roofs and Green Roofs in Urban Areas

A recent paper published in the journal Npj Urban Sustainability investigated the potential of cool roofs and green roofs in decreasing energy consumption by buildings in current and future climates.

Background

Expansive urban growth in the 21st century has led to increased per capita urban greenhouse gas emissions, resulting in significant changes to global climate patterns and temperatures. The urban heat island effect further exacerbates this issue by raising urban temperatures and increasing building energy consumption for cooling. The built environment plays a major role in the increased energy demands and greenhouse gas emissions, necessitating effective methods for reducing building energy consumption.

Role of Roofs

Green roofs and cool roofs are two strategies commonly used to decrease building energy consumption. Green roofs utilize vegetation to reduce heat absorption and provide insulation, while cool roofs use reflective surfaces to reduce heat absorption.

Several studies have confirmed the effectiveness of cool and green roofs in reducing energy consumption. However, most research has not considered how variations in the built environment and ambient climate affect the energy-saving potential of these roofs. Additionally, studies have not focused on evaluating the performance of these roofs under future climate scenarios. Therefore, a comprehensive study is necessary to compare these technologies in various climate settings under both current and future conditions.

The Study

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In this study, researchers proposed an integrated approach to assess the energy-saving potential of cool roofs and green roofs in current and future climate conditions. The approach combined building energy simulation and climate change modeling to quantify the climate change effect on building energy demand.

The objectives of the study were to develop an integrated modeling approach for detailed building energy simulation, identify the effects of local ambient climate on the energy-saving performance of these roof technologies, assess the impacts of the urban built environment on roof performance, and predict the energy-saving performance of these roofs under various shared socioeconomic pathways (SSPs).

Six global cities were selected for evaluation: São Paulo, Los Angeles, London, Seoul, Hong Kong, and Cairo. These cities represent various climatic zones and were used to assess the performance of green and cool roofs under future and current climate change scenarios.

In the sensitivity analysis, researchers considered temperature variations for three SSP scenarios from 2021 to 2100. The future energy-saving potentials of green and cool roofs were investigated under these scenarios. Additionally, the future energy-saving performance trends were projected until 2200 based on the relationship between climate projections until 2100 and the energy-saving of both roof systems.

Research Findings

The integrated modeling framework provided more accurate estimations of monthly and hourly building energy use compared to conventional methods. The energy-saving effects of green and cool roofs depended on the climate zones where they were implemented. Both roof types showed significant potential in decreasing cooling energy demand across different climates.

Green roofs were more sensitive to variations in outdoor environmental conditions compared to cool roofs. The health and growth of plants on green roofs are affected by factors like sunlight and precipitation. However, the energy-saving effectiveness of cool roofs remained unaffected by environmental variations.

Cool roofs were more suitable than green roofs for decreasing cooling energy consumption in regions with cloudy or rainy conditions and warm climates. Both roof types were suitable for reducing energy demands year-round in hot and sunny regions. However, cool roofs led to an increase in heating energy demand in cities with colder climates.

The energy-saving effects of cool and green roofs were also influenced by the urban built environment. Low-rise buildings displayed the best energy-saving performance, while the energy-saving performance decreased with increasing building height.

In future climate conditions with high greenhouse gas emissions, both green roofs and cool roofs could save more cooling energy for buildings. For example, by 2100, the implementation of green and cool roofs resulted in a 65.51% and 71.72% reduction in heating, ventilation, and air conditioning (HVAC) consumption, respectively, at the city level. The global warming effect on decreasing building heating energy demand must be considered in future climate projections.

In conclusion, this study demonstrated the feasibility of using cool roofs and green roofs for building energy savings in current and future climates. However, their effectiveness varies based on climate zones and the urban built environment.

Journal Reference

Jia, S., Weng, Q., Yoo, C., Xiao, H., Zhong, Q. (2024). Building energy savings by green roofs and cool roofs in current and future climates. Npj Urban Sustainability, 4(1), 1-13. https://doi.org/10.1038/s42949-024-00159-8, https://www.nature.com/articles/s42949-024-00159-8

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SDGs, Targets, and Indicators

1. SDG 7: Affordable and Clean Energy

   • Target 7.3: By 2030, double the global rate of improvement in energy efficiency
   • Indicator 7.3.1: Energy intensity measured in terms of primary energy and GDP

2. SDG 11: Sustainable Cities and Communities

   • Target 11.2: By 2030, provide access to safe, affordable, accessible and sustainable transport systems for all, improving road safety, notably by expanding public transport, with special attention to the needs of those in vulnerable situations, women, children, persons with disabilities and older persons
   • Indicator 11.2.1: Proportion of population that has convenient access to public transport, by sex, age and persons with disabilities

3. SDG 13: Climate Action

   • Target 13.2: Integrate climate change measures into national policies, strategies and planning
   • Indicator 13.2.1: Number of countries that have integrated mitigation, adaptation, impact reduction and early warning into primary, secondary and tertiary curricula

4. SDG 15: Life on Land

   • Target 15.1: By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services, in particular forests, wetlands, mountains and drylands, in line with obligations under international agreements
   • Indicator 15.1.2: Proportion of important sites for terrestrial and freshwater biodiversity that are covered by protected areas, by ecosystem type

Analysis

The article discusses the potential of cool roofs and green roofs in decreasing energy consumption by buildings in current and future climates. Based on this information, the following analysis can be made:

1. SDG 7: Affordable and Clean Energy

   • The article addresses the issue of energy consumption in buildings and how cool roofs and green roofs can contribute to reducing energy demand.
   • The specific target under this SDG that can be identified is Target 7.3, which aims to improve energy efficiency globally.
   • The indicator mentioned in the article is Indicator 7.3.1, which measures energy intensity in terms of primary energy and GDP. This indicator can be used to measure progress towards improving energy efficiency.

2. SDG 11: Sustainable Cities and Communities

   • The article highlights the role of cool roofs and green roofs in urban areas to reduce building energy consumption.
   • The specific target under this SDG that can be identified is Target 11.2, which aims to provide access to safe, affordable, accessible, and sustainable transport systems for all.
   • The indicator mentioned in the article is Indicator 11.2.1, which measures the proportion of the population that has convenient access to public transport. This indicator can be used to measure progress towards providing sustainable transport systems.

3. SDG 13: Climate Action

   • The article discusses the impact of cool roofs and green roofs on reducing greenhouse gas emissions and their potential in mitigating climate change.
   • The specific target under this SDG that can be identified is Target 13.2, which aims to integrate climate change measures into national policies, strategies, and planning.
   • The indicator mentioned in the article is Indicator 13.2.1, which measures the number of countries that have integrated climate change measures into curricula. This indicator can be used to measure progress towards integrating climate change measures into education.

4. SDG 15: Life on Land

   • The article mentions the potential of green roofs in providing habitat for plants and contributing to the conservation of terrestrial ecosystems.
   • The specific target under this SDG that can be identified is Target 15.1, which aims to ensure the conservation, restoration, and sustainable use of terrestrial ecosystems.
   • The indicator mentioned in the article is Indicator 15.1.2, which measures the proportion of important sites for biodiversity that are covered by protected areas. This indicator can be used to measure progress towards conserving terrestrial biodiversity.

Table: SDGs, Targets, and Indicators