By Lina Hassoun
Lina is a doctoral researcher at the Architecture and Building Systems group at ETH Zürich. Her work focuses on low-impact cooling technologies and targeted building retrofits to improve urban heat resilience. She is particularly interested in the intersection of climate adaptation, energy systems, and social equity.
Green and white roofs are sustainable solutions for reducing urban heat and improving building energy performance. When combined with photovoltaic (PV) systems, they are often promoted as technologies that also enhance solar energy yield. However, our comprehensive analysis reveals that previous studies may have overstated their benefits. By evaluating year-round performance across diverse climates, we found that gains to PV electricity production remain modest and context-dependent.
Why This Matters
As cities accelerate their energy transitions to combat climate change, sustainable roofing solutions such as green (vegetated) and white (reflective, cool) roofs have been widely promoted as tools for improving the yield of rooftop photovoltaic (PV) systems by reducing roof surface temperatures. These cooler surfaces are expected to enhance PV efficiency, as solar panels perform better when they’re not overheated. But how reliable are the frequently cited benefits of these combined systems (e.g. Fleck et al. (2022), Ramkiran et al. (2021), Hui & Chan (2011))? Our study analyzed the year-round performance of PV-green and PV-white roofs across 13 cities with diverse climates, considering current and future climate scenarios. The findings reveal that their impact has been inflated by experimental studies conducted in controlled, favorable conditions.
Our Approach
We used building and solar system simulations to evaluate and compare the performance of three roof types (gravel, green, and white) paired with solar panels across 13 cities with different climates. Gravel roofs typically consist of a layer of loose gravel placed over a dark roofing membrane, providing basic protection but minimal thermal regulation. Green roofs incorporate vegetation planted on a substrate (e.g. soil) layer. White roofs are covered with reflective materials, such as white paint, specialized coatings, or white membranes, which reduce heat absorption. In this study, white roofing is achieved through the use of a reflective membrane. When paired with PV panels, sustainable roof types like green and white roofs are referred to as ‘combined roofs’.
Our analysis covered a whole year to capture seasonal variations and included future climate scenarios to assess how these roofs might perform in a hotter, changing world. We modeled green roofs without additional irrigation across all cities to reflect conditions where irrigation may be impractical, costly, or unsustainable. This approach is especially relevant in regions where large-scale adoption of green roofs would likely exclude expensive irrigation systems, particularly in water-scarce areas.
Figure 1: Roof constructions used in this study
First, we validated and calibrated the models using real roof temperature and solar energy yield data from roofs in Zürich, Switzerland. Then, we applied them to 13 cities with different climates, from the consistently warm and humid conditions of Singapore to the seasonal extremes of Montreal. For each city, we ran simulations for current conditions and a future climate scenario (year 2090), based on a high-emissions trajectory, assuming greenhouse gas emissions continue at present levels. This allowed us to see how climate change might affect solar panel performance on different roof types. By comparing the results across roof types, cities, and time periods, we could determine how each roofing solution performs in different climates, both now and in the future.
Key Findings
1. Annual Energy Gains Are Modest
Our analysis revealed that annual PV efficiency gains from sustainable roofing, relative to the baseline gravel roofing, remain modest, consistently below 2% in all considered locations. These results challenge widely cited studies reporting gains as high as 8.3%, which were often based on short-term experiments conducted under ideal conditions, such as sunny summer days or constantly saturated green roofs. This difference highlights the importance of evaluating year-round performance to better understand these technologies’ real impact.
2. Green Roofs’ Benefits Depend on Rainfall
The cooling benefits of green roofs are highly dependent on sufficient moisture levels, which are influenced by natural precipitation or irrigation. In rainy climates, such as tropical or temperate zones, green roofs provided consistent cooling benefits due to the evapotranspiration process from the vegetation. However, the lack of moisture significantly reduced their effectiveness in water-scarce or arid climates. Without irrigation, green roofs often underperformed, especially during dry or seasonal drought conditions.
3. White Roofs Outperform Green Roofs
In most scenarios and climates, white roofs provided larger reductions in rooftop temperatures and higher PV efficiency improvements than green roofs. Their performance was particularly strong in hot, arid climates where the reflective properties of white roofs effectively mitigated excessive heat. Unlike green roofs, the performance of white roofs was not dependent on moisture availability, making them a more reliable option across a broader range of climates.
4. Limited Improvements Under Future Climates
Future climate scenarios paint a mixed picture for sustainable roofing. While some cities stand to benefit from increased solar irradiation, which could enhance overall PV yields, rising temperatures are expected to offset these gains by reducing the conversion efficiency of photovoltaic systems, which declines as panel temperatures increase. For example:
- Temperature Rise: Higher global temperatures will reduce the efficiency of converting solar energy into electricity, with declines seen across all examined cities.
- Solar Irradiation: Some cities, particularly in regions with reduced cloud cover, could see a slight increase in absolute PV yields, but this alone does not compensate for the efficiency loss.
- Rainfall Changes: Reduced precipitation in certain regions could further limit the performance of green roofs, especially in future arid climates, while wetter regions may continue to support evapotranspiration.
Combined sustainable roofs help mitigate some of the negative impacts of rising temperatures by reducing rooftop heat and PV panel temperatures. However, their benefits will remain modest and cannot justify replacing existing roofing systems before the end of their functional life. Instead, replacements should be considered as part of natural building and roofing upgrades.
Figure 2: Performance comparison of PV-green and PV-white roofs in current and future climates, based on efficiency gains relative to PV-gravel
What This Means for Policy and Practice
1. Phase Out Dark Roofs
Encouraging policies to eliminate dark roofing materials like gravel and replace them with white or green roofs during natural replacement cycles.
2. Adopt Combined Roofs at Scale
The benefits of combined roofing technologies scale efficiently in urban centers with dense building stock. While PV efficiency gains remain modest, maintaining current output in some cities would require transitioning from gravel roofs. A shift towards white or green roofs in dense contexts (e.g. Cairo, Hong Kong) could enhance these benefits at scale.
3. Consider White Roofs!
White performed consistently well across various climates, particularly in arid regions where green roofs struggle without irrigation. These roofs deserve greater attention in sustainable planning.
4. Green Roofs are Multifunctional
Do not write off green roofs! These findings do not reject green roofs but advocate for thoughtful, climate-conscious design decisions that optimize performance based on location and context. While green roofs may be less impactful than white roofs for boosting PV efficiency, they provide other critical benefits such as stormwater management, biodiversity support, and urban cooling.
Conclusion
This research brings needed nuance to the discussion about climate-conscious urban planning. While sustainable roofs offer some benefits for urban cooling and energy efficiency, their actual impact is less revolutionary than previously claimed. Our study stresses the importance of context-specific application and the need for realistic expectations when incorporating these technologies into urban planning. White roofs emerge as a more reliable solution for most climates. Future research should consider multi-criteria analyses to better capture the diverse roles that sustainable roofs play in urban resilience.
Suggested citation: Lina Hassoun. “Sustainable Roofs Aren’t One-Size-Fits-All”, Energy Blog @ ETH Zurich, ETH Zurich, April 29th, 2025, https://blogs.ethz.ch/energy/sustainable-roofs/
Cover image: Michael Pointner on Unsplash
This blog post is based on Hassoun, L., & Cook, L. M. (2024). Global Analysis of Combined Photovoltaic Green and Cool Roofs Under Climate Change. Advanced Sustainable Systems, 8(12), 2400097.
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