The fact that a green roof contributes to buildings' energy efficiency is already well known. Our readers could find information about Urbanscape green roofs effect on energy efficiency in the last decade in our blogs as well as published articles. Nevertheless, the topic remains interesting, and as such constantly supported by recent research findings. In era of major contemporary challenges such as climate change, this is a topical issue, relevant more than ever.
Urban heat island effect (UHI) and our well being
We are in mid-summer time, with heat waves nowadays more or less expected to occur on regular basis. Citizens are becoming vulnerable to weather extremes in the given urban ecosystem which needs in most cases dramatic changes - in segment of urban planning on one hand and lifestyle of urban dwellers on another.
The term UHI refers to how built-up areas have a greater capacity to absorb, hold and emit the sun’s heat compared to rural areas. The more the build-up the area becomes, the greater heat island effect will occur.
Picture 1: Heat Island effect increases with urbanisation.
This absorption of energy can increase day and night time temperatures in cities by several degrees. UHI increases summertime peak energy demands, air conditioning costs, and unfortunately, heat-related illness and mortality (E2DesignLab, 2024).
Green infrastructure in cities is not questionable, it is rather a must
More and more empirical evidence shows the positive effect of the green infrastructure in cities. On-going global warming and urban growth gives more than clear indication that outdoor urban environment must be planned as thermally comfortable and resilient.
In 2023, research team led by Iungman (2023) published interesting study which accounted monitoring of death occurrence due to UHI in 93 European cities. In three months’, period June-August 2015, as much as 6700 premature deaths occurred due to hotter urban temperatures in these cities. The study revealed, that one third of these deaths could have been prevented by increasing tree cover up in cities to 30%, thereby reducing temperatures. The study highlights the substantial benefits of planting more trees in cities, although the authors acknowledge that this can be challenging in some cities due to their design, and that tree planting should be combined with other interventions such as green roofs. Bellow we show some thermal images of different green infrastructure elements, to understand how much green infrastructure (trees, roofs, walls) contributes to better and heathier outdoor urban environment.
Picture 2: Impact of UHI on streets of Melbourne with cooler side area due to tree canopy (photo courtesy: City of Melbourne)
Picture 3: Comparison of surface temperatures of a green roof and a concrete roof. Photo curtesy of Heidarinejad and Esmaili
Green roofs are one of the heat-mitigating strategies in cities, which are necessary tool to decrease the heat island effects in urban areas. In this year, the study which is revealing the effect of green roofs on city scale was published by Adilkhanova et al. (2024) in case of city of Seoul. Green roofs are shown as a promising strategy for mitigating urban heat and energy consumption, With the ability to be installed on new buildings and retrofitted, they are a scalable nature-based solution to address the challenges of urban heat. Researcher state: “…Our findings demonstrate the tremendous potential of green roofs to substantially decrease the peak temperature of a city and increase energy savings…” (UNSW, 2024).
Every building is its own story
Energy saving effect of a green roof is a fact which does a great job on city level - the more the green roofs, the higher the impact on UHI reduction and energy savings. But figures on city level do not tell much about the energy saving effect of particular green roof on individual building on a specific location. Wang et al (2024) pointed out different studies which show significant results, such as:
- a non- insulated green roof provides as much as 43,8% cooling energy savings and 40.9% winter energy savings in a Mediterranean climate.
- green roofs can reduce a building's energy use by 30.7% in hot-humid zones and 71% heating load in cold-dry zones
- optimized green roofs could increase 12.8% comfort and reduce 14% energy use.
Summer energy savings occur as a green roof successfully reduces rooftop temperatures for several 10°C compared to conventional roofs (as shown in Picture 3), helping mitigate urban heat islands. Lower rooftop temperatures lead to reduction in building energy use during the summer, cutting peak electricity demand (important in heavy burdened grid systems), and thus saving monthly cost per m2 of roof surface.
During winter time, green roofs can retain significant amount of precipitation, acting as additional insulation layer. By reducing heat loss, green roofs cut the burden of heating fuel costs during winter months. The green roof decreases the thermal loss of the roof as additional insulation and protects the surface against overcooling.
In summary, green roofs offer year-round benefits, making them an advantageous choice for energy-efficient buildings.
How much does the green roof shape the energy efficiency of an individual building?
The actual energy savings are determined by important factors, such as: climate zone, building envelope characteristics, height of the building and slope of the roof, characteristics of chosen green roof system, irrigation usage, in situ micro location and micro climate conditions.
If you would like to understand how will the green roof influence your building’s energy saving – in summer as cooling effect and in winter time as thermal insulation, contact our experts who can calculate this for any given location and more – they will show you performance in reducing local heat island effect and stormwater management: https://www.urbanscape-architecture.com/tools-documents/#pet_tool
Literature:
Adilkhanova et al., 2024. Green roofs save energy in cities and fight regional climate change, Nature cities 1, 238-249.
Iungman T. et al, 2023. Cooling cities through urban green infrastructure: a health impact assessment of European cities. The Lancet, 2023.
Heidarinejad and Esmaili, 2015. Numerical simulation of the dual effect of green roof thermal performance. Energy Conversion and Management, 2015.
UNSW, 2024. Green roofs can cool cities and save energy: modelling. https://www.unsw.edu.au/newsroom/news/2024/02/green-roofs-can-cool-cities-and-save-energy-modelling
Wang M., et al., 2024. Unlock city scale energy saving and peak load shaving potential of green roofs by GIS – informed urban building energy modelling. Applied energy vol 366, 2024.
