With The European Heatwave dominating the news, we ask how can green roofs mitigate the Urban Heat Island Effect?
Green Roofs and the Urban Heat Island effect
Watching the news of the European heatwave sweep across Italy, Spain and Greece, bringing temperatures that are expected to exceed 48 C – we consider how to make cities habitable during such temperature extremities, which are becoming ever more frequent.
Cities struggle even more in these extreme temperatures and green roofs offer one part of the solution to make cities more liveable as temperatures rise.
What is the Urban Heat Island?
The urban heat island (UHI) is a phenomenon where urban areas experience higher temperatures compared to their surrounding rural areas. This effect is primarily caused by human activities and the built environment, which alter the natural land cover and surface characteristics.
Here’s how the urban heat island effect occurs:
- Built Environment: Urban areas have high concentrations of buildings, roads, pavements, and other structures that absorb and retain heat from the sun during the day.
- Reduced Vegetation: Urbanization often leads to the removal of green spaces and vegetation, such as trees and grass, which provide shading and cooling through a process called evapotranspiration.
- Heat Generation: Human activities, industries, and vehicles in urban areas produce waste heat, contributing to the overall heat load.
- Albedo Effect: Urban surfaces, like concrete and asphalt, have lower albedo (reflectivity) compared to natural surfaces, leading to more absorption of solar radiation and, consequently, higher temperatures.
The urban heat island effect can have several impacts:
- Temperature: Urban areas can be several degrees Celsius warmer than surrounding rural areas, especially during the night (nighttime UHI) when the heat accumulated during the day is slowly released.
- Energy Consumption: Increased temperatures lead to higher energy demands for cooling buildings and indoor spaces.
- Air Quality: The UHI effect can also exacerbate air pollution, as higher temperatures can lead to increased formation of ground-level ozone and other pollutants.
- Public Health: Higher temperatures in urban areas can pose health risks, especially during heatwaves, potentially leading to heat-related illnesses and increased stress on vulnerable populations.
How can Green Roofs Mitigate the Urban Heat Island
Green roofs, also known as living roofs or vegetated roofs, are an effective way to mitigate the urban heat island effect, which refers to the increased temperature in urban areas compared to their rural surroundings.
- Reduced Heat Absorption: Traditional rooftops, especially those made of dark materials like asphalt, concrete, or metal, absorb a significant amount of solar radiation, converting it into heat. In contrast, green roofs have vegetation that absorbs some of this heat and uses it for processes like photosynthesis, reducing the amount of heat that is transferred into the building below.
- Evapotranspiration: Plants on green roofs release water vapor through a process called transpiration, and this moisture evaporates into the surrounding air. Evapotranspiration has a cooling effect, similar to how sweating cools our bodies. By releasing water vapor, green roofs help to cool down the ambient temperature in the immediate area.
- Thermal Insulation: The vegetation, soil, and other layers of a green roof act as insulation for the building, reducing the amount of heat that penetrates through the roof into the interior spaces. This can lead to reduced energy consumption for cooling during hot weather.
- Shading Effect: The plants on green roofs provide shade to the building’s surface, reducing the direct exposure to sunlight. This shading effect helps in lowering the temperature of the rooftop and the surrounding environment.
- Reduction of Heat Re-radiation: Traditional roofs, especially those made of dark materials, can re-radiate absorbed heat into the surroundings during the evening and nighttime, contributing to higher nocturnal temperatures. Green roofs, with their reduced heat absorption, re-radiate less heat, thus reducing nighttime temperatures.
- Air Quality Improvement: Green roofs can absorb and filter pollutants from the air, leading to cleaner and cooler air in the urban environment. This can have a positive impact on local air quality and the overall health and well-being of city residents.
- Stormwater Management: Green roofs can retain rainwater, preventing it from quickly running off into stormwater drains. This helps to moderate the urban heat island effect because the evaporation of stored water further cools the roof and surrounding air.
These traditional benefits of green roofs are further influenced by the green roof system selected and the volume of plant content.
Add Bio-solar to the equation and you get a ‘double-bubble’ cooling benefits AND clean energy.
Green Roof Study Sydney
‘Like an iPhone that switches off when left in the heat, the output of solar panels slows down over 25 degrees by 0.5 percentage points for every degree rise in temperature, it says.
Sydney scientists have proven that a green roof with plants and solar panels can mitigate that impact – and cool the building and the area, and reduce the need for air-conditioning in the heat of the day.
Most rooftops in Sydney exceed 50 degrees on a hot day, said Dr Peter Irga, an atmospheric scientist in the University of Technology Sydney’s Faculty of Engineering. “Solar works better on a rooftop in Scotland or Wales than it does in Sydney,” he said.
But a study by Irga and his colleagues found a bio-solar green roof (BSGR) – where plants were located next to solar panels – cooled the surface of the roof by as much as 20 degrees, reduced ambient heat, and increased solar output by an average of 4.5 per cent.’
Green roof applications which are a mix of green/blue and biosolar are therefore a magic bullet in the battle against the urban heat island.
Skilled with the installation of all roof types, the Pritchard and Pritchard team can support construction specialists and property developers with achieving optimal green roof solutions.
