Green façades are a beneficial design approach using vegetated architectural features on previously unused outdoor vertical surfaces. Theyc an enhance the urban environment, and improve the internal and external thermal performance.
The expansion of urban areas has necessitated the loss of green spaces and exacerbated the heat island effect, which can impact the quality of the surrounding environment, micro-climate, and ecosystems.
There is significant potential to mitigate this effect through green façades, which consume heat energy through evapotranspiration and contribute to the vertical mixing of air.
Green façades integrate plants directly or indirectly along a vertical supporting structure, as opposed to green or living walls constructed in a variety of continuous, modular, and linear systems – but often the terms are used interchangeably.
Green façades consist of materials with minimal heat transfer and embodied energy during the construction and installation phases, and provide thermal insulation. By reducing the heat flux, or the amount of heat that is transferred indoors through hard surfaces, the inner temperature of a building can be reduced substantially.
Numerous studies show greenery used in façades can absorb or reflect 40 to 80 per cent of sun radiation during summer months. A study conducted at the University of WesternAustralia showed that during hot sunny days, the external wall temperatures behind a green façade wereup to 18 degrees Celsius cooler than those behind a shade sail and without a façade, respectively.
The temperatures behind the green façade and shade sail were always cooler than the ambient air, by up to 11 and 6.8 per cent, respectively.
This indicated that the evapotranspirative cooling contributed between 25 and 35 per cent of the overall gap cooling induced by the green façade.
The study explained the thermal benefits provided by the green façades indicated that they could contribute to building sustainability, and saw an effective naturebased reduction in individual building energy use and city-scale urban heat.
It said: “The findings also demonstrated that under regular and limited-watering settings, Australian Waterwise plants could transpire more (both evapotranspiration rate and diurnal evapotranspiration)and produce greater evapotranspirative cooling.”
More recently, a study published in the journal Energies last year suggested it had become a consensus that the integration of plantations and greenery systems for building envelopes provided a sustainable solution and addressed unsustainable energy consumption.
The researchers said: “The presence of plantations on green roofs plays a pivotal role in altering local microclimates, primarily through evapotranspiration.
“This natural mechanism significantly contributes to the humidification and cooling of the ambient air in urban environments, effectively mitigating the urban heat island effect.”Their analysis of the interaction between incoming solar radiation and the components of a green system revealed a complex energy exchange process, with about27 per cent of the incoming solar radiation reflected into the atmosphere by the vegetative systems.
Meanwhile, the vegetation and substrate layer collectively absorbed about 60 per cent of the radiation.The remaining 13 per cent penetrated through the substrate layer, contributing to a lesser extent to the thermal dynamics of the system.
The researchers added: “This elaborate balance of reflection, absorption, and transmission results in a significant reduction in solar heat gain, estimated to range between 70 and 90 per cent during the summer months.
“Concurrently, during the winter period, the green roof system effectively decreases heat loss from the building envelope by approximately 10 to 30 per cent.”
Along with a much lower cost than other thermal cooling solutions such as air-conditioning, green façades improve air quality through car exhaust absorption; reduce noise pollution; reduce symptoms like cough, fatigue, and dry or itchy skin; and even improve the recovery rate for patients with various physical and mental ailments.
Green façades and green areas also provide opportunities for social interaction and help overcome the loss of collective memory, image, and identity.
Despite the benefits, widespread adoption has faced several challenges, including maintenance difficulties, design complexities, and high costs. Dr Ozgur Gocer, a Senior Lecturer in SustainableDesign at the University of Sydney, explained green façade systems require specialised care, particularly in
Australia’s climate, which faces extreme heat and drought.She said: “Resilient plant species and smart irrigation systems are critical for the success of living green walls, and due to this vast climatic diversity, custom-tailored solutions are essential to meet the specific environmental and operational requirements of each climate zone.”
A study published this year in Urban Science evaluated the impact of green walls and roof applications on energy consumption and thermal comfort in an education building in Ankara, Turkey, which is in the Csb climate zone (also known as the warm-summer Mediterranean climate zone). Notably, the researchers used a validated building energy simulation model to assess the effects of 21different plant species to determine their potential to reduce energy demand and improve indoor thermal conditions.
The findings indicated that green walls and roofs could significantly enhance building performance, with energy consumption reductions of up to 9.21 per cent and thermal discomfort hours decreasing by 23.24 per cent, depending on the plant selection.
With its detailed analysis of plant species, the study suggested that plant selection is crucial in optimising energy efficiency and thermal comfort, particularly intemperate climates.
They also highlighted the seasonal performance variations of green systems, showing that while they were highly effective in summer for cooling, their insulating effects in winter required further investigation to avoid potential increases in heating loads.
