Can Exterior Green Walls Contribute to a Carbon Neutral Architecture?

A carbon neutral building is achieved when the amount of CO2 emissions is balanced by climate-positive initiatives so that the net carbon footprint over time is zero. Considering their unmatched ability to absorb CO2, planting trees is often viewed as the best carbon offsetting solution. But as cities become denser and the amount of available horizontal space for green areas drastically reduces, architects have been forced to explore other approaches. Therefore, to address these climatic challenges and connect people to nature, exterior green walls have become a rising trend in increasingly vertical cities. Even if there is research to claim that these can positively impact the environment, many question if they can actually contribute to a carbon neutral architecture. Although the answer may be quite complex, there seems to be a consensus: green walls can be effective, but only through good design.

Surprisingly, green walls are a relatively new concept; in fact, the idea was first popularized by French botanist Patrick Blanc in the 1980’s. Hence, this might explain why the design community still hasn’t reached a consensus on their environmental role. Nonetheless, to get a deeper insight on the current debate, below we explore their potential benefits, challenges and critiques from a sustainability standpoint – as well as design decisions that could contribute to more climate-friendly solutions.

What is a green wall?

A green wall is a vertical structure covered with vegetation. Essentially, a living cladding system. Instead of being adorned with climbing plants rooted from the ground like in traditional green facades, these provide a vertical growing surface in the form of modular panels, tray systems or freestanding walls. The greenery is planted in a growing medium, such as packed soil, fiber mats or other substrates, and features an integrated irrigation system.

Environmental advantages associated to exterior green walls  

Of course, an evident benefit of these eye-catching pieces is their biophilic effects and the visual aesthetic value they add to cities, making them greener and nicer places to live in. However, green walls are also linked to sustainability benefits. By using vertical areas that are abundant in cities, they are viewed as a way of maximizing CO2-absorbing greenery without sacrificing valuable ground space. While a tree absorbs around 5.5 kg of CO2 a year, it is claimed that 1 m² of green wall can absorb up to 2 kg, also improving air quality by eliminating harmful toxins.

Like a layer of shading, outdoor living walls can cool down interiors by lowering the temperature of the exterior surface and extend the lifetime of facades by protecting from UV radiation. Simultaneously, as this study shows, they can reduce heat loss by over 30%, just like in the case of One Central Park’s green wall. As a result, vertical vegetation systems save energy by decreasing the amount required to cool or heat a building. Plants also have a powerful ability to lower temperature in cities, reducing urban heat islands that cause air pollution and greenhouse gas emissions. Furthermore, urban greening is known to boost biodiversity. One Central Park, for instance, includes 250 species of native Australian flowers and plants that are known to attract local wildlife.

Lack of consensus regarding sustainability

Despite the environmental benefits linked to green walls, they haven’t been immune to criticism, preventing them from gaining a solid reputation from a sustainability standpoint. Some argue, for example, that the main advantages that horizontal green spaces bring to cities (like shading or soil drainage) are lost by raising them from the ground – in other words, that cities must rather focus on improving or creating more open spaces.

Peter Massini, natural environment policy-maker, suggests that “green walls are a poor cousin to green roofs,” yet he acknowledges the value of more recent attempts. Others argue that living walls should be considered solely as visual elements, since although they reduce temperatures and insulate buildings, their success is limited by climatic conditions and surrounding circumstances that can harm the structure and plants. Another common critique relates to excessive resource consumption – water, materials and energy –, which could eventually counteract environmental advantages. Of course, this can be avoided or minimized with good design, but some state that there is still a high risk, especially in ambitious cases that stretch over several floors.

The role of green walls in the road towards carbon neutrality

Nonetheless, others from the design world assert that green walls are still a good opportunity for vegetation in cities. Even though they certainly cannot replace the benefits of ground-level greenery, it is also true that land is less and approaches like these will be needed in denser urban centers.

 “Wherever we have an opportunity to bring more green, more landscape into the city, we should be ensuring that happens.” – Linda Corkery, national president of the Australian Institute of Landscape Architects

Moreover, these challenges and doubts do not necessarily mean that vertical greenery shouldn’t be applied or encouraged, but rather that it alone is not sufficient to promote greener, healthier cities. After all, green walls must come in hand with other efforts and initiatives, such as green roofs or parks, in order to make a significant collective impact. And, of course, if living walls are incorporated, they should be properly studied and designed – as well as integrate new climate-smart technologies – to be successful in the path towards carbon neutral architecture. With this in mind, below we present some design criteria to consider when looking to maximize sustainability:

• First, study the all the local conditions, such as air currents, sun exposure, temperature and humidity. 
• Choose the system that is most suitable for a successful project, which starts by understanding modular, tray and freestanding solutions.  
• Select adequate plant species to ensure healthy and long-lasting growth. This heavily depends on climate settings – for example, walls may require plants that can survive droughts in places with a lack of rainwater.
• Incorporate efficient water management systems to avoid manual and excessive irrigation. These can be in the form of drip irrigation, tank systems or automatic irrigation.
• Integrate technological innovations to increase efficiency, automation and monitoring of living walls, including air circulation methods and effective growth mediums that can create smart, active and more sustainable structures.
• Proper plant maintenance, such as cleaning, trimming and irrigating is crucial for plants to stay alive.
• Ensure adequate sunlight exposure to avoid the risk of moisture retention or leaves and debris clogging drainage systems.

This article is part of the ArchDaily Topics: The Road to Net Zero Architecture presented by Randers Tegl.

Randers Tegl aims to take responsibility and think sustainable as a part of reaching the goal of Net Zero. Both in terms of how building materials impact the climate and how the materials age, but also with a focus on architecture. That is why Randers Tegl created their sustainable series GREENER, which comes with full documentation in the form of EPD, so it is possible to use the product in technical calculation programs.

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