Implications of green façades on historic building materials

Cities are increasingly implementing nature-based solutions (NbS) to mitigate current climate stressors in urban environments, e.g. elevated temperatures and air pollutions levels. Among NbS, ground-based green façades are well-known for their ability to cover a large surface area of vegetation while using minimal ground space. This green initiative consists of climbing plants growing along a vertical surface by either attaching themselves to the surface or using a climbing aid.

Despite the proven benefits of green façades, historic buildings, crucial components of urban environments, are often neglected in urban mitigation strategies. Concerns about potential adverse effects of greening on materials durability and structural integrity, which is currently poorly understood, have limited their implementation on historic buildings. Our research aims to understand the impact of green façades on the degradation processes of historic building materials to unlock the co-benefits of greening built heritage and to explore the potential of green façades as a preventive conservation method.

To achieve this, we analyse the interaction of green initiatives and stone-built heritage in both outdoor and controlled environments. Case studies conducted in the historic city centre of Antwerp (Belgium) over the past two years provided valuable insights on how green façades have an impact on the local microclimate. Monitoring a wide range of environmental parameters, relevant for common degradation processes of built heritage, e.g. surface and air temperature, solar irradiation,  moisture content, amount of wind-driven rain and relative humidity, enables an understanding of the mechanisms of green façades responsible for changes in the local microclimate and identifies the key extrinsic and intrinsic factors affecting the effectiveness of this greenery.  

Our findings highlight the potential of green façades to reduce the risk of common degradation processes affecting built heritage. Green façades significantly reduces solar irradiation and moisture accumulation on wall surfaces, thereby lowering the risk of biodeterioration. It also lowers the maximum surface temperatures by providing shade and tempers the relative humidity fluctuations. This buffering effect reduces the likelihood of salt crystallisation, most common during summer. In winter, evergreen vegetation enhances resistance to freeze-thaw cycles by maintaining higher minimum surface temperatures through thermal blanketing and reducing moisture accumulation on wall surfaces.

This research provides valuable insights into the potential of green façades as a sustainable and preventive conservation method for stone-built heritage. By buffering environmental conditions at the wall surface, green façades can unlock co-benefits: improving urban environments while enhancing the durability of historic building materials. A deeper understanding of the behaviour of green façades on the degradation of historic building materials can support their implementation in urban environments, ensuring that stone-built heritage becomes more resilient to current and future climate challenges.