Impact of vertical greening on urban microclimate and historic building materials: A meta-analysis

An urban environment is subject to elevated temperatures and higher pollution levels compared to less densely populated areas. Implementing green infrastructures, such as vertical greening, is one method to mitigate this effect. Vertical greening is especially suitable for built heritage in city centres due to the limited space required for plant growth, while still providing substantial green surface area. However, built heritage is often excluded from mitigation strategies due to the unknown potential risks of vertical greening on the degradation of historic building materials. This paper provides a meta-analysis of the literature to establish a current understanding of how the introduction of vertical greening affects microclimates near the surfaces of built heritage and associates those changes with common degradation mechanisms of historic building materials including salt crystallization, freeze-thaw weathering, biodeterioration and chemical weathering resulting from pollutant dispersion. Vertical greening can reduce the fluctuations of surface temperature, air temperature, relative humidity and the amount of solar irradiation and particulate matter on a wall, which is likely to reduce the risk of most common degradation mechanisms in historic building materials induced by salts and frost. Even though degradation induced by particulate matter and bio-activity has received less attention in the literature, our analysis suggests that these factors can also be influenced by vertical greening. The risk of chemical degradation appears to decrease with vertical greening while bioactivity may increase.