What's ‘cool’ in the world of green façades? How plant choice influences the cooling properties of green walls

Abstract

Green walls provide an option for reducing the thermal load on buildings, reducing the requirement for mechanised air conditioning and helping to mitigate urban heat islands. The range and extent of benefits can vary with green wall typology. This research investigated green façades utilising wall shrubs and climbing plants to reduce air temperature adjacent to, and surface temperatures of, brick walls. Artificial wall sections were used to provide replicated data sets in both outdoor and controlled environmental conditions. During periods of high solar irradiance outdoors, the presence of live Prunus laurocerasus plants placed against walls significantly reduced air and surface temperatures compared to blank walls, but also in comparison to excised (non-transpiring) plant sections. Largest temperature differentials were recorded mid-late afternoon, where air adjacent to vegetated walls was 3 °C cooler than non-vegetated walls. Prunus also provided significant wall cooling in controlled environment studies, but was intermediate in its surface cooling capacity (6.3 °C) compared to other species; Stachys and Hedera providing >7.0 °C cooling. When evaluated on a per leaf area basis, however, other species demonstrated greater cooling potential with Fuchsia, Jasminum and Lonicera out-performing others. Not only was it evident that different species varied in their cooling capacity, but that the mechanisms for providing wall cooling varied between species. Fuchsia promoted evapo-transpiration cooling, whereas shade cooling was more important in Jasminum and Lonicera. Plant physiology and leaf area/morphology should be considered when selecting species to maximise cooling in green wall applications.