Overcoming the barriers to green walls in urban areas of the UK

Green infrastructure is seen as a tool to mitigate a host of environmental challenges in urban areas. Vertical greening solutions such as direct greening are gaining popularity due to relatively low cost and the fact that they have a minimal ground footprint. There are still, however, a range of barriers to their uptake, including worries about potential wall damage (physically and via RH increase). This research had sponsors from multiple disciplines and as such covers a wide range of topics aimed at reducing barriers to installations of direct greening. The impact of several popular and widely-used plant species (Hedera helix (English ivy), Parthenocissus tricuspidata (Boston creeper), and Pileostegia viburnoides (climbing hydrangea)), on the internal/external temperature and relative humidity (RH) of replicated experimental model ‘buildings’ (three per plant species, plus bare buildings) was studied over two summers and winters. All the plant species reduced both the air temperature internally/externally during the summer daytimes by at least 1 o C (Hedera produced the greatest cooling effect internally and externally, 7.2 o C and 8.3 o C reduction, respectively). All plant species reduced the daily ‘variation’ (morning to afternoon) in external RH, and external and internal temperature during summer (Hedera reduced variation most and Pileostegia least). During night-time, foliage maintained a higher temperature on the external building surface which could reduce the risk of freeze-thaw damage in winter. Additionally, while all plants increased the external RH around the building envelope, that increase should not transfer internally in buildings designed to current standards as the walls include protective layers such as damp-proof membranes. The ‘buildings’ were then modelled using building energy simulation software (IES), which accurately simulated the bare ‘buildings’ in both summer and winter with a weather file containing the data for the local weather. Model validation was also performed for a Hedera helix layer based on data from experiments and literature, which was accurately simulated for both summer and winter conditions (mean standard deviation between the simulated and experimental internal temperatures was ± 2.03 o C for bare buildings, and ± 1.74 o C for Hedera-covered ‘buildings’). The simulation of a Hedera helix layer will allow energy savings to be calculated in the future from the use of direct greening in retrofits and new builds. Experiments on the management of a widely spread, but often misunderstood plant species, Hedera (ivy), were also conducted. Hedera aerial root attachment was prevented or reduced (by at least 30%) by altering wall surfaces through application of copper and zinc sheets and meshes or silanebased anti-graffiti paints. Furthermore, a combination of root restriction and water deficit reduced aerial root number by 88%. This could significantly ease ivy pruning and other management, alleviating the fears around its potential damage to wall surfaces. Finally, a desktop study into the practicalities of increasing direct greening was conducted, whereby an overview of the green wall industry showed that it is growing, as is the understanding and acceptance of the benefits of direct greening in the UK. Both life cycle assessment and cost-benefit analysis show that direct greening is sustainable in the UK. Regarding increasing green wall uptake, there is currently no mandatory policy in place in the UK; however, a combination of non-mandatory policies and new funding streams for green infrastructure are working well and should continue to increase green wall uptake and installations