The impact of green facades and vegetative cover on the temperature and relative humidity within model buildings

Vertical greening solutions such as direct greening are gaining popularity due to their relatively low cost and minimal ground footprint. However, concerns about increases in relative humidity (RH) can reduce implementation. The impact of several widely-used plant species (Hedera helix, Parthenocissus tricuspidata and Pileostegia viburnoides) on the internal/external temperature and RH on the south-facing wall of replicated experimental model 'buildings' was studied during summer and winter. All plant species reduced the air temperature internally/externally during the summer daytimes by at least 1 degrees C compared to bare 'buildings'. Hedera produced the greatest cooling effect internally and externally, by 7.2 degrees C and 5.7 degrees C respectively. All plant species reduced daily variation in external RH and external/internal temperature during summer; Hedera reduced variation most and Pileostegia least. During night-time in both seasons, the temperature behind Hedera foliage typically remained higher, which could reduce the risk of freeze-thaw damage in winter. The RH was not significantly elevated by vegetation except during warm afternoons in summer and winter. During warm afternoons, the external RH was significantly higher only behind the Hedera foliage compared to the bare `buildings', 11% and 3.7%, summer and winter respectively. However, inside all vegetated `buildings' the RH was at least 11% higher compared to the bare `buildings' during summer. Yet, in winter the internal RH was 5.7% lower in the Hedera-covered compared to the bare 'buildings'. Current building standards would prevent the transfer of external RH in this range internally, as the walls include protective layers such as damppro of membranes.