Assessing green walls' effects on outdoor human thermal exposure in temperate climate cities

Green walls or vertical gardens are becoming increasingly popular urban heat mitigation measures. However, empirical evidence regarding their efficiency in a temperate European climate remains scarce. In this study, micrometeorological measurements and established biometeorological indices were utilised to investigate the impact of green walls on outdoor thermal exposure and thermal comfort in five case studies in Czech cities. The results indicate that during hot days, green walls can contribute to reducing outdoor heat stress; however, the effect under the European temperate climate is limited. The magnitudes of changes in comparison with conventional facades 50 cm from the walls during hot summer days were: ≈ - 1.2 to + 0.1 K for air temperature, ≈- 6.4 to + 3.3 K for Mean Radiant Temperature, ≈ - 2.1 to + 0.2 K for Universal Thermal Climate Index, and ≈ - 4.1 to + 0.2 K in the case of Physiologically Equivalent Temperature. The cooling effect is primarily observable in direct greening systems without any supporting structure, rather than in green living wall systems utilising black plant pots. Consequently, the sufficient effectiveness of one of the widely used climate change adaptation measures could not be generally confirmed, suggesting that further investigations in diverse settings should be considered.