Effects of green walls on outdoor human thermal environment: Evidence from Czech cities

Abstract

Green walls, facades, or vertical gardens are gaining prominence as nature-based climate change adaptation measures in urban areas. Despite their growing popularity, empirical evidence regarding the effects of these systems on the outdoor human thermal environment in moderate-climate cities remains limited. We conducted micrometeorological measurements and applied biometeorological indices to quantify the impact of green walls on outdoor thermal exposure and thermal comfort in five urban sites in Czech cities. Our results demonstrate that green walls can reduce heat stress during hot summer days, however, their effect under European moderate climate conditions is modest. Observed differences in thermal conditions at a 50 cm distance from greened versus conventional facades were: ≈ –1.2 to +0.1 K for air temperature (Ta), ≈ –6.4 to +3.3 K for Mean Radiant Temperature (MRT), ≈ –2.1 to +0.2 K for the Universal Thermal Climate Index (UTCI), and ≈ –4.1 to +0.2 K for the Physiologically Equivalent Temperature (PET). Direct greening systems without any supporting structure exhibited substantially stronger cooling effects than green living wall systems utilising black plant pots. Our findings emphasise that the effectiveness of these widely used adaptation measures is highly system- dependent and underscore the need for comprehensive assessments across varied urban environments, particularly incorporating detailed radiation flux measurements.