Abstract New and retrofitted green roofs and green walls (GRGW) represent an opportunity to attenuate excessive heat produced in increasingly densely developed urban environments. This paper reports on the results of an original experiment in Sydney Australia in 2016 to evaluate seasonally the heat stress attenuation through green roofs and green walls. Data was collected from mid-summer 2016 (January) to early summer (November 2016) the following season. Two scaled-down structures representing a considerable percentage of housing stock were used to compare heat attenuation in a traditional design compared to a structure covered with a lightweight GRGW on two elevations. Importantly, the results inform our knowledge and understanding of the fluctuations in GRGW performance over an extended period. The combination of relative humidity and temperature plays an important role in establishing heat stress levels in terms of Wet Bulb Globe Temperature (WBGT). The higher levels of WBGT occurred in summer, whereas the lower levels occurred in winter. The WBGT of both prototypes was similar during winter, showing no significant relevance of GRGW for heating an indoor environment. However, during the summer the vegetation had a major role in reducing WBGT. Heat stress is seasonally evaluated according to the percentage of the time, which the thresholds for different metabolic activities are reached. During warm conditions, GRGW attenuated heat stress and the associated health-related risks substantially.