Attenuating heat stress through green roof and green wall retrofit

The process of rapid urbanisation is becoming problematic due to the reduction in, and lack of compensation of, previously vegetated areas. With a combination of green roofs and green walls, adopted on a large scale, it is possible to attenuate the urban heat island effect and internal temperatures in buildings. Tall buildings are becoming a common housing type in many cities, and considering the role of external walls in heat gain, it is expected that the combination of green roofs and green walls have great potential to improve thermal performance. As only 1-2% is added to the total stock of buildings annually, the focus should be on the retrofit of existing buildings to deliver maximum thermal benefits. In the present work lightweight, modular vegetated systems were adopted for roofs and walls. Instead of considering only the temperature influence in heat stress, this research adopted the use of heat index that encompasses the combined effect of temperature and relative humidity. For this purpose, the thermal benefits of green roof and green wall retrofit is evaluated in two small scale experiments, where identical prototypes (vegetated and non-vegetated) are compared using block work and timber framed drywall structures for Rio de Janeiro, Brazil and Sydney, Australia, respectively. The results show a different understanding in heat stress evaluation regarding heat index rather than temperature itself, especially under high levels of relative humidity. This evidence demonstrates green roof and green wall retrofit offer a proven role in heat stress attenuation in residential buildings.