Impact of plants transpiration, grey and clean water irrigation on the thermal resistance of green roofs

Evapotranspiration is one of the important processes in green roof heat and mass transfers. It is usually given in the literature by direct measurement or developed models to estimate both transpiration of plants and evaporation of soil at same time. In this paper, transpiration and evaporation were studied and evaluated separately. The thermal performance of green roof was evaluated in controlled weather conditions using a low-speed wind tunnel and the plants transpiration was measured in term of green roof thermal resistance. Green roof samples with two types of plants were tested. The results showed that plants evapotranspiration represented about 13% of the thermal resistance for ryegrass and about 37% of the thermal resistance for periwinkle. As an application, the impact of grey water on evapotranspiration and green roof thermal resistance was studied using the same laboratory facility. Green roof samples were tested in side-by-side studies using either grey water or clean water for long-term irrigation. Synthetic grey water of repeatable composition was used. Results showed visible effects of grey water on periwinkle but not on ryegrass. The thermal performance for green roofs irrigated with grey water was about 30% lower than those irrigated with clean water.