Do green roofs cool the air?

Publication date: Available online 2 November 2016
Source:Building and Environment
Author(s): Anna Solcerova, Frans van de Ven, Mengyu Wang, Michiel Rijsdijk, Nick van de Giesen
Rapid urbanization and an increasing number and duration of heat waves poses a need to mitigate extremely high temperatures. One of the repeatedly suggested measures to moderate the so called urban heat island are green roofs. This study investigates several extensive sedum-covered green roofs in Utrecht (NL) and their effect on air temperature right above the roof surface. The air temperature was measured 15 and 30 cm above the roof surface and also in the substrate. We show that under normal conditions, air above the green roof, compared to the white gravel roof, is colder at night and warmer during the day. This suggests that green roofs might help decrease air temperatures at night, when the urban heat island is strongest, but possibly contribute to high daytime temperatures. The average 24 h effect of sedum-covered green roof was a 0.2 °C increase of air the temperature. Under extremely dry conditions the green roof exhibits behavior similar to conventional white gravel roof. Interestingly, the pattern of soil temperature remains almost the same for both dry and well-prospering green roofs, colder during the day and warmer at night.