A Roof of Greenery, but a Sky of Unexplored Relations—Meta-Analysis of Factors and Properties That Affect Green Roof Hydrological and Thermal Performances

Abstract

Green roofs are expected to contribute to the mitigation of multiple environmental issues that affect urban areas. Owing to their composition, organization, and external factors, the performances of green roofs have been demonstrated to be overall positive but strongly variable. Our work first aims at proposing consensual definitions and a frame adapted to these biotic-abiotic systems. It also aims at shedding light on the qualitative relations between various internal properties and external factors of green roofs on their hydrological and thermal performances. One hundred relevant study papers were filtered from 395 papers as per our defined search criteria based on originality and precision. The expectations were to be capable of hierarchizing factors and properties that would influence the performances of green roofs. The main findings highlighted that most factors and properties have a positive influence on the performances of green roofs, showing there are many existing levers to enhance the green roof performances and tackle some of the main urban environmental issues. However, even if previous research has already explored various relations, in the final filtered consideration of 6 performances and 30 factors and properties, there was a possibility of 180 combined factor–property–performance relations studies overall. Out of these possibilities, only 82 have been studied at least once, leaving the other 98 relations (54%) unexplored. Considering that these lists were far from exhaustive, a huge potential in determining green roof performances remains unearthed. In this regard, various proposals have been made regarding: (i) identification of levers to enhance the performances of green roofs; (ii) filling the gaps: the exploration of the unstudied relations; (iii) promotion of deeper and innovative experimental approaches for research on green roof performances; and (iv) the shift from mono to transdisciplinary research about green roofs.