Given the large amount of worldwide energy use associated to buildings' life cycle, in recent years various energy efficient strategies have been proposed to reduce buildings' environmental impact, e.g. green and cool roofs. The purpose of this work is to analyze an innovative type of green roof, named Cool-Green Roof combining the features of both green and cool roofs. In fact, it is characterized by a specific vegetative layer able to optimize the quote of short-wave radiation reflected by the selected vegetation. After the analytical dissertation of the system, the developed solution is studied when applied in a case study building represented by a multifamily XVI century building in central Italy, characterized by cement based roof ceiling needing to be retrofitted. Both in-lab and in-field experimental analyses were carried out for evaluating the building thermal-physics and the solar reflectance and thermal emittance of the selected plant compared to other flat roof materials and greeneries. Additionally, the year-round performance of Cool-Green Roof is assessed through calibrated and validated dynamic thermal-energy simulation. Main findings of the study show how the Cool-Green Roof is able to reduce the number of indoor overheating hours in summer by 98.2%, with negligible penalties in winter, given its high insulation capability, typical of green roof solutions. Therefore, the Cool-Green Roof could be considered as (i) a strategy for roof retrofitting in existing (even historic) buildings, and (ii) a solution for improving urban environment quality. Finally, the Cool-Green Roof could represent a promising mitigation strategy against urban heat island phenomenon, suitable for application even in those dense historical cities where other invasive mitigation techniques are unlikely applicable. (c) 2015 Elsevier Ltd. All rights reserved.