•We monitored a high insulted real-scale green roof in a Mediterranean climate.•It was monitored during three summers characterized by diverse vegetation density.•Results show that high vegetation density green roof is a passive cooling system.•It reduces thermal load of about 60% compared with the roof with no vegetation.•A roofing resistance numerical model was developed and experimentally validated. This paper presents the results of an experimental study carried out on an extensive green roof situated in a Mediterranean coastal climate zone. The aim of the study is to analyze the thermal energy behavior of a green roof during the summer so as to evaluate the effect of vegetation density on the energy performance of the roof and to identify the characteristics of the plants and substrate that have the greatest impact. The paper describes the results of monitoring carried out during the summer in 2010, 2011 and 2012, the development of a numerical model for calculating the thermal resistance of the substrate and the vegetation and the procedure for validating the model using the experimental data. The results show that a green roof which has high vegetation density acts as a passive cooling system when the roof is highly insulated (U value=0.24W/(m2K)) and that in these conditions the incoming thermal gain is about 60% lower than when the roof has no vegetation.