Experimental investigation of the thermal performances of an extensive green roof in the Mediterranean area

• Experimental analysis of an extensive green roof in Mediterranean climate.
• From 26% to 35% of incident solar radiation lost by evapotranspiration in summer.
• A reduction of 100% of the thermal energy entering the indoor environment in summer.
• A reduction between 30% and 37% of the outgoing thermal energy in winter.
• A minimum decrement factor of 0.135 and a maximum time lag of 4.8 h are obtained.

Nowadays green roofs are a well-consolidated technology used in several countries since these systems can be employed both to generate considerable energy savings and to improve the thermal performances of buildings. This paper deals with an experimental analysis of an extensive green roof, installed on a building of the University of Calabria (Italy, Lat. 39.3° N). The thermal performances of different layering solutions are analysed under typical Mediterranean climate conditions with reference to a traditional roof system. The analysis showed that the green roof is able to reduce the temperature at the interface with the structural roof, on average, by 12 °C with respect to a black bituminous roof in summer and to maintain, on average, a value that is 4 °C higher in winter. The measured temperatures allowed to calculate the transferred heat through the building roof, showing negative heat fluxes for the whole period and a reduction of the thermal energy entering the indoor environment of 100% in summer, proving the passive cooling effect of the green roof, and a reduction between 30% and 37% of the thermal energy exiting the indoor environment in winter. The experimental data have been employed also to conduct a dynamic characterization of the different vegetated solutions, showing that the different green roofs solutions are able to generate values of decrement factor varying between 0.135 and 0.193 and a time lag between 3.1 h and 4.8 h, which can substantially improve the dynamic properties of traditional roof structures, especially in the case of roofs with limited dynamic performances.

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