Urban green infrastructures contribute to increase sustainability in cities. Among green infrastructures, vertical green systems applied on buildings envelope are promising solutions. Green façades, applied to building walls, allow a sustainable passive climate control in buildings. Energy modelling of green façade systems allows their effective design, simulation and application. Upstream of the modelling there is the mathematical description of the energy transfer that takes place in the system. Radiative heat exchanges in the long-wave infrared range is one of the mechanisms to be considered. This implies the knowledge of geometric configuration factors, surfaces radiometric properties and sky radiation models. This paper focuses on the infrared exchanges occurring on the external surfaces of a green façade, compared with those of a bare wall. A mathematical procedure was followed to define the configuration factor between ground and vertical surface. Nine different sky radiation models were applied to find out, through statistical indices, the best fitting one. The analytical study was supported by empirical data, gathered on an experimental green façade built at the University of Bari. Calculations of the infrared fluxes were made for a summer, an autumn and a winter period, both daytime and night-time. It resulted that the sky model of Walton provided the best agreement with the measured data. In winter, the covered wall showed a reduction of long-wave infrared radiative energy losses equal to 79% compared to the bare wall, so the green façade acted as a thermal barrier.
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