Increasing urban green infrastructure (UGI) in a city contributes to improve urban air quality and urban climate in summer, reducing buildings surface temperature, especially in southern Europe. UGI includes green walls and their correct application requires the knowledge of the energy performance of the applied green system. The effects of the green walls application on the building depend on the local climate, water availability, building shape. The presence of green walls affects the building microclimate all day, by reducing heat waves during the warm periods and heat losses from the building in cold periods. The heat and mass transfer between the external environment, the green wall and the building surface determine the building microclimate. Solar radiation, long wave infrared radiation, convective heat transfer and evapotranspiration are the main mechanism of heat transfer in a green wall. The paper describes the main parameters concerning heat flow in green walls that can be used in simulation models for predicting temperatures in buildings using the external weather conditions as model inputs. The green wall system was described by a schematic representation, the following layers were defined: the green wall, the gap air, the external surface of the building wall, the internal surface of the building wall, the air inside the building. The energy balance was defined for each layer and all the terms involved in the energy exchange between the layers were defined.