Green walls for solar shading: a novel thermal performance model applied to a case study

The increasing global focus on green buildings and green façades reflects the growing need to reduce energy consumption and enhance the environmental sustainability of buildings. European guidelines, particularly those from the European Parliament’s Energy Performance of Buildings Directive (EPBD), emphasize improving the efficiency of the building stock to achieve near-zero or net-zero energy goals. A key strategy in this effort is the renovation of existing buildings using green surfaces, such as green roofs and façades, though widespread implementation has yet to be fully realized.

This study investigates the thermal performance of green walls as a means of solar radiation shading during the summer, using a case study of a vertical green wall on a building in Casaccia, Rome. A “black-box” modeling approach was employed, leveraging input-output data mapping to offer advantages such as lower engineering costs, reduced need for domain-specific knowledge, and adaptability as the model evolves with new data. The model, developed with MATLAB and Simulink, was validated with data from the green wall installed on the Casaccia building.

The dynamic energy behavior of the building during summer, when vegetation is most effective at limiting solar radiation, was simulated using an electro-thermal analogy in the model. Multi-layered walls were represented by resistors and capacitors to simulate thermal resistance and capacitance. The mathematical model accurately predicted heat flow through the building envelope, validated by sensor data from various points on the building’s green layer, façade, and internal environment.

The model was applied to study the thermal behavior of five common wall types, both with and without green coverage, under typical summer conditions in 2022. Key parameters, including the green factor (Kv), green attenuation factor (Fa_v), green phase shift (t_v), and periodic green transmission (YIE_v), were used to evaluate the efficiency of the green wall. The results demonstrated that green walls can substantially decrease heat flow into a building, thereby improving energy efficiency. Specifically, the green factor (Kv), which represents the ratio of the total heat flux entering and exiting the internal wall over a 24-hour period with and without the green wall, confirms that green walls can dramatically reduce heat transfer into the building. These findings also provide valuable reduction coefficients that can be used in early design stages, offering designers a practical tool for improving building performance.