Reducing summer thermal discomfort in historical buildings: comparative application of synthetic indices in Southern and Northern Europe

This study presents an integrated approach to optimize thermal comfort and energy efficiency in historical buildings, aligned with the objectives of the European Energy Performance of Buildings Directive (EPBD) and the vision of net-zero energy buildings. The research work includes a comparative analysis of two historical office buildings – one located in Catania, Italy, and the other in Copenhagen, Denmark – to assess the effectiveness of ventilation and shading strategies under current climatic conditions. Using a methodology that combines dynamic thermal simulations in TRNSYS with adaptive thermal comfort indices – namely Frequency of Thermal Discomfort (FTD), Intensity of Thermal Discomfort (ITD) and Fluctuation of Thermal Discomfort (FD) – the study first identifies those thermal zones that are affected by intense summer overheating. Then, the paper evaluates the effectiveness of passive solutions to minimize thermal discomfort: the solutions include Free Cooling, implemented through natural ventilation strategies, combined with optimized mechanical ventilation systems designed to maximize energy efficiency and occupant thermal comfort. The results demonstrate that the integrated approach, which combines passive and advanced mechanical solutions, substantially improves indoor conditions while preserving the architectural integrity of the building. The highest level of overheating discomfort is observed in the thermal zones at the highest floors, since they do not benefit from the thermal inertia of the ground and are subjected to intense solar irradiation. Thanks to the implemented solutions, including internal shading (curtains), free cooling by nighttime ventilation and air movement through fans, a reduction by about 80-85% in the ITD index was achieved in the most problematic thermal zone in Catania, while a more interesting 95-100% reduction was observed in the most critical thermal zone in Copenhagen. This work contributes to achieving climate-neutral goals and promotes a sustainable vision for integrating historical buildings into future urban energy systems.