Resilience and adaptability of historical buildings to climate change in a Scandinavian context: Synthetic indices for assessing summer thermal comfort

This article aims to assess summer thermal comfort in historical buildings through a methodology that uses synthetic comfort indices to integrate thermal modelling and future climate predictions. The approach includes detailed modelling in TRNSYS validated through on-site measurement of indoor air temperature and relative humidity. Three indices (Frequency (FTD), Intensity (ITD) and Fluctuation (FD) of Thermal Discomfort) are introduced to quantify overheating during occupancy hours, providing a framework suitable for both non-adaptive and adaptive comfort theories, in line with major thermal comfort standards. The methodology is applied to a 17 th century office building near the Rosenborg Castle in Copenhagen. Indeed, in temperate climate zones such as Denmark the design of building retrofit strategies focus more on improving energy performance in the cold winter season, often neglecting the potential impact of summer overheating, which is likely to worsen in the future due to climate change. The results highlight significant differences in thermal comfort performance under present and future climatic conditions. For example, ITD values reach up to 1500°C·h under current climate conditions, but they double when projecting climate data to 2050 under high-emission scenario, i.e. Representative Concentration Pathway (RCP) 8.5. However, by applying passive mitigation strategies - e.g. internal shading devices, ceiling fans with moderate air movement during occupancy hours and “ Free-cooling ” through nighttime ventilation - ITD values drop below 100°C·h, and they even become null in most zones. In a few thermal zones, ITD values remain above 100°C·h but never exceed 300°C·h, even under the most severe future scenarios. The approach offers a replicable framework to identify critical thermal zones and to evaluate retrofit options in historical buildings. • Synthetic indices quantify summer overheating in heritage buildings. • Adaptive comfort theory guides passive retrofit design decisions. • TRNSYS simulations show climate change doubles overheating intensity. • Passive strategies cut thermal discomfort by over 90% in key zones. • The method supports multi-objective optimization for future retrofits.