Long-term evaluation of residential summer thermal comfort: Measured vs. perceived thermal conditions in nZEB houses in Wallonia

In a context of global warming, summer thermal comfort is a key issue for the design and operation of nearly zero-energy buildings (nZEBs). Although there are various methods and benchmarks for the long-term evaluation of summer thermal conditions in free-running buildings, their application to the residential sector is still debated. Based on data from post-occupancy measurements and survey campaigns, this paper evaluates the compliance with commonly used overheating criteria defined by existing standards (CIBSE and Passive House Institute) of 23 nZEB houses in Wallonia (Belgium). The quantitative relationship between measured and perceived comfort is statistically analysed, and building characteristics leading to the most critical overheating situations are qualitatively discussed. The results show that summer thermal discomfort is frequent in nZEB houses in Wallonia, despite its temperate climate. In living rooms, the long-term appraisal of thermal conditions appears to be related to the frequency of extreme temperatures, while occupants’ thermal perception in bedrooms seems to be more affected by smaller temperature deviations. These results imply a critical review of the capacity of the criteria commonly used for the evaluation of overheating risks in free-running residential buildings to predict the satisfaction of occupants with their summer thermal comfort. More generally, the findings reinforce the importance of a better integration, even in temperate climates, of environmental controls – and their required operation by the occupants – to properly mitigate overheating risks in nZEB houses.