Thermal comfort, summertime temperatures and overheating in prefabricated timber housing

Timber is increasingly used in construction of buildings due to its green credentials and ability to reduce the overall construction period when compared with conventional materials. However, the lack of thermal mass along with the low U-values can be a risk factor in increasing overheating. This paper investigates the indoor thermal conditions and overheating risk in prefabricated timber buildings focusing on two buildings built in the last decade in the UK, Oxley Woods and Bridport. The study employs a combination of different methods: post-occupancy evaluation, thermal comfort surveys, monitoring and simulation. The results reveal high satisfaction rates in both buildings, with lower thermal sensation in Oxley Woods where monitored internal temperatures were higher, demonstrating higher adaptive capacity due to the increased use of controls. Overheating analysis through the use of the CIBSE comfort model revealed extreme summertime overheating in 67% of the spaces during the monitoring periods, while for the simulations in just 22% of the spaces. With the adaptive thermal comfort model (BSEN15251) overheating is more frequent at Oxley Woods with cold discomfort also becoming an issue in both buildings. Comparison of the two comfort models suggests that the CIBSE model is more sensitive predicting extreme occurrence of overheating, while the adaptive BSEN15251 model is closer to the results of the thermal comfort evaluations, with availability of controls enhancing adaptation further. Comparing the findings with those from previous studies, which were mostly built with heavyweight materials, indicate that high temperatures were more frequent in the current study, highlighting that the lack of thermal mass in prefabricated timber developments increases the overheating risk, even in mild summer weather conditions as occurring in the UK.