Window opening behaviour in summer: evidence from urban bedrooms and implications for UK overheating modelling

<p dir="ltr">Occupant behaviour fundamentally influences both indoor environmental quality and energy efficiency in residential buildings. This study investigates summertime window-opening behaviour in 13 non-air-conditioned bedrooms in East London during summer 2024. Continuous monitoring of indoor temperature and window status was paired with outdoor data from a local weather station. Two five‐day periods, a baseline (18–22 June) and a hot spell (29 July–2 August), were compared. Key metrics include the percentage of time windows were open during day and night, total window-opening duration per bedroom, and mean hourly opening duration across all rooms. Indoor and outdoor temperature trends were also compared across the two periods. Observed window behaviours were evaluated against the assumptions of Approved Document O (UK Building Regulation on overheating and ventilation), highlighting discrepancies between actual window opening behaviour and modelling assumptions used in overheating assessment. The findings highlight significant variation in window operation patterns across bedrooms and between the two periods, notably between daytime and nighttime. These results underline the limitations of standardised behavioural assumptions and the need to integrate real occupant behaviour into building simulations. Incorporating such real‐world complexity can enhance thermal comfort, indoor air quality, and the effectiveness of adaptive design strategies in urban apartments.</p>