In Ireland, the Passive House standard is gaining popularity. As of 2024 report indicates 45 Passive House-certified homes have been completed nationwide, reflecting a significant rise in demand for energy-efficient construction (PHAI Homepage. https://phai.ie/ [1]). While passive house standards are designed to optimise energy efficiency and indoor comfort, performance can vary depending on occupancy lifestyle, habits, and how the residents live. Factors such as heating preferences, ventilation management, and energy consumption habits can influence the building’s energy use and overall performance. This paper explores how the performance of Passive House buildings can differ based on occupant behaviour and habits, highlighting the gap between theoretical design principles and real-world usage. Operational data was collected from two social housing units in County Monaghan, Ireland, built to Passivhaus standards—one occupied home and one unoccupied home acting as a control. Data collection spanned a minimum of one year, gathering environmental performance metrics and energy consumption data. By comparing occupied and unoccupied Passive Homes, this study examines the impact of human behaviour and its effect on these low-energy buildings, offering insights into the importance of occupant engagement in achieving the desired sustainability outcomes. This study contributes to the growing body of knowledge on low-energy buildings and emphasises the critical role of energy-efficient standards in achieving carbon neutrality within the domestic sector.