This study investigates the impact of housing energy efficiency on summer overheating. Using indoor sensor data, energy performance certificates (EPC), and household surveys, it explores temperature variation among energy efficiency levels and the existence of communal heating. Applying two- sample t-tests, results reveal that energy-efficient homes could “trap” heat for good insulation, while inefficient ones experience fluctuating temperatures in Southwark, London. Moreover, the non- adjustable communal heating deteriorates summer heat. These findings suggest future energy retrofit practices as well as policies to adapt to overheating, by adopting better ventilation systems, adjustable communal heating systems as well as scientific EPC rating systems.