Better understanding is needed of building thermal performance as a mediator between climate and health outcomes. There is concern that current dwelling energy conservation strategies that focus on keeping warm in winter may lead to overheating and heat stress in summer or an increased use of cooling energy. Drawing on public health methodologies to predict heat related health outcomes, this study has standardised three-day averaged mean indoor to outdoor temperatures from 107 homes in Melbourne, Australia, to test the influence of the residential energy efficiency rating on the living room temperatures in summer. At the heat wave threshold of 25 degrees C, on average, better rated 6-Star homes were 0.89 degrees C warmer than 4- or 5-Star rated homes. At this reference temperature, air-conditioned 6-Star homes used 15.84 kWh/day electric cooling energy more to achieve the same living room temperature as 3-Star rated homes. The findings confirm the results of simulation studies that found increased fabric insulation may be associated with increased summer indoor temperatures, risk of heat stress and cooling energy in a mild temperate climate. Hence, it is recommended that residential thermal performance ratings should evaluate the dwelling's performance for each season independently and that cooling through natural ventilation and shading be promoted. (C) 2015 Elsevier B.V. All rights reserved.