The Household Energy End-use Project (HEEP) has collected energy and temperaturedata on 397 randomly selected houses throughout New Zealand, providing a statisticallyrepresentative national sample. This database has been used to explore the drivers ofsummer indoor temperatures. The general lack of air-conditioning means that summertemperatures are affected by passive influences (e.g. house design, construction)compared to winter temperatures where space heating is used. Summer temperatures arestrongly influenced by the house age and the local climate together these variablesexplain 69% of the variation in daytime (9 am to 5 pm) living room temperatures. Newerhouses tend to have larger glazing areas, higher levels of thermal insulation and reducedeaves. Preliminary analysis suggests that as newer houses are warmer, climate changemay be more important than in older houses. Human actions such as the opening ofwindows may have a large effect on the maximum temperature.