Abstract The energy statistics of OECD Countries show that between 30 and 50% of primary energy is consumed in non-industrial buildings (i.e. in dwellings, offices, hospitals, schools etc.) Of this, as much as 50% is dissipated from the building in the departing air stream. As buildings become more thermally efficient, the proportion of energy loss (either heating or cooling losses) associated with ventilation and air infiltration is expected to become the dominant thermal loss mechanism. Additional losses may be associated with the energy needed to operate mechanical ventilation systems. It is, therefore, essential to understand the role that air change plays in contributing to energy loss, and to identify methods to improve the energy efficiency of ventilation. Much of this area of activity is still ongoing, but the purpose of this paper is to present an overview of the current level of understanding and summarise the results of recent analytical work. Taking the building stock as a whole, the present analysis shows that airchange accounts for approximately 36% of total space conditioning energy and contributes to almost half of heating equipment losses. If fresh air supply was controlled to meet current recommended ventilation rates for health, then it is estimated that ventilation heat energy could be reduced to a quarter of its present value.