Approximately 30% of the energy delivered to buildings is dissipated in the departing ventilation and exfiltration air streams. In buildings constructed to very high Standards of thermal insulation, the proportion of airborne energy loss can be much higher. This loss has important implications both at the consumer level, where the cost must be met, and at the strategic level, where it impacts on primary energy need and environmental pollution. The amount of energy consumed is dependent on the flow rate of ventilation and the amount of conditioning of the air that is necessary to achieve thermal comfort. Additional energy is needed to drive mechanical ventilation systems, cool air by refrigeration or evaporation and maintain acceptable humidity levels.