Photo Source: Keeping the heat in, Figure 2-3, pg.11

• Air flow in buildings is complex, time dependent and multi-directional
• A perfectly airtight air barrier system is unlikely to be achieved in practice, so it is also desirable to control the air pressure differences driving the air flow
• A tight building envelope minimizes the amount of air required to provide pressurization. In winter, cold air coming in from the exterior through leakage is heated when entering the conditioned space.The more air that is brought in, the greater the heating load, the greater the operating cost and the greater the drying effect (referred to as over-ventilation)
• In parts of Canada during the summer, outdoor air brought in from the exterior must be dehumidified and cooled
• Air pressure differentials contribute to the transfer of pollutants, radon gas ingress, and spillage and back-drafting of combustion appliances