Uncontrolled airflow (i.e. leakage) through the envelope can be the major source of heat loss and can lead to other problems. Since warm air can carry large amounts of water vapour, airflow is also the main means by which moisture is carried into the envelope.

Under winter conditions, inside air is forced out through the building envelope, carrying heat and moisture, while incoming replacement air brings drafts and dry winter air. For air to move from one side of the building envelope to the other, there must be holes in the envelope and a difference in air pressure between the inside and outside. The difference in air pressure can result from any combination of wind, a temperature difference creating a stack effect in the home, and combustion appliances or exhaust fans.

• Wind effect: When wind blows against the house, it creates a high-pressure area on the windward side and forces air into the house. There is a low-pressure area on the downwind side (and sometimes other sides) where inside air is forced out.
• Stack effect: In a heated home, less dense warm air rises and expands, creating a higher-pressure area near the top of the house. Air escapes through holes in the ceiling and cracks in the walls and around upper-storey windows. The force of the rising air creates lower pressure near the bottom of the house, which draws outside air in through cracks and openings (e.g. basement windows and rim joist space). Stack effect increases as a result of higher leakage rates, higher building heights and larger differences between the indoor and outdoor temperatures.
• Combustion and ventilation effect: Fuel-burning appliances, such as those that use wood, oil, natural gas or propane, need air to support combustion and provide the draft in the chimney. Open chimneys and fireplaces exhaust a lot of air, reducing pressure within the house. Because this air must be replaced, outside air is drawn in through leaks in the envelope. (See Section 9.4, Ventilation and combustion air).