Air always flows from a high to a low-pressure area like water running downhill. Without an effective barrier, air outside a home at a higher pressure will attempt to enter the home. Similarly, inside air at high pressure with reference to the outside will tend to exit the house. Air needs an opening or hole to flow through and a driving force to move it. Leakage into and out of a home depends on the pressure difference and hole size. We need air flow through our buildings but not through our structures.
Unintended holes in the structure or the ductwork allow uncontrolled air leakage and rob a home of its efficiency and healthful environment. Improper air flow can led to adverse effects on the health and safety of the people in the building by promoting mold growth, spreading pollutants, and possibly creating backdrafting of combustion appliances, causing possible carbon dioxide poisoning. It can draw in moist air from outside, or force moist interior air out into the walls, ceilings, and other structural assemblies affecting the house’s structural integrity.