NC State Extension

Infiltration (Also Called Air Leakage) – Defined

The unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. In the summer, infiltration can bring humid outdoor air into the building. Whenever there is infiltration, there is corresponding exfiltration elsewhere in the building. In the winter, this can result in warm, moist indoor air moving into cold envelope cavities. In either case, condensation can occur in the structure, resulting in mold or rot. Infiltration is caused by wind, stack effect, and mechanical equipment in the building. (more details below)

Air leakage is sometimes called infiltration, which is the unintentional or accidental
introduction of outside air into a building, typically through cracks in the building envelope
and through use of doors for passage. In the summer, infiltration can bring humid outdoor
air into the building. Whenever there is infiltration, there is corresponding exfiltration
elsewhere in the building. In the winter, this can result in warm, moist indoor air moving into
cold envelope cavities. In either case, condensation can occur in the structure, resulting in
mold or rot. Infiltration is caused by wind, stack effect, and mechanical equipment in the
building (see Figure 1 below). Wind creates a positive pressure on the windward face and negative pressure on the non-windward (leeward) facing walls, which pulls the air out of the building. Wind causes infiltration on one side of a building and exfiltration on the other. Wind effects can vary by surrounding terrain, shrubs, and trees.

The “stack effect” is when warm air moves upward in a building. This happens in summer and winter, but is most pronounced in the winter because indoor-outdoor temperature differences are the greatest. Warm air rises because it’s lighter than cold air. So when indoor air is warmer than the outdoor air, it escapes out of the upper levels of the building, through open windows, ventilation openings, or penetrations and cracks in the building envelope. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through open doors, windows, or other openings. The stack effect basically causes air infiltration on the lower portion of a building and exfiltration on the upper part. Mechanical equipment such as fans and blowers causes the movement of air within buildings and through enclosures, which can generate pressure differences. If more air is exhausted from a building than is supplied, a net negative pressure is generated, which can induce unwanted airflow through the building envelope. Bathroom exhaust fans, clothes dryers, built-in vacuum cleaners, dust collection systems, and range hoods
all exhaust air from a building. This creates a negative pressure inside the building. If the enclosure is airtight or the exhaust flow rate high, large negative pressures can be generated.

InfiltrationFigure 1: Examples of infiltration. Image courtesy: Building Science Corporation, www.buildingscience.com

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