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Air Leakage – Defined

WHAT IS AIR LEAKAGE? (sometimes called infiltration)

Ventilation is fresh air that enters a house in a controlled manner to exhaust excess moisture and reduce odors and stuffiness. Air leakage, or infiltration, is outside air that enters a house uncontrollably through cracks and openings. It is unwise to rely on air leakage for ventilation.

During cold or windy weather, too much air may enter the house and, during warm or calm weather, too little. Also, a leaky house that allows moldy, dusty crawlspace or attic air to enter is not healthy.

The recommended strategy in both new and old homes is to reduce air leakage as much as possible and to provide controlled ventilation as needed. For simple house designs, effective spot ventilation, such as kitchen and bath fans that exhaust to the outside, may be adequate. For more complex houses or ones in colder climates, whole house ventilation systems may be appropriate. Such systems may incorporate heat recovery, moisture control, or air filtering.

Definition: 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.

Air Leak

Sealing and Insulation – Resources to help you set project priorities, detect and seal air leaks, choose appropriate products, and complete Do-It-Yourself projects.

Detecting Air Leaks: Resources, videos, and helpful links

Prioritizing Air Sealing Projects: Explains where air sealing is most effective in the home and helps homeowners set priorities for air sealing projects.

Sealing Air Leaks: Tips, DIY Projects, Materials, Diagrams, Examples, and Instructional Videos

Sealing and Insulating your home is one of the most cost-effective ways to make home more comfortable and energy efficient — and you can do it yourself. Use This Guide to:

  1. Learn how to find and seal hidden attic and basement air leaks
  2. Determine if your attic insulation is adequate, and learn how to add more
  3. Make sure your improvements are done safely
  4. Reduce energy bills and help protect the environment

Air leakage control is an important but commonly misunderstood component of the
energy efficient house. Tightening the structure with caulking and sealants has several
positive impacts.
A tight house will:

  • Have lower heating bills due to less heat loss
  • Have fewer drafts and be more comfortable
  • Reduce the chance of mold and rot because moisture is less likely to
  • enter and become trapped in cavities
  • Have a better performing ventilation system
  • Potentially require smaller heating and cooling equipment capacities.

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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