Definitions covering ventilation and the flow of air into and out of a space include:

Purpose provided (intentional) ventilation: Ventilation is the process by which ‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. This may be accomplished by either natural or mechanical means.

Air infiltration and exfiltration: In addition to intentional ventilation, air inevitably enters a building by the process of ‘air infiltration’. This is the uncontrolled flow of air into a space through adventitious or unintentional gaps and cracks in the building envelope. The corresponding loss of air from an enclosed space is termed ‘exfiltration’. The rate of air infiltration is dependent on the porosity of the building shell and the magnitude of the natural driving forces of wind and temperature. Vents and other openings incorporated into a building as part of ventilation design can also become routes for unintentional air flow when the pressures acting across such openings are dominated by weather conditions rather than intentionally (e.g. mechanically) induced driving forces. Air infiltration not only adds to the quantity of air entering the building but may also distort the intended air flow pattern to the detriment of overall indoor air quality and comfort. Moreover, infiltration can result in inferior performance, excessive energy consumption, an inability to provide adequate heating (or cooling) and drastically impaired performance from heat recovery devices. Building airtightness standards exist which limit infiltration losses.

Other air losses, e.g. duct leakage: Air leakage from the seams and joints of ventilation, heating and air conditioning circulation ducts can be substantial. When, as is common, such ducting passes through unconditioned spaces, significant energy loss may occur. A US study from 2005 indicated that 10%–30% of the conditioned air in an average central air conditioning system escapes from the ducts (Modera, 2005). Pollutants may also be drawn into the building through these openings. Some countries have introduced ductwork airtightness standards and requirements to limit duct leakage.

Air recirculation: Air recirculation is frequently used in commercial buildings to provide for thermal conditioning. Recirculated air is usually filtered for dust removal but, since oxygen is not replenished and metabolic pollutants are not removed, recirculation should not usually be considered as contributing towards ventilation need.

Ventilation is needed to provide oxygen for metabolism and to dilute metabolic pollutants (carbon dioxide and odour). It is also used to assist in maintaining good indoor air quality by diluting and removing other pollutants emitted within a space but should not be used as a substitute for proper source control of pollutants. Ventilation is additionally used for cooling and (particularly in dwellings) to provide oxygen to combustion appliances. Good ventilation is a major contributor to the health and comfort of building occupants.


As ventilation clearly plays an important role in the context of COVID-19, the AIVC board decided in their last (online) meeting of September 2020 to start a project to collect, discuss and disseminate information about COVID‐19 in relation to ventilation and airtightness.

We would thus like to encourage you to check the outcomes of the project and:


  1. AIVC (1996). “GV: A Guide to Energy Efficient Ventilation”.
  2. M. Modera (2005). Fixing duct leaks in commercial buildings. ASHRAE journal, pp. 22-28.
  3. AIVC (2020). “VIP 40: Ductwork airtightness - A review”

See also

  1. V. Leprince, F. R. Carrié, M. Kapsalaki (2017). Building and ductwork airtightness requirements in Europe – Comparison of 10 European countries. Proceedings from the 38th AIVC Conference "Ventilating healthy low-energy buildings", Nottingham, UK, 13-14 September 2017.
  2. TightVent (2019). “List of applicable standards for building and ductwork airtightness”