Infiltration 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
[1]. Infiltration is sometimes called
air leakage. The leakage of room air out of a building, intentionally or not, is called
exfiltration. Infiltration is caused by
wind, building pressurization and by
air buoyancy forces known commonly as the
stack effect.
The infiltration rate is the volumetric flow rate of outside air into a building, typically in cubic feet per minute (CFMs) or liters per second (LPSs). The air exchange rate, (I), is the number of interior volume air changes that occur per hour, and has units of 1/h. The air exchange rate is also known as air changes per hour (ACHs).
ACH can be calculated by multiplying the building's CFM by 60, and then dividing by the building volume. (CFM x 60)/volume
In many smaller buildings, 'forced' or 'mechanical' ventilation is not used to introduce ventilation air. Instead, natural ventilation, via operable windows and other openings, exhaust fans, and infiltration are used to provide ventilation air. Typically, at least one-third air change per hour is considered the minimum. Recently, ASHRAE Standard 62.2 has been adopted (2004); it clarifies the ventilation air requirements for low-rise residences. The Standard specifies when, for example, forced ventilation is required in houses.
