Air leakage is the movement of air into and out of a building that is not controlled through an exhaust system but rather through factors such as wind blowing through gaps in the building envelope. Air leakage is a concern for the following reasons:
1. Air exfiltration (defined as the unintentional movement of air from the interior of the building to the outdoors) can transport significant amounts of moisture into the building envelope where it may condense and form interstitial condensation. This can lead to a wide range of damaging and ultimately expensive conditions such as structural distress, corrosion, mould development, etc.
2. Air infiltration (the opposite of air exfiltration) can create cold drafts and an uncomfortable indoor environment.
3. Air infiltration has the potential to move outdoor pollutants into the building.
4. Increased air infiltration can lead to unnecessarily high energy costs.
5. A leaky structure is usually a noisy structure since the transmission of outdoor noise into the building occurs primarily through the same cracks and openings by which air leakage occurs. Since no structure is perfectly airtight, air leakage, and its undesirable effects, cannot be eliminated but can only be controlled within manageable limits.image: Kebabknight via Wikimedia Commons (Creative Commons BY-SA)