Air leakage through gaps in a building’s envelope can be caused by either the buoyancy effect of air or by the pressure differential created by wind blowing on a
building. Typically both of these factors will influence the total rate of air leakage at a given point in time.
As air warms up within the building it expands and becomes less dense, causing it to rise upwards and leak out through gaps in the fabric at a higher level. The air that is lost from the building is then replaced by colder external air, which is drawn into the dwelling through gaps in the fabric at a lower level. This uncontrolled air leakage is experienced by the building occupants as cold draughts.
As wind blows against the building colder outside air will be forced under pressure through any gaps in the envelope. On the leeward side of the building the external air pressure is lower and will draw warmer air from inside the building out through any gaps in the envelope. The stronger the wind the greater this pressure differential is and the higher the resultant background air leakage rate will be.
Moisture ingress and interstitial condensation
When warm moist air flows out through a gap in the building fabric the air cools down and as the air cools its capacity to hold water vapour is reduced. When the warm air comes in to contact with a sufficiently cold surface the water vapour condenses to a liquid state. When condensation occurs within the build-up of the building fabric it is known as interstitial condensation. Over time, if the interstitial condensate remains trapped in the building fabric, it will lead to a deterioration in the fabric U-values and possibly structural damage and mould growth.
Excerpt from “Passivhaus primer: Airtightness Guide” by BRE