Air leakage can be defined as:

The uncontrolled movement of air in to and out of a building which is not for the specific and planned purpose of exhausting stale air or bringing in fresh air

There are three main driving forces of envelope air leakage: -

• Wind. Wind exerts constantly-changing positive and negative pressures on the building envelope.
• Stack Effect. Rising warm air causes pressure differentials through the building envelope, which are generally positive at high level and negative at low levels.
• Mechanical Systems. Heating and ventilation systems create positive or negative pressures within the building.

Any one of these driving forces, or a combination of all three, will lead to air leakage through any cracks or gaps in the building envelope. This leads to cold external air moving into the building, and warm internal air moving out of the building.

It is now widely accepted in the UK that there is little merit in improving the effective U value standards required for envelope assemblies unless the levels of uncontrolled air leakage through such assemblies are significantly reduced.

The significant energy penalty which uncontrolled air leakage causes has been recognised within the amendments to Part L (Conservation of Fuel and Power) of the Building Regulations 2006. These amendments introduce maximum envelope Air Leakage Standards for domestic and non domestic buildings.

As well as the direct energy penalty imposed by air leakage, other associated problems include: -

• Occupier discomfort because of drafts.
• Degradation of the building fabric due to interstitial condensation.
• Poor indoor air quality due to the ingress of fumes, dust, etc.
• Inability to achieve required pressure conditions in controlled environments.
• Difficulties in balancing air handling systems.
• Noise transfer through leakage paths.

Air leakage should never be considered as acceptable natural ventilation because it cannot be controlled or filtered, and will not provide adequate or evenly-distributed ventilation. It is generally at its most severe during the colder, windier, winter months and has least impact during the warmer, calmer, summer periods. This is generally the opposite of the requirements for ventilation within buildings. Ventilation of a building should rely on a designed strategy based on the assumption that the envelope will be relatively airtight.