The term ‘Air Leakage’ refers to the escape of air through gaps in the envelope of a building, while the term ‘Airtightness’ refers to the resistance of the building envelope to air leakage. Airtightness is desirable from the perspective of the energy performance as it minimises heat loss. The term ‘Air Permeability’ refers the physical property used to measure the airtightness of the building fabric and it is defined as air leakage rate per hour per square metre of envelope at a test reference pressure differential across the building envelope of 50 Pascal (50 N/m2). Air permeability can be no more than 10m³/h/m²@50pa to comply with Part L of the UK Building Regulations, which means that no more than 10 cubic metres of air can escape per hour for every square metre of the envelope surface area, with an internal air pressure of 50 Pascal. ‘Air Testing’ is required to establish the airtightness of a buildings. A dwelling achieving an air permeability of 5 will typically use 40 per cent less energy on space heating than one with an air permeability of 10. The PASSIVHAUS standard uses a slightly different measure to determine airtightness, expressed as air changes per hour (ACH) @50pa, or the number of times per hour that the air in the building changes when it is pressurised at 50 Pascal. Buildings meeting the standard are required to have an airtightness of less than or equal to 0.6 ACH @ 50pa, roughly equivalent to an air permeability of 1 or less. Concerns have been raised regarding greater levels of airtightness and it is the case that mechanical ventilation is required when air permeability is 7 or better in order to maintain a healthy indoor air quality.