Airtightness is measured using a blower door test. This is undertaken by first closing all doors, windows and any other openings. Then, a fabric door in an adjustable frame is inserted into the front door frame. A fan is mounted within the frame, which pressurises and depressurises the building.
After this process, a highly sensitive instrument is used to measure the difference in pressure between the inside and outside the building at various fan speeds. These sensors measure pressure differentials to determine the air leakage rate at a range of different pressure gradients.
Using the measured building volume and surface area, an accurate rate of air leakage is determined.
This is commonly described as Air Changes per House (ACH) at 50 pascals. 50 Pascales is the equivalent of a 32km/h wind blowing onto the building. The National Construction Codes Building Code of Australia (BCA) Part 1 requires a maximum of 10m3/h/m2, which is broadly equivalent to 10ACH at 50 Pascals.
The uncontrolled leakage of air through the building envelope can be the source of draughtiness, leading to an uncomfortable house. This can contribute to substantial extra heating and cooling costs. It also allows dust, pollens, and other airborne particles to enter the house in an uncontrolled way.
Air leaks in the building envelope may also lead to problems with water vapour condensing in wall cavities, creating problems with mould and structural damage. An airtight building envelope, combined with the correct continuous insulation and a well thought out ventilation strategy, are the keys to a comfortable and energy efficient home.