Envelope permeability is the term given to building leakage and is measured in cubic meter (m3), per hour (h), per m2 (m2) of building envelope, or m3/h/m2. This measurement is taken at a differential pressure of 50Pa between inside and outside the building. It is tested with a blower door, which is a special type of fan designed to measure building leakage. Depending on how large the building is and permeability requirement, fans can be used in many different configurations.
All buildings leak air and the direct relationship of air infiltration and energy consumption is clear, proven and measurable. Controlling the airtightness of a building enables the mechanical system to maintain the internal conditions with a greater accuracy and better efficiency because it does not have to work against unconditioned air leaking in, or the loss of conditioned air leaking out.
Building efficiency in Australia has not changed significantly since the introduction of Section J in the National Construction Code of Australia. Prior to this there was no real measure of efficiency that buildings had to meet in order to comply. Mandatory building airtightness is the last of the low hanging fruits on improving building efficient without increasing the cost of construction.
The National Construction Code 2019 Section JV4 is the first step on improving building airtightness through measuring the actual buildings, not simulating and assuming the works will be done according to code. It is a direct measurement on the performance of a building envelope and therefore a direct measurement on the quality of construction undertaken because poor quality buildings rarely pass a building leakage test.