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Measuring buildings – Air-tightness of new house construction (June 2016)

How well houses are sealed against air leakage can have a significant impact on their energy efficiency. Until now, little data has been collected on newly built houses to quantify their air-tightness. CSIRO has recently completed a project to quantify the air-tightness of current housing construction around Australia. Results were mixed, however, the average air change rate was found to be close to what one would expect from the NatHERS software, so the regulatory settings are consistent with average practice.


Concerns have been raised in various forums that the energy efficiency features of houses currently being built in Australia is being compromised by poor air-tightness. CSIRO recently completed a study to quantify the air-tightness of new house construction around Australia. The houses in most cities were up to 3 years old and the Melbourne houses were up to 10 years old.

Blower door testing was carried out on 129 volunteer homes. In the testing, all external doors and windows are closed, an internal pressure of 50 Pa is applied via a fan positioned in the front door way and the flow rate at that pressure is measured. The resulting air changes per hour at 50 Pa pressure (ACH@50Pa) for each house was then determined. It should be noted that to roughly estimate the average ACH under normal environmental conditions, the ACH@50Pa figure from the test results should be divided by 20.

In addition, inspections of each house was undertaken to assess the quality of the installation of the insulation and the heating and cooling. Weather sealing around windows and doors was also inspected. Details of these additional inspections are contained in the full report.


Figure 1 (below) shows the spread of ACH results for each city along with the averages.

The NCC does not specifically quantify an air leakage rate, but has a performance requirement that states:

“A building must have, to the degree necessary, a level of thermal performance to facilitate the efficient use of energy for artificial heating and cooling appropriate to the sealing of the building envelope against air leakage.” NCC Volume 2 (2015) – clause P2.6.1

Consequently, the blower door test results were compared with the level of air-tightness that would be expected from NatHERS software modelling that is used to demonstrate compliance with the NCC. Overall, the average air change rate measured was 15.4 ACH@50Pa which is slightly higher than around 10‑12 ACH@50Pa that one would expect for a reasonably well sealed modern house modelled in the NatHERS software.

Air change rates by city including averages


  • Overall the project has found that newly constructed houses in Australia have a broad range of air tightness levels ranging from world’s best practice through to much more leaky than the assumed air tightness levels in the NatHERS software.
  • The average air change rate was 15.4 ACH@50Pa. This is slightly above the air change rate that one might expect from NatHERS modelling.  A third of the houses had results lower than 10 ACH@50Pa which demonstrates that well sealed houses are possible and occurring across Australia.
  • Several houses recorded air change rates above 30 ACH@50Pa which is common amongst old poorly sealed houses, but is considered to be leaky for a newly constructed house.
  • Adelaide and Hobart houses recorded results significantly lower than all of the other cities tested. On average, Adelaide and Hobart houses had an ACH@50Pa of 8.5 and 7.9 respectively.
  • No immediate cause for the variations in air change rates has been identified. General build quality and attention to detail may be significant factors.
  • Houses with uPVC window frames recorded much lower air change rates than most other houses.