Reducing air leakage is one of the most cost-effective ways to improve home comfort and the energy performance of residential buildings. This is particularly true for Australia where buildings are extremely leaky compared to the rest of the world.

Currently buildings in Australia have wide variability, between 1.4 and 39 air changes per hour. This is perhaps in part a result of our relatively moderate climate, but also the fact that, unlike many of our international peers, the National Construction Code does not set a quantified minimum.

In its report entitled "The Bottom Line" the Australian Sustainable Built Environment Council (ASBEC) and ClimateWorks point out that reduced air leakage can be achieved at low cost through improved workmanship to prevent loss of conditioned indoor air and infiltration of unwanted outside air.

The report wants immediate action taken as part of the current review of the National Construction Code.

While the Code requires residential buildings to include construction features that minimise air leakage - such as specific requirements for common air leakage points in a building to be sealed, including exhaust fans, windows and doors - the Code does not set a minimum overall air leakage performance level.

Until recently there had been relatively little practical testing of the air tightness of Australian dwellings. However, standard ‘blower door’ testing from 2015 in capital cities around Australia found that newly constructed buildings (less than three years old) had an average air leakage rate of approximately 15 air changes per hour at 50 pascal pressure (ACH@50Pa, a standard measure for air tightness).

These homes have been assumed as representative of those currently being built to current Code requirements. In order to properly assess the opportunity for improved air tightness, the Trajectory Project modelling assumes approximately 15 ACH@50Pa as the base case air leakage benchmark for the residential archetypes modelled.

There is significant opportunities to cost-effectively improve energy performance by improving air tightness to around 6 ACH@50Pa. An air leakage rate of 6 ACH@50Pa as modelled is a reasonable level for the Code to target in the short term.

The CSIRO study found that 18 per cent of the houses tested were already at less than 8 ACH@50Pa, and around one third were less than 10 ACH@50Pa, even in the absence of specific infiltration rate requirements.

This level of air tightness is also comparable with international standards and is what some leading builders have stated they are achieving as standard practice . At this level of air tightness, indoor moisture and pollutant levels can be managed by effective ventilation fans in the kitchen and bathroom - heat recovery ventilation is not generally needed to ensure energy efficient fresh air delivery at this level.

Other cost effective opportunities identified in the report include: Increasing roof insulation for detached housing; Using ceiling fans in warm and hot climates to reduce the need for air conditioning; Increased requirements for wall insulation; Stronger specifications for window performance; Installation of roller shutters and larger eaves in certain orientations; and tighter standards for lighting and dometic hot water systems.