Airtightness & Blower Door Results

The leakage and/or entry of air from or into a building (commonly called infiltration or airtightness but it is also exfiltration) is a very important influencer of the passive thermal performance of a home, and therefore in building energy simulation.

Infiltration occurs through the leakage of air through the walls, windows, floors, ceilings and other penetrations of a home and is influenced by construction methods and detailing, and local weather conditions. Losing conditioned air to the external environment via air-leakage can increase energy, reduce thermal comfort and can change the moisture balance and air quality of a home.

With a well insulated thermal fabric, infiltration can become one of the most significant thermal loads on a home, and is one of the last remaining "low hanging fruits" for further energy and thermal performance improvement available to building practitioners. However, the effects of infiltration are complicated and can effect many aspects of a building's performance, including condensation, mould and vapour flows. 

While it traditionally has not received significant attention in Australia, there is increasingly a focus on the effects of airtightness in our built environment and methods to improve. Disregarding airtightness in the design of a home does not mean that problems are avoided, as the final built product may actually be very tight or very leaky and therefore cause additional problems.

Airtightness in Chenath & NatHERS

The Chenath engine that is used to calculate the hourly thermal results of a home accounts for airtightness and infiltration in several ways. Infiltration is primarily assessed on a zone by zone basis, where each zone receives a infiltration value with two components, one a static leakage rate, and one that is related to stack or wind driven leakage rates. 

Chenath is interesting and somewhat unique in having this approach where infiltration is influenced by the hourly weather data (i.e. the hourly wind speed and direction of the site), as many calculation methodologies rely on static infiltration rates (i.e. a fixed 'ACH' or Air Changes per Hour rate). This has however meant that it hasn't been easy for Chenath to simply report the overall air-change rate of the dwelling in a useful metric that the industry is used to, such as the ACH at 50 Pascals or natural pressure.

The zone infiltration values in NatHERS regulatory mode are based on a background air-leakage rate per zone which is based on area as well as the dwelling's Site Exposure, and then any additional air-leakage sources from windows, doors, openings and penetrations are then accounted for in the calculation. See the Ceiling Fans & Penetrations article for further information on leakage rates from penetrations.

NatHERS Regulatory Mode Airtightness

As described above, the airtightness rate is influenced by both the construction, site exposure and weather (i.e. wind-speeds) of the site, and therefore there is no default fixed air-change rate that is utilised or can be reported in the NatHERS scheme.

Since Hero v4.0, with the introduction of Chenath v3.23 engine, an annual average air-tightness rate has now been introduced and will be reported even for regulatory mode simulations. This is shown in the Airtightness section of the Project Data-grid tab. Note this value will not be reported for NatHERS 2019 profile projects that utilise Chenath v3.21

Air tightness is reported since Hero v4.0 even in regulatory mode, and generally ranges from 8 to 12 ACH @ 50 Pa which is quite leaky.

Varying Airtightness in Hero

Since Hero v3.0, you can vary the overall airtightness of a dwelling in non-regulatory mode in the Project Data-grid tab. This relies on entering the expected final air leakage rate at 50 Pa such as what would be obtained by a blower door test. After a simulation is run (note this simulation will take a little bit longer than a normal simulation), the natural pressure air-change rate is also reported.

You can also vary the air-tightness in the Optimise feature for batch simulations which is an important way of studying the impacts of various air tightness levels on a project, and informing your clients of it's impact.

The Future of Airtightness in NatHERS

For many years Hero has been strongly advocating for the introduction of airtightness modelling into the NatHERS scheme alongside other industry partners. As of early 2024, it is expected that we may see a methodology to claim a more airtight home introduced into the scheme however there is no firm timeframe. The ability to vary airtightness in a NatHERS Rating away from the defaults requires NatHERS to ensure that that airtightness is actually achieved in the built product, and therefore will likely require some form of "As Built" certificate or report. We will continue to advocate for this sensible and cost effective measure to improve thermal performance and assist industry in achieving such an outcome.