There’s no ‘as built’ verification process in standard residential construction. You can design, model and specify your home’s energy performance, and move in without ever knowing whether it was built the way it was drawn.

A blower door test gives you an objective air tightness measurement for your home, while there’s still time to fix or improve it. And I believe it’s essential for every build and renovation.

Listen to the episode now.

Hello! This is Episode 435. This is Way #35 of the ‘44 Ways to Create Your Sustainable Home’ series.

In this episode, we’re kicking off Section Eight: Sustainability During Construction. Because, if you’re planning to create a sustainable home, your goals can come completely undone during construction.

This is largely because there is no ‘as built’ verification process inherently included in how we deliver construction of residential homes. 

However, a certified Passive House does have as-built verification requirements, and so, there are certain tests that can be adopted from the passive house methodology into traditional construction practices.

Way #35 is: Get a Blower Door Test.

What is a blower door test?

A blower door test is a diagnostic tool used to measure how airtight your home is. It gives you an objective, measured result for how air tight your home is, and measures the movement of air in and out of the building envelope, helping identify any gaps or issue areas that will mean air leaks long term.

Conventional construction practices mean that homeowners move into a home without ever knowing what their home’s actual airtightness is. 

A blower door test helps you identify the leakage pathways before a home is finished, so rectification work can be done to improve the building performance.

You may have seen air tightness measured in something called ACH @ 50Pa. ACH stands for Air Changes per Hour, and 50Pa is 50 pascals of pressure. 

It’s the test pressure used during a blower door test, and simulates real-world wind forces. It’s essentially telling you how many times the entire volume of air inside a building leaves it and is completely replaced in an hour, when subjected to constant pressure of 50Pa.

A 50Pa pressure is roughly equivalent to a 32 km/h or 20mph breeze blowing against all sides of the house at once. 

If you’re not familiar with wind speeds, 32 km / h is enough to blow small trees, and is often the type of gusts you’ll get during afternoon breezes, cold fronts or storm transitions. 

If you’re thinking, ‘Oh, we don’t get winds that strong very often’ (which, to be frank, I thought too when I heard 32 km / h, because I’m not great with wind speeds and what they feel like)… well, when I looked at the data for Sydney for 2026, the average wind speed has exceeded this for 5 months of this year already, and the max wind speeds have been near or above this every month so far.

So, a blower door test is working at a pressure that’s similar to this wind speed, and the rating is then measuring how many times the entire volume of air inside a building leaves it and is completely replaced in an hour. 

And the average Australian home measures between 15  and 36 ACH @ 50Pa.

A CSIRO Report published in 2024 called ‘Air Infiltration of New Dwellings in Australia’, written by Michael Ambrose and Sean Maxwell, documents research done to test the air tightness of 233 newly built homes in Melbourne, Sydney, Brisbane, Canberra and Adelaide, of which, 128 were detached homes constructed within the last 4 years as ‘typical’ builds that didn’t have a specific air tightness objective.

The CSIRO report comments that the air tightness level assumed in NatHERS when doing the energy modelling of a home is 10ACH @ 50Pa. And ‘in fact, more than 60 percent of results in that study were greater than 10 ACH50.’

The CSIRO report says this: ‘There will frequently be a difference between modelled and as-built results for building air tightness. This may be due to a range of factors including design issues, build quality and material selection… In the case where air tightness is leakier than modelled, energy performance may be worse than expected.’

In some cases, performance can be as much as one or two stars worse than predicted due to excess air infiltration alone. The opportunity to realise real-world savings in energy use and occupant comfort by including air tightness in compliance can be significant.

Nothing in standard Australian construction practice requires a builder to actually hit that assumed airtightness level. There’s no mandatory test and there’s no verification step.

Your home is modelled at a certain performance that correlates to 10 ACH @ 50Pa, a star rating is generated, with 7 stars being the benchmark in Australia, and then your home is built with no requirement to check whether the build quality actually delivered the performance the model assumed. 

And you live with that gap, regardless of how it turns out.

How is a blower door test done?

A calibrated fan is fitted into an external door frame, temporarily sealed in place with a flexible panel. Every other external opening in the home is closed: all windows, doors, exhaust fans, fireplaces, air conditioning ductwork, anything that connects interior to exterior. 

The fan then either depressurises or pressurises the building, creating a controlled pressure difference between the inside and the outside. While the test is running, you can be in the house. This is when the consultant or builder walks around searching for those air leakage pathways. Sometimes they use smoke pen to easily see where air is leaking.

The most common sources of uncontrolled air leakage in residential construction aren’t dramatic failures. 

Air leakage pathways are usually small gaps and penetrations that appear at junctions between different building elements and around services. Common sources of air leakage are at the penetrations for light fittings, power outlets, exhaust fans and ductwork. Wall and ceiling junctions can also be a source of leakage.

Ideally, a blower door test is done twice.

Once as a diagnostic test when the building wrap, exterior cladding and roof is on, and the windows are in. This is when the building is water-tight, and when air leaks around penetrations or gaps can be identified and rectified fairly simply. 

Then another blower door test can be done when the internal lining is in so you can get the final measure, and also check anything that might have happened when fit off of exhaust fans, light fittings or other items occurred.

When interviewing potential builders for your project, a blower door test is a great way to see whether the builder is familiar with building for air tightness and building envelope performance, or is willing to upskill in understanding this information. 

If the builder has never done one before, ideally they get excited about it, and see it as an opportunity to learn and improve the projects they build. 

If they’re resistant, or dismissive of it, or simply say ‘sure, you can do that, but it can be managed separately to our scope of work, and we won’t be involved’ then PLEASE see this as a red flag that the builder is not aligned with your goals, and you better off finding someone else to work with.

In this Episode, I cover:

• What a blower door test actually is, and why there’s no ‘as built’ verification baked into standard Australian residential construction
• How ACH @ 50Pa works, and what 50Pa represents in real world conditions
• What the CSIRO ‘Air Infiltration of New Dwellings in Australia’ report from 2024 found, and how it compares to the 10ACH @ 50Pa NatHERS assumption in your design and energy efficiency modelling
• How much a one- or two-star drop in energy performance can cost you in heating and cooling energy over time
• How a blower door test physically works on site, and what the consultant or builder does with the smoke pen while the fan is running
• The two most useful times to do a blower door test during construction
• Where air typically leaks in a home
• How to navigate including a blower door test with your builder, and the red flags to watch for in their response
• Why a home being more air tight than expected can also be an issue, especially without mechanical ventilation

Plus a whole lot more.

LISTEN TO THE EPISODE NOW.

RESOURCES:

Episode 419, on building tight and ventilating right >>> 44 Ways #19

Episode 417, on getting a WUFI analysis done on your floors, walls and roof >>> 44 Ways #17

Episode 422, on reviewing building wraps to manage moisture for climate >>> 44 Ways #22

Episode 296 with Jessica Allen of Climasure, on air tightness, blower door tests and WUFI analysis >>> Air Tightness

Episode 297, continuing the conversation with Jessica Allen of Climasure >>> WUFI Analysis

Aerotight’s guide to blower door tests >>> Everything you need to know about blower door tests

CSIRO report on how much air leaks through new Australian homes >>> Air Infiltration of New Dwellings in Australia

Sydney wind data by year >>> Live Sydney Weather

A free guide to simplifying sustainability across research, planning, design, materials, and building, from Amelia Lee’s 30+ years in residential design and construction >>> 44 Ways to Create a Sustainable Home

Access the support and guidance you need to be confident and empowered when renovating and building your family home inside my signature online program >>> HOME Method