Beautiful, Sustainable Building Design

Blower door test results

Posted by admin on 28/04/2017 at 10:24 pm

Blower door test PROJECT

Envirotecture recently undertook a blower door test on an Inner West terrace house. The retrofit project intends to use the Passive House EnerPHit Step-by-Step system to renovate the project over a couple of stages; this test was being done to establish a baseline for air tightness.

A big thanks to Sean Maxwell of ProClima for doing the test and allowing so many others to watch.


The air tightness test utilizes a blower door whereby a fan either pressurizes or depressurizes the building. The test is usually conducted as 50Pa of pressure which is the equivalent of a 20-25km/h wind being applied to the building i.e. not too much.

The fan is connected to a computer which is preprogrammed with the building geometry. The software can then calculate how many cubic metres of air per hour would pass through the building envelope and translate that to a number of air changes per hour (ACH).

The house

The house has a floor area of 76m2 and an entire envelope area of 248m2. While the blower door test takes into account the entire surface area of the building it is interesting to note that in this instance 103m2 of that are common walls.

For the purpose of this test the volume is taken to be the ‘conditioned’ area defined as the volume between floor and ceiling.

The first stage of the project is the conversion of the attic at the front of the home to usable space. This volume was excluded from the initial test.

The results

The building currently experiences 15 air changes per hour at 50Pascals (15 ACH50).

For fun, we attempted to test the house including the existing attic space as we wished to later quantify the difference attained through the current air tight construction methods being applied to the attic. Unsurprisingly this was not possible as the current attic space has two whirlybirds (no dampers).

To better visualise the air leakage we used talcum powder in a small bottle to create a small puff of dust. As the video below shows the unsealed sashes of the double hung windows were one source of air leakage.

The house contains one non-original window, a double glazed double hung that does have a brush seal. No air flow could be felt from between the sashes.

To augment the air tightnesss results we also used a thermal imaging camera to see the differences in surface temperatures. Unfortunately the outdoor temperature was ~20ºC, with such a small temperature difference the distinction between surfaces was harder to see than of we had performed the test at 6am in winter.

There is a large leak behind the washing machine, the air could be felt rushing past it during the air tightness test. The thermal image below shows the relative coolth of the masonry wall where the air flow over the brickwork is cooling the surface. A similar but smaller effect was visible at the top of the kitchen cabinetry where air appears to be leaking behind the trims and into the unsealed roof cavity above.

washing machine thermal image

What is ‘normal’?

Data on the air tightness of Australian houses is relatively thin. There is much quoted CSIRO study (LINK) that concludes an average air tightness of 15ACH, making this terrace ‘average’.

It is important to note that only detached homes were tested in the CSIRO study and they were predominantly three years old or less; our terrace house is ~100years old!

Australia currently has no legal requirement for air tightness in any building type hence the paucity of data (and performance). The UK requires a maximum of 10ACH for new homes, New York state requires 3ACH


Passive House buildings are required to achieve an air tightness of 0.6ACH50 with a slight dispensation for the EnerPHit retrofit projects to 1ACH50 i.e. 1/10 of the UK requirements and an infinite improvement on the Australian requirements.

As a holistic approach Passive House also ensures that buildings have an adequate supply of fresh air as well as appropriate levels of insulation to ensure energy efficiency. It is this trifecta that provides high comfort and high efficiency.

There is much debate about the implications of air tightness. If not coupled with appropriate ventilation then the risk of condensation increases, the old adage ‘built tight, ventilate right’ rings true! Passive House buildings use mechanical heat recovery ventilation (MHRV) systems to replace the internal air volume with fresh air approximately twice per hour but without losing the heat. These systems are not ‘normal’ in Australia although this will change rapidly in the next few years.

Windows are an important part of a ventilation strategy too. On the day of our test the external temperature was lovely so opening windows as a way to get air exchange makes sense. At 6am in winter or in a heatwave I would counsel against it.

How is air tightness achieved?

An air tight strategy operates at the building scale so making each room air tight is unnecessary. In a new home is it common to use an air tight membrane fixed to the internal face of all external surfaces (roofs, walls) with the insulation outside of that membrane. A cavity is often created inside of that membrane to allow for the running of services (water, power) without having to penetrate the air tight membrane too often.

The large flat areas of walls, roofs and floors (if necessary) can be relatively easy to deal with, it is usually the junctions and penetrations that require more thought and effort.

In our terrace retrofit we are faced with additional challenges that the building already exists so installing membranes in certain locations is impossible, in others, incredibly difficult.

For the attic conversion currently underway we have developed some details to air seal between existing ceiling joists, hanging beams and masonry party walls. Another challenging junction is at the rafter/wall connection, while sealing that connection in the attic space is possible (but fiddly) a continuous air barrier to the room below is not possible without removing the ceiling itself. The practical realities of this is part of the reasoning behind the slightly less onerous requirements of EnerPHit.

We will continue to publish on the progress of the retrofit including the lessons learned. If you wish to know more please get in touch via the website, to stay in touch subscribe to the newsletter here.

Here is the video of Sean introducing the test and explaining some of the technicalities.


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