Air tightness....

I get a reasonable number of questions about air tightness which is one of the key passive house principles.

Australia has no air tightness requirements... despite some solid suggestions and some solid International examples.

The UK requires new houses to have < 10 ACH50. The USA Department of Energy has a Zero Energy Ready Home national program certification that outlines required ACH50 values ranging from 1.5 to 3 depending on climate zone. Certified Passive Houses are required to have < 0.6 ACH50.

Air tightness is measured using a blower door test. Using a calibrated fan and pressure gauges they increase and decrease the pressure in the house relative to outside measuring how much air has to be pumped in or out to maintain the desired pressure at a range of pressures. From these measurements they determine how much air leaks in when the house is 50Pa lower than outside and how much air leaks out when the house is 50Pa higher than outside. To account for the fact that houses are of difference sizes the air leakage is standardised relative to the house volume. This number is the number of Air Changes per Hour at 50Pa is shortened to ACH50. Fifty pascals is a little bit arbitrary, although it is comparable to the pressure caused by the wind on a moderately windy day. It is a pressure difference that a typical house is likely to experience.

We have had 3 blower door tests:

During construction by Arjun of Thermal Environmental to test the building envelope prior to any of the interior plasterboard, etc being fitted. This was so that any leaks could be easily identified and fixed.

Just after construction by Arjun of Thermal Environmental to test the finished building for Passive House certification.

And a couple year later... just because I get asked about these things and I ran into Peter of Blower Door Services at the Asquith Passive House open house.

We can pick at the various numbers and maybe two decimal points is getting a bit carried away, but out house is a little bit tighter (on the negative pressure test at least) then it was when the house was certified. Why? Who knows, but the important bit is that we are no leakier now than when they finished construction.


Air tightness results.
DatePositiveNegativeAverage
2019.May.150.480.450.47
2019.Aug.210.300.410.35
2021.Jun.250.270.280.27

2021 blower door test!
June 2021 Blower door test. Note the evidence of living in the house.

2019 August blower door test.
August 2019 blower door test. Note the finished interior.

May 2019 blower door test. Note the timber frame and OSB.

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