How airtight does a building need to be?

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The Passive House standard requires an airtightness that results in a maximum of 0.6 air changes per hour (ACH) based on 50 Pascals of air pressure. This is tested via an onsite blower door test required for certification once the building is finished. The reasons for such a low figure range from moisture & mould protection, to thermal comfort and energy efficiency. In Australia, we are just at the beginning of implementing airtightness in the NCC.

AIRTIGHTNESS – a leaky bucket in a leaky bucket is still a leaky bucket!

I see that a lot of people believe that the airtightness levels would improve when adding more layers. But if anything, the opposite is true.

When the airtightness system is completed, the airtightness system performs at its best. Adding plasterboard is not improving the performance at all.


During construction, the airtightness system is getting damaged, penetrated or changed. A common practice of plasterboard installers in Germany was cutting the tape around the windows to fit the plasterboard. NOT GOOD!

This is the reason the blower door test for the Passive House certification needs to be done when the building is finished. (I actually would do 3 tests. First, when the airtightness system is completed. Second, just before everything is getting covered. Third, at completion.)

I highly recommend that Builders should have their own Blower Door System.

So, how airtight does a building need for it to work?

Everything above 3 air exchanges is asking for trouble. This is the reason building regulations worldwide have quickly moved from 15, 10 or 5 air exchanges to 3 or lower.

In Australia, we are just at the beginning of implementing airtightness in the NCC. Ireland, England and many other countries started ten or more years ago.

Moreover, we need to ventilate our buildings mechanically; there is no way around it. I’ve been trying to avoid mechanical ventilation for the past 10 years. Trust me, there is no guaranteed solution for good air quality without some force.

We know that mechanical ventilation manufacturers with heat recovery always say the systems only work if the buildings are airtight. The most common figure you get is less than 1.5 air exchanges per hour.

In this case, the bare maximum is 1.5 ACH.

To achieve the Passive House standard, a maximum of 0.6 air exchanges per hour are needed. The reason for such a low figure are:

  • Moisture protection by air movement through the buildups
  • Stop air pollution getting into the building
  • Drafts free and comfortable living spaces
  • Reduction mould issues,
  • Improve energy efficiency

Again, a leaky bucket in a leaky bucket is still a leaky bucket. So, let’s build tight and ventilate right!

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