If you have read anything about the Passiv Haus standard, you will have come across airtightness and the blower door test. They sound technical, and they are, but the idea behind them is simple: a home that leaks air is harder to keep comfortable and more expensive to run. This article explains what airtightness is, how the blower door test measures it, and how much it actually matters for a home in Perth.

It is worth understanding because airtightness is one of the clearest dividing lines between a genuine Passiv Haus and a well-built passive solar home. Knowing what the test does helps you judge how far down that path your own project needs to go.

What airtightness actually means

Airtightness is a measure of how much uncontrolled air leaks in and out of a home through gaps in the building fabric: around windows and doors, at wall and roof junctions, through service penetrations, and anywhere the construction is not sealed. Every home leaks to some degree. The question is how much.

Uncontrolled leakage matters for two reasons. First, comfort: draughts, hot air pushing in on a 40 degree day, and cool air escaping when you are trying to heat. Second, efficiency: air you have paid to cool or warm slips out and is replaced by outside air at the wrong temperature, so your heating and cooling work harder than they should.

There is an important distinction here. Airtightness is not the same as having no fresh air. A well-sealed home is deliberately ventilated through a controlled system, so you get filtered fresh air on purpose rather than random leakage you cannot manage. Sealing the leaks and ventilating deliberately is the whole point.

What the blower door test does

The blower door test is the standard way to measure airtightness. A powerful, calibrated fan is mounted in an external doorway on a temporary airtight panel. The fan either pushes air into the house or draws it out, holding the interior at a set pressure difference to the outside, usually 50 pascals, which is roughly the effect of a moderate wind on every face of the house at once.

To hold that pressure, the fan has to replace exactly the air that is leaking out through the gaps. By measuring how hard the fan has to work, the test calculates the total leakage. The technician can also walk the house while it is under pressure and feel or use a smoke pen to find precisely where the leaks are, which is as useful as the number itself during construction.

How the result is expressed

The result is usually given as air changes per hour at 50 pascals, written as ACH50. It describes how many times the entire volume of air in the house is replaced in an hour under that test pressure. Lower is tighter.

For reference, the Passiv Haus standard requires 0.6 ACH50 or lower, which is very tight. A typical new Australian home built without any airtightness focus might sit somewhere between 5 and 15 ACH50. The gap between those numbers is the gap between a leaky home and a sealed one.

Trying to work out how airtight your home really needs to be? Start a conversation and we will talk through the right target for your project.

Start your design conversation

How much airtightness does a Perth home need?

This is where local climate matters. The 0.6 ACH50 Passiv Haus target was developed for cold central European winters, where uncontrolled leakage is a large and constant heat loss. In that setting, extreme airtightness pays for itself.

Perth's climate is milder and the case is more nuanced. Hitting 0.6 ACH50 requires a fully sealed envelope, membranes and tapes, careful detailing at every junction, and mechanical ventilation with heat recovery to supply fresh air. That is a genuine Passiv Haus level of build, and it commonly adds 20 to 35 per cent to the cost over a comparable passive solar home, depending on how complex the design is to seal and detail.

For most Perth homes, a sensible improvement in airtightness, combined with good insulation, orientation and reverse brick veneer, captures most of the comfort and efficiency benefit without the full Passiv Haus premium. A passive solar home built this way reaches 9 to 10 star NatHERS ratings and very low running costs. Chasing the last fraction of airtightness to 0.6 delivers real but diminishing returns in our climate, and it is a decision to make deliberately rather than by default.

The short version

Airtightness is about sealing the gaps so you control the air coming into your home rather than letting it leak wherever it likes. The blower door test puts a number on how well you have done it. Full Passiv Haus airtightness is the tightest standard there is and it is worth it for a specific kind of build, but for most Perth homes a well-sealed passive solar design gives you the bulk of the comfort and savings for a much smaller premium.

You can read our overview of Passiv Haus, weigh up whether certification is worth it, or get in touch to talk through your project.

Frequently asked questions

It depends on your target. The Passiv Haus standard requires 0.6 air changes per hour at 50 pascals, which is very tight. A typical new Australian home built without any airtightness focus might sit between 5 and 15. Any deliberate effort to seal the envelope will bring that number down markedly, and for most Perth homes a moderate result combined with good insulation and orientation captures most of the benefit.

No, when it is done properly. Airtightness removes uncontrolled leakage, not fresh air. A sealed home is ventilated deliberately, usually through a mechanical system with heat recovery, so it delivers a steady supply of filtered fresh air. The result is generally better air quality than a leaky home, not worse, because you control where the air comes from.

Only if you are targeting a specific airtightness level, such as full Passiv Haus certification, where the test is required to prove the result. Most passive solar homes in Perth are not built to that standard and do not need a formal test. That said, the test is a useful quality check during construction because it finds leaks while they can still be sealed. Whether it is worthwhile is a decision to make early in design.

For a specific kind of client, yes. The 0.6 target was developed for cold European winters, where leakage is a major heat loss. In Perth's milder climate a well-sealed passive solar home with reverse brick veneer already delivers most of the comfort and efficiency for a much smaller premium. Reaching full Passiv Haus airtightness adds real but diminishing benefit here, and commonly 20 to 35 per cent to the cost depending on design complexity, so it is a deliberate choice rather than a default.