Airtightness and blower door testing: the quality check more houses are getting

Australian houses have historically been leaky. Gaps around windows and doors, unsealed downlights, vented skirtings and open fireplaces meant plenty of air came and went whether the owner liked it or not. As energy standards have risen and insulation has improved, attention has turned to air leakage, because a well-insulated house that leaks air loses a large share of the benefit.

Blower door testing, standard practice in some countries for years, is becoming more common in Australia, particularly for high-performance homes, Passivhaus projects and builders who want to prove their quality. This guide explains why airtightness matters, how testing works, where houses leak, and how to fix it.

Why air leakage matters

Energy

Every gap in the building envelope lets heated or cooled air escape and outside air leak in. In a leaky house, the heating or air conditioning is working against a constant flow of outside air. That is energy wasted and money spent, and it means the house performs worse than its energy rating suggested, because the rating assumed a certain level of sealing.

Comfort

Draughts make people feel cold even when the air temperature is reasonable. Leaks also let in dust, noise and outside pollutants. A tighter house is quieter, cleaner and more even in temperature.

Moisture

This is the hidden one. Warm indoor air carries moisture. When it leaks into a wall or roof through a gap around a downlight or a pipe penetration, it can condense on cold surfaces inside the construction. Air leakage can carry far more moisture into a wall than vapour diffusion through the linings. Sealing the envelope is therefore part of managing condensation, a subject the NCC has focused on increasingly.

What the NCC asks for

The NCC’s energy efficiency provisions for housing include building sealing requirements: sealing around windows and doors, exhaust fans with self-closing dampers or similar, sealing of chimneys and flues, and treatment of gaps in the building fabric, among others. These are described as requirements to limit unwanted air infiltration. For commercial buildings, the NCC also recognises envelope sealing tests as a way to verify performance.

In practice, most houses in Australia are not tested, and the sealing requirements are met by following the prescriptive details. Testing is increasingly used where builders want to demonstrate performance, where clients ask for it, or where a high-performance standard such as Passivhaus requires it.

How a blower door test works

The test follows a standardised fan pressurisation method. A calibrated fan is fitted into an external doorway using an airtight frame. The fan depressurises (or pressurises) the house to a set pressure difference, commonly 50 pascals, which is roughly the pressure of a strong wind on all sides of the house at once. The airflow needed to maintain that pressure shows how leaky the house is.

Results are commonly expressed as air changes per hour at the test pressure: the number of times the house’s volume of air would be replaced each hour at 50 pascals. They can also be expressed as air permeability, relative to the envelope area. Lower numbers mean a tighter house.

While the house is under pressure, the tester and builder can find the leaks. Air rushing through gaps can be felt by hand, seen with a smoke pencil, or seen with a thermal camera when there is a temperature difference.

When to test

The most useful test is often done before the linings go on, once the air barrier is complete. At that stage, leaks can be found and sealed easily. A final test at completion confirms the result. Testing only at the end tells you how leaky the house is, but by then many leaks are hidden behind plasterboard and much harder to fix.

Where houses leak

Downlights are the classic Australian leak. Older-style downlights cut into the ceiling can leave a significant gap between the room and the roof space, and they also displace ceiling insulation. Sealed, insulation-contact-rated downlights address both problems. The wall-to-ceiling junction, where the top plate meets the ceiling lining, is another major path, especially where cornices are not sealed. Every penetration through the ceiling, from plumbing vents to electrical cables, is a potential leak.

The air barrier

An airtight house has a continuous air barrier: a layer that stops air moving through the envelope. In a typical Australian house, the air barrier is usually the internal linings, plasterboard walls and ceilings, together with sealed junctions and penetrations. In high-performance homes, a dedicated airtight membrane may be installed on the inside of the frame, with taped joints and sealed penetrations, before linings go on.

The key word is continuous. An air barrier with gaps at every junction and penetration is not an air barrier. Designing where the barrier is, and how it continues around corners, windows and services, is the first step. Making sure every trade who penetrates it knows how to seal it is the second.

Build tight, ventilate right

A tighter house needs deliberate ventilation. Without it, moisture and indoor pollutants build up, and condensation and mould follow. That is why the phrase “build tight, ventilate right” is used.

For most houses, ventilation means exhaust fans in kitchens, bathrooms and laundries that discharge outdoors, windows that can be opened, and sensible use. For very tight homes, including Passivhaus, mechanical ventilation with heat recovery supplies fresh, filtered air continuously and recovers much of the heat from the outgoing air. Either way, ventilation must be designed, not left to leaks.

A worked example: the pre-lining test

A builder doing their first high-performance home arranges a blower door test after the frame, windows, insulation and services are in, but before plasterboard. The house has an airtight membrane taped on the inside of the frame.

The test shows the house is tighter than typical, but there are clear leaks: around several electrical penetrations where the electrician cut the membrane, at the junction of the membrane and two window frames, around a plumbing vent in the ceiling, and at the cavity slider. The builder and the tester walk the house with a smoke pencil while the house is under pressure and mark every leak.

The electrician returns to fit grommets and tape the penetrations, the window junctions are re-taped, the plumbing vent gets a proper collar, and the cavity slider is sealed. A quick retest shows a big improvement. The final test at completion confirms it. The lesson the builder takes away is that most leaks were made by trades who did not know the membrane mattered, which leads to a new rule: every trade gets a five-minute airtightness briefing at induction.

Getting trades on board

Airtightness is a team effort. Electricians, plumbers, air conditioning installers and carpenters all penetrate the envelope. If they do not know it matters, they will cut holes and leave them. Brief each trade, supply the right grommets, tapes and collars, and make the test result something the whole team can see. Builders who get good results usually treat the test as a quality scorecard for the site.

Renovations and existing homes

Airtightness improvements are one of the most cost-effective upgrades in existing homes: sealing old downlights, fitting dampers to exhaust fans, weatherstripping doors and windows, sealing gaps at skirtings and floors, and blocking unused fireplaces. A blower door test before and after shows the client what was achieved. But the same warning applies: as an older house is tightened, make sure exhaust fans work, discharge outside and are used, and that any combustion appliances have the air supply and flueing they need.

Airtightness and high-performance standards

Voluntary standards such as Passivhaus set a very demanding airtightness target and require testing to prove it. Some Australian builders are using those standards to differentiate their homes. Even for builders who do not pursue certification, the techniques, a continuous air barrier, taped membranes, sealed penetrations and planned ventilation, can be applied to any house to improve its real performance.

What it costs

A blower door test is typically a modest cost per test, done by a trained tester with calibrated equipment. The bigger costs are the time to seal properly and, for high-performance homes, the airtight membrane and tapes. Against that, the energy savings, better comfort and reduced condensation risk accrue for the life of the house.

Marketing a tested home

A blower door result is a rare thing in residential building: an objective measure of build quality that a client can understand. Builders who test routinely can show clients their results, compare them over time, and use them to demonstrate that their homes perform as promised.

The bottom line

Insulation keeps heat from conducting through the envelope; airtightness stops it leaking out through the gaps. Design a continuous air barrier, seal every penetration and junction, brief every trade, test before lining if you can, and ventilate deliberately. The result is a house that performs like its energy rating, feels better to live in, and stays dry inside the walls.

Questions builders ask

• Condensation
• Energy efficiency
• Insulation
• NCC

Published 21 August 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.