Airtightness and the blower door

What is measured

In the test the building is put under pressure or suction with a fan, and the air that has to be blown in or drawn out to maintain the pressure difference is measured. That gives the air change rate.

Beyond that, the leakage points can be located during the test — with smoke, an anemometer or a thermal camera.

That is the real benefit: it does not just give a number, it shows where the structure is faulty.

Why it matters

For energy

Heated air leaving through gaps is a direct loss.

For moisture

Warm, humid air flowing out through a gap condenses in the colder part of the structure. That is a far greater moisture load than diffusion — in other words a gap is more dangerous than a poor vapour layer.

For comfort

Draughts, cold sensation, dust ingress.

For acoustics

A gap lets sound through too.

Why it connects to vapour control

Because in light gauge steel construction air sealing and vapour control are largely done by the same layer: the internal membrane and its continuity.

So the airtightness test is an indirect check that the membrane is sound. The principle of vapour control.

When to test

During construction

When the membrane is finished but not yet covered. A fault found then can still be repaired.

That is the most useful moment, and the one most often skipped.

At handover

As verification, with a record.

Where leakage typically arises

• at the installation plane of windows and doors,
• at the wall-to-floor and wall-to-roof junctions,
• at mechanical and electrical penetrations,
• at laps in the vapour control layer, where the tape has not held,
• at shutter boxes and socket boxes.

The ventilation question

An airtight building needs controlled ventilation. That is not a side effect but a design task: airtightness and mechanical or deliberate ventilation work together.

Instead of “let the house breathe”, the aim is for air change to happen where and in the quantity we want.

The point

The blower door is not only a test but a fault-finding tool. It gives most when it is carried out before the lining goes on.