The principle of vapour control

The task

There is always water vapour in the internal air — from cooking, from washing, from people. That vapour migrates towards the colder layers. If it reaches the dew point on the way, it condenses.

The job of the vapour control layer is to stop that migration starting into the interior of the structure.

Why this is different in light gauge steel

You can chase six to eight centimetres into a 300 mm brick wall and the wall still seals, because the brick itself does part of the work.

In light gauge steel construction vapour control is done solely by the membrane. There is no solid material behind it to take over the role.

It follows that if anything punctures that envelope, moisture will collect at that point — and a hole drilled for every light switch is exactly such a puncture.

What that means in practice

The membrane has to be continuous

With laps, taped joints and connections carried into the adjoining structures.

No cables inside it

Mechanical and electrical installation goes on the inside face of the vapour control layer, in a separate plane. Service cavity.

Penetrations have to be sealed

Where something does have to pass through — a ventilation duct, say — a verified seal is needed.

The connections are the critical points

Wall to floor, wall to roof, window installation, the foundation connection: the membrane is hardest to carry through continuously here, and this is where it tends to break.

Vapour and corrosion

Where vapour collects, moisture reaches the structure. That is precisely what corrosion of steel requires. Corrosion and service life.

Vapour control is therefore not a comfort question but a durability one.

Checking it

The continuity of the membrane cannot be seen once the lining is on. So:

• photograph and record it before it is covered,
• the airtightness test gives indirect feedback, because air sealing and vapour control largely rest on the same layer. Airtightness and blower door.

The point

In this technology the vapour control membrane is not one layer among many: it is the only one doing that job. Every puncture is a real risk.