Wind barrier and breather membranes

The two membranes in the build-up

Inside: vapour control

It stops internal vapour getting into the structure. The principle of vapour control.

Outside: wind barrier, vapour permeable

It has three jobs:

1. Wind barrier: stopping air from moving through the insulation. Insulation blown through performs substantially worse than the laboratory figure promises.
2. Rain protection: keeping water that gets behind the cladding off the insulation.
3. Vapour permeability: whatever the structure has taken up can leave outwards.

The basic rule

Vapour permeability should increase outwards through the build-up. If the outer layer is tighter than the inner one, moisture is trapped.

So what goes on the outside of the insulation matters: a closed, vapour-resistant board there is as much a risk as bad vapour control inside.

The ventilated cavity

Many build-ups have a ventilated cavity behind the cladding. Its role:

• it drains away moisture that gets behind the cladding,
• it ventilates out the vapour heading out of the structure,
• it reduces the summer heat load.

In exchange it raises fire questions: the cavity needs cavity barriers. Fire spread on façades.

What to watch on site

• Laps and taping to the manufacturer’s instructions.
• Connection into the adjoining structures: windows and doors, the foundation junction, the roof.
• It must not be stretched taut between the sections, or it can tear at the fixings.
• Exposure time is limited: the UV resistance of the membrane is finite, so it cannot stay uncovered for months.

The drainage role

In some build-ups the external membrane is also a secondary drainage layer: it collects and leads out the water that gets behind the cladding. The detailing then has to provide falls and an outlet — that is junction design.

The point

The external membrane is not “just a sheet of plastic”: it protects the performance of the insulation and it lets the structure dry. The wrong order here does as much harm as bad vapour control.