Air movement behind the cladding

Some claddings incorporate natural ventilation gaps, such as the gaps at the lap of weatherboards and at the lapped edges of profiled metal sheets. Cladding systems that allow natural air circulation are known as air permeable systems (they also deflect water as a rainscreen), and these have good potential for natural ventilation.

Monolithic systems that rely on a face seal coating for weathertightness are not air permeable (barrier cladding) and offer little potential for natural air ventilation when they are direct-fixed. Consequently, when the components of these types of cladding assemblies get wet, there is little opportunity for air drying to occur.

Drained and vented cavities require a minimum of 1000 mm² of clear ventilation/drainage opening per metre of exterior wall length to allow sufficient ventilation within the cavity (E2/AS1). Air permeable claddings offer significant infiltration paths, which increases ventilation in bottom-only vented cavities.

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Air pressure moderation

Air pressure is typically higher on the exterior surface of a wall than behind the cladding or in the building interior. If there are any gaps, air can be ‘sucked’ through, potentially carrying moisture with it.

Pressure moderation is achieved by:

• wall assemblies designed so that the air pressure behind the cladding and within the wall assembly (i.e. within the framing cavity, trim cavity, and drainage cavity if there is one) is as close as possible to the air pressure on the exterior surface
• an air barrier (flush-stopped internal lining or rigid air barrier plus air seals) to stop the flow of air from the wall assembly into the building.

Pressure moderation.