Direct-fixed cladding construction

Claddings fixed over a drained and vented cavity are more robust with respect to weathertightness than direct-fixed cladding systems. However, the cladding system must also be well installed and finished to provide an effective primary line of defence. 

Ensure that direct-fixed claddings are well installed and finished. This type of construction has limited potential to manage water that may penetrate into the wall assembly, so it must provide an effective primary line of defence.

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Drained and vented cavity construction

The inclusion of drained and vented cavities will not stop water leaking through a cladding but will help to remove this water. The cavity should only be required to drain water that gets through the primary line of defence in extreme circumstances. It should never be considered a primary drain nor an excuse for poor cladding installation.

Cavity battens can be formed in any material that meets the required durability requirements, such as H3.1 treated timber, extruded polystyrene, uPVC and durable metal. The battens must:

• be fixed vertically to wall studs over the wall underlay 
• be at intermediate centres if required for cladding support and fixing 
• form a nominal 20 mm cavity from the face of the wall underlay to the back of the cladding. 

Where the cladding requires horizontal support, packers with a 50 mm clearance to vertical battens and fixed with a 5° slope need to be installed.

Wall underlay selection

Use the type of underlay that the designer has specified around the entire building. Table 23 of E2/AS1 outlines the requirements.

Under E2/AS1 wall underlays can be flexible (kraft paper-based and synthetic compositions and in absorbent and non-absorbent forms) or rigid (plywood or fibre-cement sheet) overlaid with a flexible wall underlay. Non-absorbent wall claddings must be installed over an absorbent wall underlay. Absorbent claddings can be installed over either type.

Absorbent wall underlays absorb water as it makes contact with the underlay. Non-absorbent underlays may hold a small amount of water, but most will drain down over the surface.

Absorbent wall underlay.

Designers need to consider that, when water penetrates the cladding in direct-fixed assemblies, wall underlays will get wet.

Wall underlay must be installed to form a water-resisting drainage plane that stops water getting to the vulnerable bulk insulation or timber frame. Also flexible wall underlays must:

• be installed taut
• laps must have sufficient cover and should be taped
• any penetration or tear must be taped and sealed
• the underlay must exit the assembly at openings that will allow water to drain out.

Other critical areas to understand are:

• penetrations
• air barriers/air seals.