Ventilation and Cavities

It is particularly important that there should be no ingress of moisture where there is no air gap or cavity, and if the timber frame is untreated and kiln dried. Moisture from within the building can accumulate due to condensation, leakage or from the building use. Horizontal cladding, fixed directly to the structure without a cavity behind it, can inhibit natural venting.

When using cedar or treated timber with plain galvanised or AZ coated steel, designers should be aware that interaction between the timber and metal can occur if these materials remain wet.

Metal wall cladding should have some provision to remove condensation, which occurs when the humidity is high or when there is a significant diurnal temperature differential. Condensation form on metal more frequently when the walls are insulated and the building is heated.

All metal wall cladding must have a permeable and absorptive underlay, and ventilation must be provided when there is no air cavity behind the wall. See Condensation  and Underlay.
The minimum paved ground clearance of 100 mm must be provided to ensure that rain splash or moisture from the ground does not accumulate at the drip edge of the cladding and is not induced into the cavity.

Cavity construction is recommended for horizontal cladding in all areas that are exposed, or where ventilation is inhibited and the metal cladding may suffer from the effects of condensation.

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