By Jake Woolgar
For many builders, prospective house purchasers and building officials, the inclusion of a drained cavity behind a cladding system is a guarantee of weathertightness success. This is however a very bold and frequently incorrect assumption, as several buildings investigated by Prendos has shown.The New Zealand Building Code Acceptable Solution E2/AS1 defines a drained cavity as:
“A cavity space immediately behind a wall cladding that has vents at the base of the wall… A drained cavity assists drying by allowing water, which occasionally penetrates the wall cladding system to drain to the exterior of the building, and any remaining water to dry by evaporation.”Prendos have recently been involved in the repair of a house where cavity battens had been installed horizontally to provide fixing for wall cladding. These horizontal battens have inevitably prevented drainage from behind the cladding and restricted airflow to a point where evaporation could not occur. Horizontal cavity battens have the potential to transfer moisture from the wet cladding side of the cavity to the wall framing, as shown in the photograph over page. In this location, water was entering behind the cladding around joinery and parapets on the first floor but was quickly transferred to the untreated wall framing at the batten over the window shown.
(Photo caption – Horizontal cavity batten above head flashing prevents drainage, trapping moisture and rotting framing behind.)
Another building, this time using a castellated timber batten has shown a similar failure. Although castellations as shown in the adjacent photograph are designed to provide drainage, only a small surface area is provided to achieve it. Water is left behind the cladding sitting on the horizontal surface, wetting the timber batten. This has had two effects; firstly moisture is transferred to the rigid air barrier and untreated wall framing, and secondly causes the cavity batten to swell, cracking the cladding and allowing more water ingress.
(Photo Caption – With castellations only a small area of batten allows drainage.)
It is still common practice for builders to use diagonal battens across a cavity for fixing. These can usually be avoided. They add risk to the system by restricting ventilation and potentially transferring water on the back of the cladding to the wall framing, negating the function of the cavity.
Further examples of poorly constructed cavities are frequently observed even where horizontal battens are omitted. The photograph below shows the placement of vertical battens between and above windows. Although these battens are required for fixing they almost completely close the cavity to both drainage and ventilation.
(Photo Caption – Is this a cavity?)
In order to allow a cavity to drain and adequately vent, only vertical battens should be used. Placement of the battens should be at centres as wide as the manufacturer suggests for the cladding, taking note of the required wind loadings. To maintain the cavity at both internal and external corners, cavity battens should be spaced away from those corners to allow for drainage. As with all aspects of building, a cavity is only as good as the design and workmanship that goes into it and the water that comes out of it.