The cladding over the cavity can also be brick, fibre-cement sheets, laminate panels or other materials such as aluminium panels.
Whatever the construction material, think of your cladding as the first line of defence and a cavity as the second line of defence, says Ball.
The point of the cavity, says Ball, is to:
• Prevent the direct transmission of moisture absorbed into or that passes through joints, gaps or penetrations in the external cladding to the internal wall structure where it might cause damage
• Provide a route for moisture that does penetrate the external cladding surface to drain safely out without causing damage
• Deliver ventilation to assist in the drying out of moisture that may have penetrated through the external cladding
• Improve the thermal and acoustic efficiency of external walls.
Without a cavity there are a lot of potential problems lurking.
"Because we know that it is almost impossible to entirely prevent moisture from getting through the external "skin" of a building, cavities are designed to manage that moisture so that it doesn't cause any damage," says Ball.
Cavities can go wrong for a variety of reasons. One of the big issues that came to the fore with the leaky building crisis was that water can penetrate into a house from a variety of different entry points.
That can be through doors, window joinery, balustrades, cladding joints or decks. In fact, any feature of a building has the potential for moisture to flow or be pushed by wind pressure into a building's inner wall structure.
As any leaky home owner will know, if the cavity failure is widespread and systemic, the damage may be significant, resulting in the need for substantial remedial works.
It's not just the cost of remedial works that matters. A poor or non-existent cavity can destroy the occupants' health.
"We know that long-term saturation of timber framing frequently precipitates decay fungi and mould growth, both of which have been widely associated with the weather tightness failure of buildings in New Zealand," says Ball.
These moulds can be very detrimental to your health.
Cavities are one of the reasons that building work should be done by a licenced practitioner with experience. For a cavity to work well it needs the battens to be placed correctly, says Ball.
"Also any external or internal flashings installed must be designed to direct moisture outwards as opposed to inwards."
Where cavities fail it is usually because they have been poorly constructed. For example, with brick veneer claddings failure can occur due to wall ties sloping down and inwards allowing moisture to flow from the back of the bricks and into framing, says Ball.
"Alternatively, mortar from the brick joints falls down the cavity and if not cleaned out by the brick layer, sits on wall ties creating a bridge or blocks the 'weep holes' at the bottom of the wall that allow moisture to drain out."
Some cladding systems, such as those often installed on high-rise apartment buildings, are designed to allow moisture through the external cladding, says Ball.
"These are called 'pressure-equalised' systems and permit managed volumes of moisture to pass through engineered gaps in the cladding or 'rain-screen' and drain out at each interstorey joint.
"Such moisture will either flow down the back of the external cladding material and drain away or dry out on the back of the external cladding through natural ventilation."
Ironically, even a poorly constructed cavity is better than none and can provide a building with some degree of protection against weather tightness failure, says Ball.
Questions have been raised about urea formaldehyde foam insulation, which for 25 years was often retrofitted into cavities.
BRANZ says injected or blown-in insulation should only be retrofitted into cavities if there is undamaged wall underlay or building paper installed Older houses and those with brick veneer cladding often did not have the required building paper.
If you are unsure about how a property you plan to buy is constructed or have concerns about the cladding, then you should seek the advice of a chartered or registered building surveyor.
