Cladding - Direct Fix or Cavity?

Defining the terms

The standard method of attaching weatherboard or sheet claddings to wall frames for decades has been to wrap the outside of the wall in a sarking or building wrap then screw or nail fix the cladding straight over this, allowing no sort of ventilation cavity. The wrap used to be a foil, but in recent years vapour-permeable wraps have become the norm. This is still the minimum required standard in the building codes as of NCC 2022.

The alternative method of fixing requires the addition of cavity battens. These are 20-35mm battens which get fixed over the wall wrap. The cladding then gets fixed over the battens, leaving an air-space which provides ventilation.

Defining the Problem

So if direct fix cladding has been the norm for so long why is this now a problem? To understand this you need to understand the purpose of the wall wrap itself and how increased building standards are changing how important it is. Modern wall wraps are intended to provide weather protection from water ingress from the external facade of the building AND allow vapour to pass through it from inside the building. The building materials inside the wrap (the insulation and plasterboard) are vapour permeable so some water vapour can pass through from the inside air of the building. In addition to this condensation can build up inside the wall itself, initially from timber framing that hasn’t yet fully dried, but mostly as a result of condensation caused by temperature differentials between the external façade and the inside particularly in winter when the inside of the house is being heated.

In previous decades this did not necessarily cause huge problems as homes were more poorly insulated and more poorly constructed so temperature differentials were less and there were more ‘gaps’ in the construction to provide ventilation (air changes) for the inside of the house. As building standards and methods have improved and houses have become better insulated and better sealed there is now much more of a risk for condensation to build up inside the house AND inside the external walls. This can potentially degrade the insulation over time and in a worse cause scenario cause mould growth and timber frame rot.

The colder the climate the worse this risk is. Famously New Zealand had a wet house problem that prompted huge amounts of remediation work and a change in building standards as a direct result of this issue. While large areas of Australia fall within warmer climate zones there is still potential for problems to occur in the temperate zones of southern Australia.

How does the cavity help?

Fixing cladding over cavity battens creates an airspace between the air-permeable wrap and the cladding. Correctly detailed and installed this allows for airflow to enter at the base of the cladding and (with a chimney effect) flow up the wall and out the top. Any vapour or moisture that does come through the vapour permeable wrap can then be dried out and carried away, keeping the wall system dry. This ensures the longevity of the insulation and the health of the building. It doesn’t solve all the condensation issues of the building, but it is a fundamental construction detail that should be included in all high-quality framed construction homes.

More information

A full white paper on this issue can be downloaded here if you are interested.