Cavity Barriers and Their Application Here in Australia

What Are “Cavity Barriers”?

The concept behind “cavity barriers” isn’t very complicated; they’re a fire and smoke containment system used to stop the spread of fire and smoke through cavities. The most prevalent and widely used cavity barriers are used to stop vertical fire and smoke spread within the cavities formed around the perimeter of an external building envelope. These come in two forms:

• Horizontal Cavity Barriers—used to seal the edges of the horizontal internal floor slabs, and
• Verticle Cavity Barriers—used again to seal the vertical floor to floor edges to divide the cavity on one given floor vertically into predetermined zones.

Some vertical and horizontal cavity barriers are also used to protect window and door openings from fires in the building envelope cavity.

Why do we need Cavity Barriers?

There are three very different “cavity barrier” applications for different construction types used here in Australia:

• Curtain wall,
• Rainscreen “cladding” or ventilated facades, and
• Cross Laminate Timber (CLT)

I’ll discuss each point in turn, explaining why different terminology is used to compare and contrast the different “cavity barriers” (which relates in principle to the different fire spread mechanisms and associated fire testing developed to deal with these for each construction type).  

1. Curtain Wall Type Buildings

Curtain wall is defined in our NCC as a non-load bearing external wall that is not a panel wall. I will deal with panel walls later. Curtain wall construction has been with us for a very long time; we all recognise the large, glazed curtain wall buildings which are prevalent in our Australian cities. Fires have shown us that we need to provide some means to stop fire from spreading up and between the floor slab edge and the perimeter of the curtain wall assembly itself, and also to avoid leapfrogging from level to level. The slab edge fire stopping is termed perimeter fire stopping or slab edge fire stopping, to differentiate it to cavity barriers which are reserved for use in cladding systems, not curtain walls. Typically, the perimeter fire stopping works together with a spandrel to provide some fire resistance for the slab to glazed curtain wall interface.

2. Rainscreen Cladding

Cladding on buildings are defined in the NCC as panel walls, meaning non-load-bearing external wall in a frame (or similar construction) that is wholly supported at each storey. This type of building envelope construction is typically referred to as rain-screen cladding (or a ventilated façade in other parts of the world). This construction uses internal walls on the perimeter of the building envelope, and the so-called rain-screen or cladding hides a void or cavity where thermal insulation, framing, sarking and the like live. The cavities are also drainage of moisture or rain, and for ventilation to help dry and stop any mould and the like growing in what could potentially otherwise be a damp or water-logged environment. Cavity barriers are used here to stop fire spread in these cavities, but a special challenge is required, as we need to utilise what are termed open state or intumescent cavity barriers to allow for everyday ventilation, but to close and seal adequately in a timely manner in the event of a fire.  

3. Cross Laminate Timber

The sustainability initiative across the globe has seen the onset of Cross Laminated Timber (CLT) used in construction including so called CLT fire barriers or panels. Personally, I am not convinced this is wise, but the masses have spoken and CLT seems to be gaining popularity in construction. Cavity barriers are required under NCC for CLT construction and some fire testing requirements have been included along with some FRLs or fire ratings.