Cavity Barriers and Their Application Here in Australia


Passive fire compartments, fire containment barriers and separate fire cells (or zones) are well known in the industry. For those who don't know, the principles are relatively simple; if a fire breaks out in one part of a building, we want to contain the fire to that part of the building using fire walls, floors, ceilings, shafts and other so called “fire stopping” products and systems. Most passive fire protection systems have detailed fire test methods, allowing fire ratings to be deduced for regulatory purposes. The required Fire Resistance Levels (FRLs) are provided in the National Construction Code (NCC), based on the class of building (use) and type of construction (rise in storeys).


This article deals with so called “cavity barriers” and is a compilation of my research into the application of SIDERISE perimeter fire stop and cavity barrier systems, for specification, design advice and ultimate sale here in Australia by Trafalgar Fire, my company.






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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:

  1. Curtain wall,
  2. Rainscreen "cladding" or ventilated facades, and
  3. 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 spreading up and between the floor slab edge and the perimeter of the curtain wall assembly itself, and also to avoid leap frogging 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 rainscreen 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 rainscreen 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.



Want to read more about cavity barriers and their application to Australian building construction? Download the full article here.


Fire Testing


For the sake of time and length, I'll be summarising the following points. However, I go into more detail in my full article which you can download here.


Curtain Wall Perimeter Fire Stop

It can be a struggle determining how to apply AS1530 Part 4 and conventional FRL's to slab edge or perimeter fire stop materials. Many consultants I speak to point me to NCC Clause C3.16 (see below) for construction joints—and without anywhere else, this is probably a good start.


The problem with this approach is that control joints are tested between two fire rated building materials, and while our slab edge is a fire rated building material, the curtain wall is not fire rated. The Europeans have a dedicated fire test method (EN1364-4) for this application, where the fire rated slab edge meets the non fire rated curtain wall building envelope. However, curtain wall fire stop is covered in NCC Clause C2.6, along with spandrels, and are currently only required in non-sprinkler protected buildings, except for the new fire protective timber construction requirement in C1.13. If it was intended to use C3.16, I guess it would say so in C2.6 and at present it does not.


Watch the successful 3 hour fire test incorporating an Aluminium glazed curtain wall façade/concrete floor slab interface sealed with SIDERISE CW-FS perimeter fire stop system, or download the full article to see the fire test images.


Open State Cavity Barriers for Cladding



We are all aware of the cladding pandemic we find ourselves in, and it's sad that the NCC requirements are still ambiguous and not in line with international best practice. Cavity barriers are a proven and cost effective means of stopping fire spread behind the cavities when we clad a building envelope.


Let’s hope logic prevails in terms of cavity systems and we see some more definite technical guidance including fire and requisite movement criteria incorporated into NCC and associated Australian Standards or industry guides sooner rather than later. I would hope our insurance industry reads this and puts out some guidance to minimise damage to building façade fires by way of cavity passive fire protection or cavity fire stopping measures.


Our NCC provide no provisions for so called open state cavity barriers (which you will remember are those that allow ventilation through the cavity barrier in the cold state, and close up using high performance intumescent materials in the hot or fire state). The open state cavity is typically only 25mm or 50mm, which allows the high performance intumescent materials to close off very quickly as proven by fire testing.


Just so you are not confused, a rainscreen cladding system requires provisions for drainage and ventilation, hence why a small section of the cavity needs to be open; the so called open state. Of course in fire it changes by way of the intumescent material expanding and closing the gap changes state to one of closed, stopping fire spread and excessive smoke spread.




BS8414 Full Scale Building Envelope

The BS8414 fire test is a full scale fire test for “cladding systems”, simulating an external fire adjacent to the cladding, and assists to measure vertical and horizontal flame spread, either up the outside of the cladding itself, or inside the cavities behind the cladding where insulation, framing and other building envelope assembly components hide. It’s a big fire test simulating several levels of construction and include a wing wall.


For the fire testing purist, nothing beats a full scale system fire test. This shows the full interaction of the entire façade or building envelope ASSEMBLY and how it react to an external fire.


The importance of cavity barriers in these fire tests cannot be underestimated. Full scale fire tests in accordance with BS8414 show the importance of well designed cavity barriers. 


To me the best analogy I can think of is building a brick wall without mortar. We just do not do it. The same applies to cladding on our buildings; we need cavity barriers. 


Design Considerations


I have been critical of the NCC in terms of definitive requirements for fire and smoke and I make no apologies for this. I think most people will agree that “cavity barriers” are required, and are a relatively small cost, but important SYSTEM element to mitigate fire spreading up the outside, or inside the cavities of a building envelope. However, we seem to focus on the fire and smoke, but what other performance attributes are important for “cavity barriers”?


Once again, I go into more detail in the full article (available for download here). But to summarise:


  • Vertical Mineral Fibre Orientation. Mineral fibre materials (aka Rockwool or Stone wool) used in “cavity barriers” require their fibre orientation to be vertical to allow repetitive movement without breaking down the fibre structure. This is sadly the opposite orientation to which the fibres run in a manufactured mineral fibre slab. For this reason, most reputable “cavity barrier” systems will have vertical fibre orientation.


  • Compression during installation. The “cavity barrier” materials need to typically be inserted into a slab edge or building perimeter under compression, so that in a fire condition, they do not fall out, and of course so they can cater for expansion movement in normal practice.


  • Mechanical fixing. To provide durability and allow for serious movement without dislodgement, a mechanical form of attachment to the edge of the floor slab, that can cater for movement, will provide the best design and performance.


  • Independent and third party cyclic testing for movement. Nothing beats real life in the field experience and independent laboratory movement testing, rather than relying on theoretical calculations or supplier unsubstantiated claims in their marketing literature. There are strict requirements in Europe to do cycle testing on “cavity barriers” and then subject the same specimen to the fire test!


Why did I choose SIDERISE for Trafalgar Fire?


To be honest, I always thought “cavity barriers” were just some Rockwool or Bradford Insulation shoved into the edge of a slab. Over the last 25 years in passive fire protection, and pre Grenfell fire, I only had a handful of technical enquires for building envelopes, mainly around FyreBOARD Maxilite® for spandrels and the odd question about sealing the slab edge for noise; not fire and smoke!


The Grenfell Tower, our Lacrosse Tower and Neo 200 Tower fires in Australia, changes to the NCC around cladding, insurance industry concerns and influence over those designing for fire and facades, and over 4.2 million sqm of Aluminium Composite cladding with a combustible PE core being about to be stripped off our buildings for recladding, solution enquiries have gone through the roof.


SIDERISE offered me pedigree with clever simple product and system designs, patented production, from a globally recognised market leader boasting a staggering amount of independent testing with third party certifications.


Let’s just review the movement and durability criteria I listed above. SIDERISE tick and exceed all four of the key design and performance attributes. The clever manufacturing process ensures the correct fibre orientation by cutting Rockwool into lamella or strips, rotating them through 90 degrees, pre-compressing the strips, and then adding a thin aluminium scrim or foil to both sides of a factory made and laminated SIDERISE CW-FS board.


The clever mounting tabs provide a simple, cost effective, quick to install and mechanical means to fixing the fire stop or “cavity barrier” to the edge of a floor slab. It allows for serious movement, even Seismic type movement, and is proven with years of real life service, and many different cycling test regimes and some including fire testing post cycling.



How else, but with independent cyclic test for movement can you specify and approve a “cavity barrier”? Ask for independent movement testing or specify SIDERISE! 

Read and download the full article today.

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