Following numerous and significant fires in the recent past, no one doubts the importance and critical nature of the fire protection of high rise building external cladding systems. Such cladding systems are often referred to as rainscreen or curtain wall systems.
Tenmat has been at the forefront of developing the highest performing Ventilated Fire Barriers (also referred to as “Open State Cavity Barriers”) for external cladding systems for over 10 years, with products continuously being improved for even greater fire protection performance.
The problem
Many types of cladding, rely upon an open cavity behind the cladding to function correctly both in terms of keeping out the elements and optimising thermal efficiency. This type of cladding system results in a vertical open cavity which acts exactly like a chimney in the event of a fire. The “chimney effect” of the open cavity multiplies the flame length up the cavity and thus causes rapid fire spread up the outside of the building. Even if the cavity itself is fully non-combustible then the extended length of flames created in the “chimney” still allows the flames to reach the next floor level, where windows and other wall penetrations will allow the fire to re-enter the building and maintain the spreading of the fire.
Many buildings use combustible insulation products within the cladding cavity and this then adds fuel to the fire, thus creating a situation where the initial fire provides a source of ignition, the chimney pulls in air and the combustible components within the cavity provide the fuel, this is the perfect scenario for rapid fire spread and intensification, unless it is STOPPED.
The cladding conumdrum
Ventilated Cladding Systems are very well established and can offer a neat solution to tired looking old buildings, as well as brand new buildings, which is hard to beat in terms of appearance and thermal performance. The need for constant ventilation of the cavity during normal use and the critical need to seal the cavity in the event of a fire initially gave the fire protection industry something of a headache. This problem has now been fully resolved for several years via the use of the latest intumescent materials and more onerous test methods to ensure that a building is made as safe as possible. It should also be noted that intumescent ventilated seals have been proven to provide improved performance even when compared to “full fill” cavity fire barriers both in terms of preventing fire spread and duration of integrity/insulation.
Benefits of intumescent cavity fire barriers
Intumescent materials are a particular category of products which react dramatically when exposed to heat and expand rapidly, often by up to 30 times their original size. This reaction has allowed products to be designed which crush pipes and seal open gaps when exposed to fire.
Intumescent fire protection products have been in Worldwide used for about 20 years and are relied upon to offer fire protection and compartmentalisation in a large proportion of buildings. These products are fully recognised by industry experts and test laboratories as a fundamental part of any fire protection system, this is a well proven and approved range of products.
When employed as part of a ventilated fire barrier system behind cladding panels, the intumescent is often attached to a mineral fibre batt which allows large cavities to be spanned leaving an open gap for the ventilation to take place.
The ventilation gap is typically between 25mm and 44mm depending upon the requirements of the airflow for the particular system. During normal use the air can move readily around the cladding system and even when exposed to high ambient temperatures the intumescent will not react.
In the event of a fire, the cladding cavity temperature will rise rapidly and at approximately 180 degrees C. The intumescent reacts quickly and seals the air gap to make a tight smoke and fire barrier, in addition to preventing the passage of fire and smoke, the activated intumescent seal also stems the flow of air needed for the fire to spread.
As the intumescent barrier solely relies upon exposure to the heat of the fire to react, there are no mechanical joints, no external power source or water supply required, this lack of complication makes intumescent seals a most reliable method of fire containment.
Unlike many forms of fire protection, an intumescent seal requires no maintenance or servicing and in the event of a fire will activate 100% of the time as soon as the temperature rises sufficiently.
Standards development for ventilated fire barriers
Over the past decade, Intumescent Cavity Barriers have been tested in a large number of full scale fire tests with both combustible and non-combustible components, these tests are often of the types BS 8414 or NFPA 285, however there has also been a development of a smaller scale test which derives from previous linear gap seal standards. This new test has been developed by the UK ASFP and is called the TGD19 standard. This utilises existing components of EN test standards in terms of furnace conditions and is currently used to compare the performance of products incorporating some key performance criteria.
As a ventilated cavity barrier is normally in the “open” position to allow airflow, a key criteria is how quickly it will close in the event of a fire and therefore a maximum closure time of 5 minutes is set in the TGD19 test standard. The closure time starts with the temperature in the furnace at ambient temperature, once the furnace temperature reaches the activation temperature of the intumescent then the closure can occur within a few tens of seconds.
Also in the TGD19 test standard are a significant amount of thermocouples which measure the temperature above the sample to ensure that even when the furnace temperature is well over 1000 degrees C, the sample does not rise more than 180 degrees C and so an insulation rating can be provided.
With the combination of closure speed requirement and increased insulation requirement, the TGD19 test is the most onerous test available and allow an integrity and insulation rating for a product to be assessed.
Ventilated fire barrier product differentiation
When it comes to the selection of the optimum ventilated cavity fire barrier for each application, it should be noted that, as with so many products, they are not all the same.
There are two fundamental types of intumescent, firstly there are rubber based materials which use rubber or derivatives as the matrix into which graphite (the active ingredient) is added. These rubber based materials are easy to extrude or press into shapes such as strips required for ventilated cavity fire barriers.
Secondly, there are mineral fibre based intumescent materials which use mineral fibres as the matrix for the graphite, the process of manufacture is to produce multiple thin layers of material to produce a sheet of finished material.
The main difference in the way that the two types of product perform is that the rubber based material tend to be multi-directional in their expansion and the mineral fibre based material tend to be unidirectional. This is due to the graphite in the rubber based material being distributed randomly however in the mineral fibre material it is layered.
In terms of performance for a Ventilated Fire Barrier, the unidirectional nature of the mineral fibre material means that all of the expansion is heading in the same direction and therefore the cavity is closed much more quickly than with the rubber based material. This also allows greater air gaps to be crossed whilst still maintaining a solid and fire resistant char therefore maximum air gaps can be up to and beyond 44mm for the mineral fibre products whereas 25mm is typically the maximum for rubber based materials.
In practice, what this additional performance actually means is that the mineral fibre material has a great amount of potential expansion to allow for movement and distortion of the external cladding whilst still providing a tight smoke and fire seal.
One of the major focal points of current and future legislation is to both reduce the amounts of smoke and toxic gas emissions of construction products during a fire. The irony here is that the rubber based intumescent fire protection materials tend to produce thick black smoke in a fire situation, whereas all of TENMAT’S mineral fibre based materials are classified as both low smoke and zero halogen due to their very low organics contents.
The requirement for low smoke and zero halogens is already required for other construction materials – fire fighters in particular would like to see this practice extended and this is already a focus for new European legislation.
Performance
Typically building regulations require that cavity fire barriers should provide 30 minute integrity and 15 minutes insulation, however market demand for ever greater performance has driven Tenmat to produce ventilated cavity barriers which dramatically exceed the normal requirements and exceed all of the competitor’s products with integrity and insulation ratings in excess of 120 minutes, EI120.
These exceptional fire performance figures outperform most of the best cladding systems themselves and give customers the confidence and peace of mind to know that they have fitted the best possible product.
Nhbc Nf51 Test program
The NHBC contracted the UK Building Research Establishment to undertake a set of fire tests to compare the performance of various types of cavity fire barrier with combustible cavity wall systems. This test program included 21 large scale fire tests and compared solid cavity fire barriers such as mineral wool cavity socks with intumescent ventilated cavity fire barriers.
In particular, the test were designed to show the effects upon performance of various barriers when they were fitted with typical on-site problems, such as gaps between the barriers and misalignment for the barriers. The results showed that the intumescent barriers were the only type tested which provided 100% fire protection, with or without on-site problems, and the other types worked well when fitted perfectly but failed when problems were introduced. The reason for the improved performance is that the intumescent barriers are “active” and will take up gaps, irregularities and distortions of the all cavity either present before or during a fire making them the highest performers available. All of the ventilated fire barriers used in NF51 were supplied by TENMAT.
Longevity
Many building will be expected to provide many decades of life, therefore the fire protection, particularly unseen products, such as ventilated fire barriers, must offer optimum protection for the same period. TENMAT have been producing and supplying intumescent products for 30 years and samples of original materials are continuously tested to ensure that they perform in the same manner as they did on the day that they were produced. In addition TENMAT undertake the longest accelerated age testing of its materials which current test methods will allow and this replicates many decades of use.
Summary
Whilst building cladding techniques and systems are changing at a high rate, architects and designers can be assured that the fire protection industry is also constantly improving and developing products to both meet and surpass the latest requirements, offering solutions which will protect the buildings and their occupants 100% of the time.
For more information, go to www.tenmat.com
About the Author
Rupert Coggon is Business Development Director for Tenmat.
