Modern Building Methods may have changed but this has driven the change for improved fire protection, particularly in respect of external wall cavity fire barriers, the latest of which have been proven to outperform all of the previous types of barrier.
The holistic approach
Fire safety should never be looked upon as being created by a single technology or product type, whilst we are discussing passive fire protection such as external wall cavity fire barriers, this should not be seen as a solution in itself.
Overall fire safety requires a combination of technologies, these include fire prevention measures such as proactive precautions and education to prevent a fire from starting in the first place, active fire protection measures such as early warning systems like fire alarms or suppression systems such as sprinklers, as well as containment systems or passive fire protection such as cavity fire barriers.
Whilst each area has its champions and lobbyists, any thoughts that the use of any one of these critical technologies can naturally negate or even reduce the need for another is fundamentally flawed. The main reason being that fire does not obey any rules except one, if there is fuel, oxygen and ignition then there will be fire but the way that fire grows and travels around a building will never be the same twice.
So where should the budgets be spent? On all three, as they all have critical roles to play in the overall fire safety of the building.
Construction techniques, through the introduction of modern methods of construction, have changed dramatically over recent years and this is particularly significant in the manner in which the building envelope is created. Requirements for fire safety, acoustic and thermal insulation as well as the aesthetic appearance of the building can often be seen to conflict. Although it seems obvious that the fire safety aspect, due to the potential catastrophic effects of getting this wrong or missing it out completely, should be a top priority; it is often still considered very late in the design process or as an afterthought.
This can particularly be apparent for areas of the building that are hidden from view such as concealed cavities behind ceilings or within roofs or external walls. These can either contain combustible materials such as electrical or mechanical services or can be an open path for unseen fire and smoke spread.
However, in the case of concealed cavities in external wall systems, the use of an open cavity to create both a thermal and moisture barrier between the external and internal components of the external envelope has been a tried and tested practice for several hundreds of years and it is a practice which simply works.
The major difference in modern wall constructions compared to more traditional methods is that the combination of insulation materials plus a mix of new or conventional building materials such as steel frame systems (SFS), weather boarding, traditional bricks or timber can result in a wall cavity which is inherently open and can contain combustible materials, thus transforming the external cavity into a chimney.
Studies have shown that fire spread within a concealed cavity where an open path and air flow is available can result in a 5-fold increase in flame height. This potential increased fire spread; combined with the fire being hidden from view means that the extent of fire spread can only be seen either once the fire breaks out at some point or by the use of thermal imagery equipment.
Such external cavity fires are inherently difficult to extinguish which is why the specification and installation of cavity fire barriers that should be installed most typically at each compartment floor and party wall level are essential to limit the fire spread as much as possible.
Horizontal fire barriers are a particularly effective method of preventing vertical fire spread within an external wall cavity. Traditional types of horizontal cavity barrier would be sections of rock mineral fibre, metal flashings or even timber battens where short duration fire protection is needed.
However, any such barrier which fully fills and closes off the cavity not only bridges the critical wall cavity, creating potential moisture and thermal transfer problems but also prevents the circulation of air which is required to prevent damp problems or indeed allow certain types of external wall systems to ventilate and function as designed.
This presents, once again, a conflict between aesthetic design and functionality. An aesthetically pleasing external finish is desirable, however closing off the external cavity would require the introduction of openings to reintroduce ventilation into the system. Such openings can be negated if ventilation can be maintained within the cavity, however traditional cavity fire barriers would not make this possible.
The use of reactive intumescent materials that are designed to rapidly expand when heated in a fire situation has provided a solution to this conflict. The intumescent material allows an air gap to be maintained within the external cavity, which not only maintains ventilation to reduce potential damp problems and allow systems to correctly function, but also prevents the inner and outer structure from being in contact and so limits thermal bridging.
Most typically these products, referred to as Ventilated Fire Barriers or Open-State Cavity Fire Barriers consist of a high density mineral fibre section to which an intumescent material is fixed to the outer edge. Intumescent materials with a uni-directional expansion characteristic are particularly well suited due to their ability to expand directly across the open air gap and maintain larger ventilation gaps of up to 50mm.
NHBC NF51 full scale fire testing
An example of the efficacy of such intumescent based systems can be seen from the results of a study to investigate the nature of fire spread within combustible cavities. As eluded to above, the creation of combustible wall cavities has long been of concern to both the fire service and building control officials, following a number of high profile fires which involved combustible wall cavities.
The Building Research Establishment (BRE) in the UK was commissioned by the NHBC Foundation to carry out and publish the “Fire in cavities in residential buildings” report which compared the performance of various types of cavity fire barrier and sheathing boards. In total 21 separate large scale fire tests were carried out which included some of the most combustible materials, typically bitumen impregnated timber based boards lined with combustible breathable membranes.
One test did not include any cavity barriers, whereas the rest all included horizontal cavity barriers in the form of either traditional timber battens, glass mineral fibre or rock mineral fibre cavity “socks” as well as intumescent ventilated cavity fire barriers.
As expected, the construction without any cavity barrier suffered rapid fire attack, whereas all of the various types of cavity barrier performed well when installed fully in accordance to manufacturer’s instructions.
However, the second half of tests were undertaken where common building faults were included in the fitting of the barriers, this included discontinuities (gaps) between the cavity barriers. When these tests were undertaken it was found that all of the traditional barriers suffered a marked reduction in performance, with timber battens in particular proving little better than the test with no barriers at all. Only the intumescent ventilated cavity fire barriers provided an acceptable fire sealing performance in these tests due to their ability to expand and fill gaps and irregularities.
In conclusion, it can be seen that whilst building practices may have changed significantly, the passive fire protection industry is also improving the performance of its products and systems to keep pace with this change.
When passive fire protection is correctly specified, installed and maintained and is considered in a holistic approach with other fire prevention methods; then it can be successfully incorporated into a building without compromising aesthetics, design or overall building performance whilst ensuring fire safety.
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