Lee Davies from CA Group discusses what is claimed to be ground-breaking technology that enables metal buildings to compete with brick when it comes to fire protection.
Over the past few years, demands on the construction industry have resulted in wet trades struggling to deliver on time. Whether through issues relating to manufacture (many kilns were mothballed during the last recession), or on-site labour (the well documented skills shortage), the need to look at alternatives to brick and block walls has never been greater.
Brick has been the go-to construction material for residential as well as many commercial, industrial and retail buildings due not only to its aesthetics but also the robust passive fire protection it affords. The brick shortage has therefore posed a significant challenge for the construction industry. Delays of up to four months have been reported, which have had a significant knock on effect on project timings. This has led architects and developers to look for alternatives.
Metal cladding systems have long been used in construction in various guises: twin skin systems, composite insulated panels and rainscreen cladding, manufactured using steel or aluminium. Other metals can also be used, including stainless steel, zinc and copper. Architects often choose metal due to the design flexibility and variety of finishes it can deliver, enabling them to put their stamp on any given building.
Furthermore, cladding systems such as those listed above meet most of the technical requirements needed for modern construction: compliance with CDM and building regulations, weather tightness, air permeability and durability, plus thermal, acoustic and structural performance. However, one area in which it can be difficult to achieve compliance is fire protection.
On balance, the metal cladding industry was a natural alternative for architects faced with delays from the wet trades. The challenge of fire protection was a problem to which the industry would have to find a solution.
Fire performance considerations
Two key areas must be considered when measuring the fire performance of any envelope solution; structural integrity and insulation integrity. The first is governed by the period the cladding system will remain in place when subjected to fire, and the second by the time it takes to reach a specific temperature on the other side of the cladding from the fire.
Structural integrity is relatively easy for designers to specify with built up metal cladding systems available that can achieve over four hours (the point at which tests are stopped). Insulation integrity with this method, however, has always been limited, due to the heat transfer through the components used to connect the outer skin to the structural steel frame. Typically, achieving more than 30 minutes was challenging and as a result more costly alternatives were needed which carried more onerous safety requirements.
Clearly this was not an acceptable solution, and research teams have continued to develop systems which achieve better performances in insulation integrity. From a specification point of view, 60 minutes of insulation integrity has always been desired for these types of buildings.
There are several aspects of system design which need to be looked at when considering fire performance – including the predominantly metal spacer system, and the overall construction methodology – factoring in the insulation. As part of any fire wall system, a robust, thermally insulating separation layer is required and it is in this area that the greatest degree of innovation has been possible.
To understand whether new products are suitable, indicative fire tests are undertaken at companies such as Exova Warringtonfire in which both the old and new products are compared, in order to establish the difference in performance.
Products have been developed that deliver on all fronts. Metal cladding systems now offer robust fire protection on a level which is comparable with brick and block (tests have demonstrated that 120 minutes of insulation integrity is achievable, alongside an overall structural integrity performance of 240 minutes). This means that architects don’t have to compromise on design in favour of fire protection. If they don’t want to wait, or if they want the flexibility that metal affords, thanks to innovation, they can now have both.
Lee Davies is technical director at building envelope manufacturer and installer CA Group