DESIGN PROCESS
Where open state cavity barriers are installed, containing an intumescent material, it is important to ensure the life expectancy of these products is considered to ensure that they can meet the life expectancy requirements of the building or are easily accessible for inspection
(and any subsequent fire testing) to demonstrate compliance during the life of the building. Open state cavity barriers contain an intumescent element that expands to form a fire barrier between the inner substrate and the back of the outer
Example detail of an open state cavity barrier
leaf of the external wall. The European Organisation for Technical Assessment has published a test method within one of their European Assessment Documents (EAD). Successful testing to this method allows a manufacturer to claim a 10 year lifetime within a European Technical Assessment (ETA). The EAD further allows that an ETA can claim up to 25 years of lifetime, if sufficient evidence can be provided. However the EAD does not specify a test method for this. Many manufacturers have their own evidence of durability testing and performance. It is therefore possible for each manufacturer to advise on the performance lifetime of their products beyond the 10 year lifetime usually documented within an ETA. Users should satisfy themselves that this evidence is relevant and substantiated. Where closed state cavity barriers are required, these will tend to be a mineral wool material, and will generally be fixed into the slab edge. The life expectancy of mineral wool is proven for more than 60 years in a wide range of climates.
LIFE EXPECTANCY (OPEN STATE CAVITY BARRIERS)
The function and material composition of cavity barriers are given in Approved Document B. This contains a number of generic materials that can in some situations be used as cavity barriers. However, for most buildings, proprietary cavity barrier products that have been fire resistance tested for integrity and insulation should be used. Typically, cavity barriers should have minimum fire resistance of 30 minutes for integrity and 15 minutes for insulation. Approved Document B also defines where cavity barriers should be located. In walls with a masonry outer leaf, cavity barriers should span across the cavity, with a cavity tray installed over. These are generally referred to as closed state cavity barriers. When installed behind a rainscreen, horizontal cavity barriers must be set back from the cladding to allow for drainage and ventilation. These barriers are usually referred to as open state and include an intumescent strip on the front edge, which expands when exposed to fire to close the cavity.
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