External wall cladding can give rise to two fire safety issues:
- fire spread through a building;
- outward fire spread via radiated heat to neighbouring building and vice versa.
This briefing note only deals with the potential for fire spread through a building via the external wall cladding.
It has been recognised that external wall cladding systems are a possible fire spread route in multi storey building and guidance regarding the construction of external walls in England & Wales is given in Approved Document ‘B’ Section 12.
12.5 The external envelope of a building should not provide a medium for fire spread if it is likely to be a risk to health or safety. The use of combustible materials in the cladding system and extensive cavities may present such a risk in tall buildings.
External walls should either meet the guidance given in paragraphs 12.6 to 12.9 or meet the performance criteria given in the BRE Report Fire performance of external thermal insulation for walls of multi storey buildings (BR 135) for cladding systems using full scale test data from BS 8414-1:2002 or BS 8414-2:2005. The total amount of combustible material may also be limited in practice by the provisions for space separation in Section 13 (see paragraph 13.7 onwards).
12.6 to 12.9 states that wall cladding must meet certain external surface requirements, that in a building with a storey 18m or more above ground level any insulation product, filler material (not including gaskets, sealants and similar) etc. used in the external wall construction should be of limited combustibility and cavity barriers must be provided in accordance with section 9 of AD’B’. Clause 8.13 also requires that every flat is separated from the rest of the building by compartment walls.
BR 135 addresses the principles and design methodologies related to the fire-spread performance characteristics of non-load bearing external cladding systems, also considers the changing drivers in this market, such as the recent increase in the new-build market for these types of system, and increasing thermal performance requirements.
BR 135 was first published in 1988 in response to the increasing use of thermal insulation as part of refurbishment programmes on existing multi-storey residential tower blocks. Subsequent fires resulted in test and classification systems being introduced for wall cladding systems – these are included in BR 135 in Annexes A & B (BS 8414 Parts 1 & 2).
BR 135 addresses the principles and design methodologies related to the fire-spread performance characteristics of non-load bearing external cladding systems. Although various potential design solutions are identified and discussed in the document, robust design details are not presented, as generic solutions were not available to this rapidly changing market, where new products and novel design solutions were/are frequently presented. The illustrations and scenarios presented in the document were/are based on typical examples of current practice, but, as this field is subject to rapidly changing designs and materials, so the guidance focuses on the issues surrounding the topic to enable designers and end users to understand better the parameters impacting on the fire-safe design and construction of external cladding systems.
Note that BS 8414-2 (Fire performance of external cladding systems – Part 2: Test method for non-load bearing external cladding systems fixed to and supported by a structural steel frame) and Annex B of the guide relate specifically to external cladding systems applied to steel-frame constructions. Consideration is not given to other construction systems such as concrete-frame or timber frame constructions. However, it is stated that the general principles given this report may still apply although suitable additional risk assessments and detail design reviews would be required
External cladding systems need to address requirements that include:
- resistance to moisture/condensation;
- wind loading;
- thermal performance – conservation of fuel and power;
- sustainability and durability;
- fire performance;
- fire resistance (Part B3 Internal fire spread (structure) of Approved Document B, Volumes 1 and 2, inEngland and Wales; Section 2 of Domestic and non-domestic technical handbooks in Scotland;Section 3 of Technical booklet E in Northern Ireland);
- external fire spread (Part B4 External fire spread of Approved Document B, Volumes 1 and 2, inEngland and Wales; Section 2 of Domestic and non-domestic technical handbooks; Section 4 ofTechnical booklet E in Northern Ireland).
The guidance discusses the potential for fire spread via the external envelope, from initiation of the fire event (fire originating internally or externally), the break out and the interaction with the external cladding including surface propagation and through cavities including cavities incorporated into the system or formed by the delamination or differential movement of the system in a fire and windows and openings in the frame envelope provide a potential route for fire spread back into the building thus by passing compartment floors.
The guidance recognises that fire barriers only around windows would not be sufficient to prevent fire spread back into the building from the envelope.
Cladding systems: application and types
External cladding systems fall into two distinct types of application:
- refurbishment of existing multi-storey housing stock, or redevelopment of commercial or residential projects based around systems that are installed on existing masonry-based substrates;
- new build market may also install systems to masonry substrates, but this sector is increasingly working with lightweight frame systems (LFS) based on either steel or concrete primary frames.
A large number of materials are available for use as external finishes: such as stone, terracotta,concrete, timber or metal including aluminium composite materials; thus although Aluminium Composite Materials have been identified as an issue, there is potential for fire spread through external wall systems immaterial of the external finish.
The range of insulation materials found in the external wall build-up can vary (see Table Below).
|Insulation Material||Typical Product||Description||Fire Performance|
|Non-combustible materials and materials of limited combustibility (as defined in Tables A6 and A7 of Approved Document B)||Generally mineral-fibre-based products such as stone fibre and glass wool, typically
formed using a resin binder.
|Produced in batts and rolls of various sizes.
The thickness and density of these products can vary widely, depending on the thermal
|All material within the fire envelope will be damaged during the course of a fire. Stone mineral wool based products tend to lose some integrity, but the material typically remains intact. Although glass mineral wool material does not exhibit fire propagation, if it has been directly exposed to the fire source it may become degraded, and in some cases may melt away.|
|Thermoset products||Polyurethane foam (PUR), polyisocyanurate foam (PIR) and phenolic foams are part of this group, used to provide insulation for external cladding systems.||These products are provided in sheet form at various sizes and thicknesses to meet thermal performance requirements. They
are often faced with materials such as glass fibre or aluminium foil.
|Unless such materials become directly exposed to the fire source, following significant delamination and cracking of the external render finish, they will typically char in the vicinity of the fire source. If the insulation is directly exposed to the fire source, and adequate fire barriers are not installed, fire spread may arise on the surface of the exposed insulation, allowing additional cracking and delamination of the external render coat to occur, and so providing a route for continued fire spread through the system away from the initial fire source|
|Thermoplastic products||Expanded polystyrene (EPS) is the most widely used product in this group, which also includes extruded polystyrene (XPS). It can be supplied in both fire-retarded and non-fire-retarded forms.||The material is generally supplied in sheet form at various sizes and thicknesses to meet thermal performance requirements.||Thermoplastic products such as expanded polystyrene (EPS) will typically soften and melt in the early stages of a fire, generating a void behind the external render finish coat. If inadequate fixings have been used, without the support of the insulating material the finish coats will sag and crack, producing a direct entry route for the fire to the insulation material. Once the material ignites, rapid fire spread can occur if suitable fire barriers and fixing details are not provided. The relatively low softening and melting points of EPS mean that damage can occur to the insulation layer well away from the seat of the fire.|
|Natural fibres||Examples such as wood fibre, cork, sheep wool, cellulose and hemp are becoming increasingly widespread. The products are generally soaked, heated and compressed to
produce the board product. In some cases binders are also used to provide the required performance characteristics.
|The material is generally supplied in sheet form at various sizes and thicknesses to meet thermal performance requirements.
These products may also be available as in situ fill products that are ‘blown’ into voids
|This group of products covers a wide range of materials, some of which will have low surface spread of flame characteristics and others relatively high. Typically, though, these products will not tend to burn away rapidly to leave voids behind the render systems during the initial fire event or in the immediate aftermath. They tend to exhibit a degree of damage in the fire plume area, with some loss of system integrity. A key issue with these products is the potential for the insulation material within the fire plume to exhibit localised, glowing hot spots. These areas of localised combustion can continue to smoulder and burn for many hours after exposure to the fire load, and can be difficult to locate. This leads to a potential risk of fire propagating unseen behind the render face. This is not necessarily a life safety issue for masonry-backed systems, but there is a potential for unseen fire spread and re-entry into the interior of a building in LFS, and this should be considered as part of any risk assessment associated with the use of these products.|
|Recycled materials||A wide range of recycled materials, such as recycled paper and newsprint, shredded
rubber and combinations of other materials, are available as insulation products, which may be treated or used with binders to achieve the required application and performance
|The form that these recycled products takes can vary from compressed boards to blown
|The fire performance will depend on the type of recycled material used – see above.|
External finish construction types
The guide identifies two types for discussion :
- non-ventilated systems;
- ventilated cavity systems.
Examples of these systems are shown below, note that non-ventilated systems may also have cavities for drainage at the rear.
The two primary systems considered in the guide are:
- masonry-backed systems – see BS 8414-1 and Annex A of the guide
- lightweight frame systems – see BS 8414-2 and Annex B of the guide.
The guide discusses a set of design principles has been developed, based on full-scale research programmes, including the use of fire barriers within cavities to prevent fire spread.
The guide recognises that all types of systems have potential for fire spread via the external envelope, unless appropriate fire barriers are provided, and gives guidance in regards to the same. e.g. for rendered systems:
- The fire barrier should be at least 100 mm high, and should form a continuous band through the insulation layer at each floor level. Any abutting of material should ensure that no cavity exists for fire to track or pass through.
- The fire barrier should be formed from non-combustible material as defined in Table A6 of Approved Document B, and should be bonded and tied back to the wall and the external render finish to ensure that no fire path can be created between the non-combustible material and the primary substrate, or between the non-combustible material and the external render finish.
- Through-fixing of the render base coat to the primary substrate with all-steel fixings should also be considered, to ensure that no movement of the external render finish away from the fire barrier is possible. It is important that there is no potential for fire between the external render coat and the fire barrier.
In regards to ventilated cavities, the key elements for producing an effective fire barrier for ventilated-cavity systems are detailed as:
- the details of the fixing of the fire barrier to the system substrate;
- that the fitting for the fire barrier is independent of the sheeting rails;
- that the fire barrier, when operating, closes across the full depth of the cavity and in some cases protrudes from the front face, to allow for movement of the panels during test;
- that the fire barrier, when operating, closes against a non-combustible structure within the system such as amineral fibre fire break.
The nature of the fire barriers required to prevent fire spread has been found to depend, in the main, on thenature of the cladding system itself. Limited experience has shown that effective fire barriers can be designed and installed for these systems. The fire barriers required the vertical sheeting rails to be cut, and therefore interrupted, at regular intervals. Certain barrier systems were found to be adequate for some sheeting materials but not for others. Fire barrier systems therefore need to be considered in the context of the complete system for each specific design, as currently there are no generic solutions that are suitable for all applications.
In practice it has been found that small-scale tests do not fully characterise the fire hazard associated with full-scale cladding systems. The only effective way to assess the fire performance of the fire barriers for this type of relatively complex system is to test the complete system at large scale.
The use of fire protection solely around the windows was generally found to be inadequate in preventing fire spread.
Taking the above guidance and the test methodology of Annex A & B of the guide, to ensure the safety of residents within high rise blocks, the following must be ascertained
- the construction of the external wall cladding;
- the method of preventing fire spread within the cavities/cladding system, i.e. the cavity barriers (mechanical or intumescent), potential for external cladding, membranes etc to bypass fire stopping systems; and
- classification of system if it was subjected to full scale testing.