Fire test methods for external wall cladding systems

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Small scale fire testing of cladding materials

Fire testing of cladding materials looks at the fundamental properties of the individual components to classify the performance. This can include the total heat energy released over a specified time (MJ or MJ/m²) or the peak heat release rate (kW/m²). The objective is to determine whether the tested materials have a higher likelihood of igniting or propagating flame.

Where the term cladding materials is used, this is used to describe fire testing of the exterior components including cladding, rigid air barriers, insulation products, sheet materials or blankets and filler materials (not including gaskets, sealants etc.). This does not include other components that make up an external wall cladding system such as the structural framing or interior linings as these tend to be away from external flames and less susceptible to the initial ignition or fire spread.

With the C/AS2 and C/VM2 requirements, there are two means to assess the performance of cladding materials.

1. Classification as non-combustible or limited combustible

The term non-combustible has been traditionally used to describe materials such as glass, concrete, steel, masonry brick, ceramic tile or aluminium. For other materials, this can also be assessed through small scale testing of the materials meeting the relevant performance criteria in AS 1530.1 (where the material is classified as non-combustible) or BS EN 13501-1 (where the material is classified as A1).

Limited combustible is a term used to describe less stringent requirements of fire performance for materials that are essentially non-combustible but contain minor amounts of combustible material. This can also be assessed through small scale testing where materials are classified as A2 in accordance with BS EN 13501-1.

2. Testing through ISO 5660-1 or AS/NZS 3837

ISO 5660-1 or AS/NZS 3837 are other small scale test methods that rely on the use of the cone calorimeter to assess the performance of cladding materials. This has historically been used in New Zealand to classify the performance of Type A or Type B cladding materials. These classifications, in themselves, do not indicate that a material does not burn. However, in specific applications, materials meeting these classification may not significantly contribute to fire spread.

Although individual materials may meet these requirements, there are situations where they may contribute to an undesired performance when trying to manage vertical fire spread risk.

Large or intermediate scale fire testing of external wall cladding systems

Where the risk (either likelihood or consequence of failure) of vertical fire spread is higher, the fire performance of the external wall cladding system must be assessed as a whole system. All cladding materials and products are considered as part of a system. Materials used to construct a building envelop must be designed as a complete cladding system rather than as separate items. For this assessment, the term external wall cladding system is used which includes all substantive components within the complete wall assembly. This includes sheet cladding materials, framing, rigid air barrier, any insulation, sheet materials or blanket and the internal lining. Where relevant, the direction of fire exposure to be considered is from the exterior side of the wall.

The amendments to the Acceptable Solution C/AS2 and Verification Method C/VM2 published in November 2020 include intermediate and large scale fire testing options that can be used to assess the performance of an external wall cladding system. This includes:

• National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components.
• BS 8414-1:2015 Fire performance of external cladding systems. Test method for non-loadbearing external cladding systems applied to the masonry face of a building. Amended by BS 8414-1:2015+A1:2017 (June 2017).
• BS 8414-2:2015 Fire performance of external cladding systems. Test method for non-loadbearing external cladding systems fixed to and supported by a structural steel frame. Amended by BS 8414-2:2015+A1:2017 (June 2017).

The NFPA 285 is described as an intermediate scale test as specimens in this test are smaller in size than other large scale fire test methods (including the BS 8414 test).

The BS 8414 test series does not include classification of performance. The performance criteria that an external wall cladding system must meet is separately described in two documents:

• BR 135 Fire performance of external thermal insulation for walls of multistorey buildings - Third edition
• AS 5113: 2016 Classification of external walls of buildings based on reaction-to-fire performance Amendment 1

BR 135 Fire performance of external thermal insulation for walls of multi-storey buildings – 3rd edition 2013

BR 135: 2013 addresses the principles and design methodologies related to the fire spread performance characteristics of non-loadbearing external wall cladding systems. Although various potential design solutions have been identified and discussed in BR 135, robust design details are not presented. In this rapidly changing market generic solutions are not available where new products and novel design solutions are frequently presented. The illustrations and scenarios presented in BR 135 are based on typical examples of current practice in the UK. To help designers and end users better understand the parameters impacting on the fire-safe design and construction of external wall cladding systems, BR 135 focuses on the issues surrounding the topic of external vertical fire spread.

BR 135 Annex A provides a classification system for the test methodology outlined in BS 8414-1 Fire performance of external cladding systems – Part 1: Test method for non-loadbearing external cladding systems applied to the face of the building.

BR 135 Annex B provides a classification system for the test methodology outlined in BS 8414-2 Fire performance of external cladding systems – Part 2: Test method for non-loadbearing external cladding systems fixed to and supported by a structural steel frame.

Other construction systems such as concrete-framed or timber-framed construction are not considered in BR 135. However, the general principles in the BR 135 guide may still apply, although suitable additional risk assessments and detail design reviews may be required.

What is specifically excluded from external wall cladding systems for compliance with C3.5 and C3.7?

For the purposes of an external wall cladding system demonstrating compliance with the Building Code for Protection from Fire, substantive components may exclude:

• signage and billboards – aggregated area up to 25 m²
• video screens up to 6 m²
• greenwalls – the acceptance of green and living walls will be dependent on the type of system proposed, its support structure and the associated management and maintenance/irrigation procedures. Generally, plants growing on metallic support systems (such as stainless steel wires) will not present an increased fire hazard provided they are adequately maintained. Other systems that include combustible support systems should be proven via fire test evidence to support compliance. For more information on greenwalls refer to:
  
  ANS Living Walls receive a Fire Safety Standard on the ANS global website
  
  Fire Performance of Green Roofs and Walls on the GOV.UK website
• sunscreens/sunshades/louvres up to 6 m² or any area if non-combustible
• any materials used as part of the external wall cladding system for the topmost floor provided the roof does not require a fire resistance rating. (Other requirements to prevent horizontal fire spread to other property may still apply e.g. limits on unprotected area and/or the ignitability of the wall cladding when located within 1 m of the relevant boundary.)
• doorsets and window frames (these are not included with the cladding requirements)
• sealants and tapes comprising < 5% of the wall area
• a canopy or balcony at ground floor level of buildings that exceed 10 m in height where it can be shown or is agreed that a fire is unlikely to spread from the area to the main external wall cladding
• minor trim and gutters, downpipes and fascias – limited amounts of materials are excluded from the requirements where it can be shown or is agreed that a fire involving the materials is unlikely to spread fire to the remaining parts of the external wall cladding or where they are remote from the main building cladding
• individual components on or within the wall assembly that are non-combustible but include a surface coating not more than 1 mm thick.

Note: the above exclusions are only relevant to each component when taken in isolation. Consideration needs to be given when the above items are combined as part of a whole system to determine the contribution of each component to the overall performance of the cladding system. For example, a video screen meeting the size limitations attached to a non-combustible cladding would require further consideration and might not be appropriate if attached to a combustible sunscreen or rainscreen system.

In-wall cavities

Continuous vertical channels and cavities within external wall cladding systems are known to promote upward vertical fire spread. Fire researchers have noted that when flames are confined within a vertical cavity or channel they elongate, leading to flame extension of up to five to ten times the expected unconfined flame lengths. This is true even in cavities without additional combustible materials present, but is made worse by the presence of combustible materials. This flame extension effect can support rapid, potentially unseen, fire spread within an external wall cladding system and must be limited.

The provision of cavity barriers within external wall cladding systems is important, particularly when combustible cladding, rigid air barriers and insulation products are used.

Cavity barriers based on fire-resisting construction tested to AS 1530.4 or similar and satisfying integrity and insulation ratings for at least 30 minutes are likely to provide an acceptable means of controlling flame spread within cavities. However, additional consideration is needed to ensure that cavity barriers within a facade system located at the junction of fire separations and the external wall assembly have adequate support, can remain in place for the period required, and provide the required level of fire resistance rating.

Examples of other potentially acceptable test standards that may be used for curtain wall systems include:

• ANSI/ASTM E2307 Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using Intermediate-Scale, Multi-story Test Apparatus, or
• BS EN 1364-4:2014 Fire resistance tests for non-loadbearing elements.

Technical assessment in place of test

Cladding products and systems range in nature and complexity. There are also a range of base wall assemblies that may impact upon how the outer weather-facing part of a cladding system product will perform. Examples include:

• Exterior Insulation Finish Systems (EIFS)
• High Pressure Laminates (HPL)
• external thermal insulation composite systems (ETICS)
• rain screen cladding
• structural insulation panel systems (SIPS)
• Expanded Polystyrene Systems (EPS)
• timber cladding.

Key system performance considerations that must be considered in a technical assessment are:

• combustibility of insulation
• combustibility of framing (e.g. timber frame)
• composition of rigid air barrier
• building underlay
• uninterrupted vertical cavity
• continuity of products.

In order for an external wall cladding system to be certified for fire safety performance, it needs to be constructed to replicate the details of the test. This includes, for example, framings, substrate, flashing details, gaskets, sealants and fixing mechanisms.

A technical assessment may be presented as part of the plans and specifications to demonstrate compliance with the performance requirements of the Building Code. Situations may arise where the proposed cladding system installation differs slightly from the absolute details of that described in a fire test report. A technical assessment must be provided by accredited testing laboratory or from a subject matter expert with knowledge and experience in fire science and fire testing.