Cavity barrier

A cavity barrier is a construction that closes openings within a concealed space against the passage of fire, or that restricts the spread of fire within such a space. A concealed space is any part of a building, other than a protected shaft, that cannot be seen from an occupied space. Typical examples are roof spaces, ceiling cavities, wall cavities, spaces under floors and platform floors. Such voids offer a hidden path for smoke and flames, so a fire inside them can go unnoticed for a long time and break out far from where it started.

Concealed spaces in a building and the cavity barriers that divide them. Based on Australian Building Codes Board (2020b), p. 75, and Ministry of Business, Innovation and Employment (2025b), pp. 41 and 62.

By closing or dividing a void, a cavity barrier limits how far a fire can travel inside the hidden space. It works alongside fire separations between firecells and fire stops around penetrations, which together maintain fire compartmentation. In New Zealand, C/VM2 (second edition, 2025) accepts cavity barriers as one way to satisfy its design scenario for a fire that starts in a concealed space. The same document requires cavity barriers at every floor level in cavities of external walls, to limit vertical fire spread on the facade.

Concealed spaces and hidden fire spread

Concealed spaces take many forms. The C/VM2 commentary lists floor plenums for IT cables, ceiling plenums, service shafts and curtain wall cavities, each with its own range of ignition sources and fuels. The International Fire Engineering Guidelines add spaces within hollow construction and the spaces behind exterior cladding on the inside of building facades. For the analysis of fire spread beyond the room of origin, these guidelines treat such features as separate enclosures.

The Australian handbook to the Fire Safety Verification Method describes why these spaces are a concern. Fire and smoke can spread unseen for a considerable period and damage the structure and the fire barriers before breaking out in a space remote from the point of ignition. Combustible linings and insulation inside the void can accelerate the spread substantially and cause multiple ignitions throughout a building, unless measures such as cavity barriers are applied. Combustible services and structural members in the void can also carry fire, but to a much smaller extent. A typical reference scenario is an unreported ignition inside a cavity, caused by maintenance work or an electrical fault, that keeps spreading undiscovered.

Hidden fires are also a problem for the fire service. Cavity fires are hard to locate and to reach, which can delay intervention and force the evacuation of large parts of a building. C/VM2 notes that fire spreading in concealed spaces may compromise the ability of firefighters to assess the threat during rescue and firefighting operations. A description of post-1968 high-rise office buildings in New York City, used by the fire department in 2001, noted that their ceiling plenums were extensive and lacked fire stopping. These plenums served as return air paths for the air-conditioning system and carried electrical and communications equipment.

Function and construction

A cavity barrier has two tasks in the C/VM2 definition: closing openings within a concealed space against the passage of fire, and restricting the spread of fire within such spaces. It is related to, but not the same as, a fire stop. C/VM2 defines a fire stop as a material or method of construction that restricts the spread of fire within or through fire separations, with a fire resistance rating no lower than that of the separation. Fire stops are mainly used to seal around penetrations, but can also close narrow gaps between building elements. The NIST reports on the World Trade Center use a broader definition, in which a firestop is "a solid or compact, tight closure to retard the spread of flames or hot gases within concealed spaces". The article Firestopping covers the sealing of penetrations and joints in more detail.

The performance of a cavity barrier is expressed in the same way as for other fire separating elements. In New Zealand the fire resistance rating (FRR) consists of three times in minutes, for structural adequacy, integrity and insulation, always in that order. Integrity is the time for which a test specimen prevents the passage of flame or hot gases, and insulation is the time for which it limits heat transmission. The rating -/30/30 that C/VM2 asks of some cavity barriers therefore sets no load-bearing requirement, but asks for 30 minutes of integrity and 30 minutes of insulation. Such ratings come from tests in which constructions are exposed to the standard fire curve, an agreed temperature course that allows tested constructions to be compared rather than a model of a real fire.

The sources describe several materials for closing hidden voids. The 1968 New York City Building Code required firestopping and fill materials to be approved noncombustible materials that could be shaped, fitted and permanently secured in place. It required hollow vertical fire divisions to be firestopped with at least 4 in. (about 100 mm) of noncombustible material, so that flame, smoke and hot gases could not pass to the storey above or below. In its report on World Trade Center Building 7, NIST lists mineral fibres, sealants, mortar and putties among the materials used for firestopping. C/VM2 adds that fire and smoke separations should be fire stopped with proprietary products that suit the orientation and the separating element in question.

Requirements in performance-based design

In New Zealand, C/VM2 (second edition, 2025) contains design scenario CS, "Fire starts in a concealed space". It applies to buildings that have concealed spaces and rooms holding more than 50 occupants. It does not apply if the concealed space contains no combustibles other than timber framing and has no more than two dimensions greater than 0.8 m. Because fire spread within concealed spaces is difficult to model, the method is prescriptive in character. The designer either confines the fire to the concealed space with cavity barriers or automatic suppression, or installs automatic heat or smoke detection inside the concealed space.

Where a concealed space has no automatic fire detection, its cavity barriers need a fire resistance rating of at least 30 minutes (-/30/30). The concealed space may then not exceed 500 m² in area. The first edition of C/VM2 contained the same rule, and its commentary explains that quantitative analysis is not expected for this scenario. In the commentary's worked hotel example, plumbing shafts of 0.8 m by 0.8 m serving each guest room are fire stopped at every level to maintain the fire resistance between floors. With that measure the design is stated to comply with scenario CS.

Design scenario CS of C/VM2 (second edition, 2025): when it applies and which solutions satisfy it. Based on Ministry of Business, Innovation and Employment (2025b), p. 41.

In Australia, the Fire Safety Verification Method Standard 2022 includes a comparable concealed space scenario. It asks the designer to show that fire spread in concealed spaces will not endanger occupants in other rooms, and that the level of safety is at least equal to the Deemed-to-Satisfy Provisions. The expected solution is separating elements or suppression to confine the fire to the concealed space, automatic detection inside it, or a combination of both. The handbook notes that the Deemed-to-Satisfy Provisions already permit unprotected voids within limits on their size and on the combustibility of linings and contents. During hazard identification, the designer records every void, its combustible boundaries and contents, and the intended mitigation, such as cavity barriers.

The handbook lists five typical mitigation measures for this scenario. These are cavity barriers, protection of service penetrations, control of materials, sprinkler protection where practicable, and automatic heat or smoke detection inside the concealed space. Sprinklers can be impractical in smaller voids, because structural members and other obstructions get in the way. The SFPE Handbook shows that sprinkler design itself must account for concealed spaces. In the US, NFPA 13 (1999) modifies the sprinkler design area where nonsprinklered combustible concealed spaces exist in a building. In risk models, a sprinkler system may be assumed to stop a fire, except for a fire that starts in a concealed space; there it only blocks the fire from entering sprinklered areas.

Cavities in external walls

The International Fire Engineering Guidelines count the spaces behind exterior cladding among the concealed spaces through which fire can spread. C/VM2 (second edition, 2025) therefore requires that fire spread through cavities in an external wall be avoided by cavity barriers at each floor level. These barriers must meet subsection 4.11.2 of the Acceptable Solution C/AS2. The rule is part of design scenario VS, external vertical fire spread, which deals with facade fire risk. For buildings taller than 10 m, and for some other buildings, this scenario aims to keep fire from spreading more than 3.5 m vertically.

Cavity barriers at each floor level in an external wall cavity (left) and a firestop in the slab edge gap of a curtain wall (right). Based on Ministry of Business, Innovation and Employment (2025b), pp. 47 to 48, and McAllister et al. (2008), p. 44.

The same subsection sets the exposure that the cladding system must withstand. The design fire exposes the facade to a radiant flux of 50 kW/m² for 15 minutes in importance levels 2 and 3, and to 90 kW/m² for 15 minutes in importance level 4. The cladding system must consist of non-combustible or limited combustible materials, or meet the acceptance criteria of the Building Product Specifications. The scenario addresses spread over the cladding materials as well as spread within the external wall cladding system, and the cavity barriers deal with the second route.

A related joint exists where a curtain wall meets the floor slab. NIST describes how gaps between a floor slab and a curtain wall are sealed with firestopping materials to reach the rating required by codes. The drawings of World Trade Center Building 7 specified a compressible "thermafibre" firestop on impaling pins between the floor slabs and the curtain wall. An inspection of Floor 23 in 1993 found gaps, and a later document reported the firestopping as satisfactory.

Code history and the World Trade Center

Prescriptive codes have long subdivided hidden spaces. The 1968 New York City Building Code required firestopping of concealed spaces within partitions, walls, floors, roofs, stairs, furring, pipe spaces and column enclosures that would let flame or hot gases pass between floors. The listed locations included hollow partitions, stair construction, ceiling spaces, exterior cornices and duct and pipe spaces. The space above a fire-rated ceiling had to be firestopped into areas of no more than 3,000 ft² (about 279 m²). No such subdivision was required where the structural members in the space were individually protected or where the space was sprinklered.

Codes of the same period differed widely. NIST found that the 1968 New York City code set out in detail when and where firestopping was required, the 1964 New York State code was less detailed, and the Chicago code had none. The NFPA Life Safety Code required firestopping of partitions at floor level and allowed a trade-off for sprinklered concealed spaces. The US construction classification quoted in NUREG-1805 does not permit concealed spaces at all in Type IV heavy timber construction, which relies on wooden members of large cross-section. The sources consulted do not discuss cavities in modern mass timber buildings.

The towers of the World Trade Center show how a cavity rule can be weakened in practice. Walls between tenants needed to continue through the concealed spaces below the floor and above the ceiling to achieve a 1 hour rating. The Port Authority instead stopped these walls at the suspended ceiling and used 10 ft strips of 1 hour rated ceiling on either side. NIST found no precedent for this approach. After a warning from the general contractor, the tenant guidelines were changed to require a continuous barrier from the floor to the underside of the slab above.

Tenant walls in the World Trade Center towers: the original detail with rated ceiling strips (left) and the later requirement of a continuous barrier to the slab (right). Based on National Institute of Standards and Technology (2005), p. 56.

Limits and reliability

Cavity barriers are mostly specified by rule rather than by calculation. The Australian handbook states that validated models for fire spread through voids and cavities are very limited. Designers therefore rely on standard tests, fire incident data, technical publications and protection measures such as cavity barriers. C/VM2 makes the same point when it gives a fixed method for concealed spaces instead of a modelling requirement. Both documents assume that the passive and active systems perform as intended for this check.

Because cavity barriers are hidden, faults are hard to find. The Australian data sheet on fire barrier effectiveness notes that sections of fire barriers in concealed spaces may be left out, because they cannot be readily inspected. No data on this defect were found, so the data sheet derived an estimate from a survey of missing fire protection on steel beams. That survey suggested 0.07 % to 0.15 % of elements with a gross defect. Applying a factor of safety of 3.33, the data sheet suggests a probability of 0.005 (0.5 %) per element that a large area of fire barrier is missing within a concealed space.

Openings in barriers also reduce their value once a fire has started. The International Fire Engineering Guidelines list openings caused by missing fire stopping or by inadequately fire stopped penetrations among the routes for fire spread. Loss of integrity through cracks, fissures and the failure of firestopping is one of the ways in which a barrier can fail. The guidelines allow barrier failure to be taken as the moment that fire spreads into the adjacent enclosure. In the Netherlands, building regulations break their main goals down into sub-goals that include keeping compartments intact and limiting the spread of fire and smoke. Cavity barriers serve these sub-goals inside the hidden parts of a building.

See also

• Passive fire protection
• Fire wall
• Fire damper
• Intumescent material

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
• Australian Building Codes Board. (2022). Fire Safety Verification Method Standard 2022.
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• Grill, R. A., & Johnson, D. A. (2005b). Fire protection and life safety provisions applied to the design and construction of WTC 1, 2, and 7 and post-construction provisions applied after occupancy (NIST NCSTAR 1-1D). National Institute of Standards and Technology.
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• Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
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