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CROSS Safety Report

Fire protection to structure by cavity barriers

Report ID: 1231 Published: 20 November 2023 Region: CROSS-UK


Overview

A reporter is concerned about a potential misconception in the construction industry regarding the role of cavity barriers and the impact their design and installation can have on the structural performance of a building.

Key Learning Outcomes

Fire and structural engineers/designers:

  • As cavity barriers have a role in protecting the structure as well as inhibiting the spread of fire and smoke, they should be specified carefully, in particular when they have a role in protecting the structure

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A reporter contacted CROSS about a potential misconception in the industry regarding the role of cavity barriers and the impact their design and installation can have on the structural performance of a building.

In the reporter's view, cavity barriers are needed for construction technologies that incorporate cavities in their final assembly. Technical guidance for such instances is outlined in Approved Document B and Regulation B3 - Internal fire spread (structure) of the Building Regulations 2010 must be satisfied:

"The building shall be designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited."

The guidance sets out two approaches to meet this regulation. The first approach is to subdivide cavities with cavity barriers (resisting fire spread within the cavity). The second is to close the edges of cavities (resisting fire spread into the cavity). The minimum recommended fire performance is E30 and I15.

the reporter would like to highlight a further intent set out in Approved Document B which is to inhibit the unseen spread of fire to reduce the likelihood of structural failure

In the reporter's view, this functional requirement is specific to limiting unseen fire and smoke spread, with the aim to protect occupants, users, and first responders situated away from the compartment of origin. This may set a specific bias to practitioners that the role of cavity barriers is solely to inhibit the spread of fire and smoke. However, the reporter would like to highlight a further intent set out in Approved Document B which is to inhibit the unseen spread of fire to reduce the likelihood of structural failure. Functional requirement B3(1) on internal fire spread (structure) states:

"The building shall be designed and constructed so that, in the event of a fire, its stability will be maintained for a reasonable period."

Some forms of construction, such as some panel walls, some floor systems, and some light framing solutions rely on their enclosing sheathing/linings to protect the structural elements located within a cavity. Such systems rely on the integrity of these linings to achieve their rated fire resistance.

in most cases the cavity barrier achieves substantially lower fire resistance compared to that which is recommended for the structural frame

In the case of external loadbearing walls, the possibility exists that the wall will be exposed on two sides at once; on the internal side by the compartment fire, and on the external side by venting flames or hot gases. This is not explicitly covered in the current technical guidance, which considers exposure on one face only. The reporter is of the mind that these walls may form part of the structural frame, and their performance should be investigated for simultaneous exposure on each side.

Fire can circumvent internal protection linings and heat structural elements by entering a cavity where there are penetrations in the wall, e.g., at a window. This route of fire spread is mitigated by installing a cavity barrier. However, the cause of concern is that in most cases the cavity barrier achieves substantially lower fire resistance compared to that which is recommended for the structural frame.

It is understood by the reporter, through experience from reviewing fire incidents, that fire or hot gases can bypass the sheathing/lining and enter the cavity early to affect structural elements. This can also occur due to either a failure (or damaged state) of the sheathing protection, or occupant interventions like a fixing that was not installed appropriately. Another possibility is that this could occur due to an unprotected ventilation mesh grill on the outside of external walls (which may be exposed to external venting flames e.g., from windows, or from balcony fires).

Even where fire and smoke do not spread extensively within a cavity, there is a possibility that the loadbearing elements will be exposed to elevated temperatures at an early stage

Incorrectly specified barriers at the edge of such a construction, e.g., around a window opening, could compromise the protection to the structure. Typical E30 and I15-rated cavity barriers, recommended for purposes of resisting unseen fire spread, are unlikely to provide sufficient protection to the sheathing studs and columns.

Even where fire and smoke do not spread extensively within a cavity, there is a possibility that the loadbearing elements (columns, beams, or slabs) will be exposed to elevated temperatures at an early stage – thermal degradation, or decomposition if combustible construction is used, will then initiate. Apart from the inherent difficulties in suppressing such unseen fires, this may also lead to localised collapse of the structural elements, which in turn may render other cavity barriers ineffective and lead to subsequent fire and smoke spread beyond the compartment’s cavity. This, of course, would be a matter of more significant concern in buildings where cavity barriers were incorrectly designed and installed.

In designs where the structure needs protection from fire, cavity barriers may be chosen to provide this protection; designers should consider the standard required for the structure rather than the, potentially lower, standard for the cavity barrier alone.

The reporter considers that more clarity in current guidance is welcome, and it could be helpful if the concern was considered in future updates for clarification and highlighting

Designers should also seek to ensure that where sheathing linings are relied upon to protect the structure, these are adequately designed and installed for fire exposure on all relevant faces, which may include external faces, to maintain overall structural performance.

The reporter considers that more clarity in current guidance is welcome, and it could be helpful if the concern was considered in future updates for clarification and highlighting. Specifically, they are of the mind that any part of the structural frame, which might be exposed on any face, needs to be considered by designers for fire protection. This includes consideration of cavity barrier performance.

In the meantime, they believe it is helpful for the issue to be widely raised in the industry so that designers and building control bodies can give appropriate thought to the matter of structural protection when cavities exist, always in proportion to the size, height, and occupancy of the building.

Expert Panel Comments

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These comments merit appropriate project specific discussion between fire and structural engineers.

This report raises concern for cases where the structural elements are protected by a system (e.g. an internally plasterboard lined external wall construction with loadbearing studs) when there is a chance for the system to have openings, for example windows, doors, etc.

In such a situation, one of the objectives of the cavity barriers could be to contribute to the protection of the loadbearing wall components in the case of a fire penetrating the opening.

Thus, the required performance of the cavity barrier for the project should be considered accordingly, notwithstanding that compliance with ADB guidance requires the fire resistance for loadbearing external walls to be considered only from the inside face and that cavity barriers have a fire resistance of 30 minutes for integrity and 15 minutes for insulation.

The maximum heat flux in a fire plume exiting from an opening in an external wall will usually be directly above the opening and in many cases the external wall construction between that opening and the next opening above will not be loadbearing. Therefore, the proximity of loadbearing structure to the external fire plume would be a relevant consideration in a fire engineered approach in consultation with the structural engineer.

Where the fire engineer decides that consideration of external fire impact on loadbearing elements is merited then it may also be appropriate for the fire engineer to consider the use of the external fire exposure curve given in BS EN 1363-2.

Note this report was updated on the 12/12/2023 with further Expert Panel comments. 

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