Cavity barriers

Where the risk comes from

A solid masonry wall has no cavity. A light gauge steel wall, however, has air space between the studs, behind the lining, in the service cavity and in the ventilated facade gap — and these are not separate boxes; they can join up.

If fire gets into such a cavity, it:

• spreads hidden: nobody sees it behind the lining,
• spreads fast: a chimney effect develops in a vertical cavity,
• is hard to extinguish: intervention requires opening up.

This phenomenon is not a fault of the technology but a consequence of it — and that is why there is an established solution.

What a cavity barrier does

A cavity barrier is an element that closes the cavity in a fire. It can be a solid mineral wool insert, a sheet, or an intumescent profile that fills the space when heated.

Two things are expected of it: that it stays in place during the fire, and that it closes the cavity across the full cross-section — a half-filled gap is not a cavity barrier.

Where they are needed

The typical locations:

• at floor junctions, so the cavity does not run between storeys,
• at compartment boundaries, in the plane of both wall and floor,
• in the facade cavity, horizontally at each storey — this is also the barrier against facade fire spread,
• in the roof space, where the cavities of the roof plane and the wall meet,
• within long, uninterrupted cavity runs, if the regulation requires it.

The practical problem

A cavity barrier is a cheap, small element that is very easy to leave out — and almost impossible to add later, because the lining is already up. So:

• it has to be on the drawings, not left to the contractor’s goodwill,
• it has to go in in the erection sequence, before the lining closes the cavity,
• it belongs among the inspection points: it can be checked before lining, not after.

The relationship with insulation

Where the structure is fully filled with insulation — injected foam or full-depth mineral wool — part of the cavity does not exist in the first place. That helps, but it does not replace cavity barriers at storey and compartment boundaries, and it solves nothing in the ventilated facade cavity.

The point

The cavity barrier is the cheapest fire safety element and the easiest to forget. If it is not on the drawings it will not be in the wall — and afterwards it can only be added by opening up.