What is a cavity barrier?
A cavity barrier is a construction fitted inside a concealed space to stop fire and smoke spreading through it. Cavity barriers subdivide voids and close the gaps around openings in them.
Concealed spaces are dangerous precisely because a fire in one is not visible and is not fought. Barriers limit how far it can travel before it emerges, which buys the time the rest of the strategy assumes.
They are used in ceiling voids, floor voids, roof spaces and within wall constructions. In external wall systems in particular their correct installation has been the subject of significant scrutiny.
They fail in the same way fire stopping does: another trade needs to get a service past one, and the barrier is cut and not restored.
In practice
A barrier installed and then cut for a duct.
The duct had to go somewhere and the barrier was in the way. The hole was never made good.
Cavity barriers fail at the point where another trade needed to pass through them, and the sequence of work makes that almost inevitable without inspection.
The inspection has to happen before the cavity is closed, which is a programme decision rather than a quality one.
What goes wrong in practice
- Cut for services with no reinstatement and no record.
- Assumed present without any survey confirming where they are.
- Compressed or displaced during other works, leaving a nominal barrier with a gap behind it.