Fire barriers in external walls do one job: they stop the hidden cavity inside the wall build-up from acting as a chimney. A modern external wall — masonry with insulated cavity, or a framed wall behind cladding — contains continuous concealed voids, and fire moves through concealed voids faster than through almost anything else in a building. Cavity barriers close those voids at the lines where fire must not pass: around openings, at compartment lines, at the junctions the design says matter.
The regulatory context
The requirements come from the Building Regulations 2010, with Approved Document B as the working guidance on where cavity barriers are needed and what they must achieve. Since Grenfell, the external wall has become the most scrutinised element in the building: regulation 7 restricts what materials may be used in the external walls of relevant buildings, and the Building Safety Act 2022 regime — with its golden thread of building information — exists in large part because nobody could show, after the fact, what had actually been installed in walls that were signed off.
That last point is the one that lands on site paperwork. For a higher-risk building the law now expects the as-built record to prove what went in; for every building, the practical position is the same — a barrier that cannot be evidenced might as well not be there when the questions start.
Why installation is where it goes wrong
A fire barrier is a fitted component doing a precision job in an unforgiving place. The failure modes are mundane:
- the barrier is missing where the design shows one — sequencing pressure, or nobody owned that interface
- it is the wrong product for the void — a barrier tested for one cavity width squeezed or stretched into another
- it is fitted with gaps, unsupported edges, or crushed behind services that came later
- it is perfect, and then the following trade cuts through it and nobody goes back
Every one of those is invisible within days, because the next operation closes the wall. That is the defining feature of this work: it can only be inspected while it is still open.
What the record has to show
Because the work disappears, the inspection record is the product. From the approving side of the desk, what a principal contractor wants against each barrier line is simple to say and demanding to keep: what was installed (product, and the certification behind it), where, by whom, when, and a photograph taken while the void was still open — then a signature from someone competent to say the installation matches the design. Stage-by-stage, closed area by closed area, not one tick at the end of the package.
Contractors sometimes read that as bureaucracy. It is the opposite: the installer who can hand over that record is the one who never gets the phone call three years later asking them to prove what is behind the cladding.