Cavity barriers
A cavity barrier closes a hidden space, such as the gap behind cladding, in a timber-frame wall, above a ceiling or in a roof, so fire and smoke cannot race through it unseen. They are needed where a cavity meets a compartment wall or floor, around openings, and at intervals along long cavities.
Missing or wrongly fitted cavity barriers were one of the most common defects found in external walls after Grenfell. Fitting them in an existing building often means taking cladding or linings off, so they are usually priced after a survey.
- Sheet
- PF-07
- Subject
- Cavity barriers
- Scope
- England, Scotland, Wales
- Prices read
- November 2026
PF-07.1
When cavity barriers need attention
- An external wall or EWS1 review has found missing or gapped barriers.
- A survey shows ceiling or roof voids running across compartment walls.
- Timber-frame flats have no barriers at party walls or floor lines.
- Barriers have been cut through for services or windows.
- Cladding or insulation is being replaced and the cavity is open.
- Building control has asked for evidence of barriers on a new build.
PF-07.2
What cavity barrier work costs
| Item | Published figure |
|---|---|
| Barriers in an open cavity during building work Lowest cost: the cavity is already exposed | Priced per metre after survey |
| Barriers in an existing external wall Cladding or render taken off, barrier fitted, wall rebuilt | Priced after an intrusive survey |
| Ceiling and roof void barriers Closing voids above compartment walls | Often part of remediation |
| Example remediation contract including compartmentation, one block East Suffolk Council, May 2023, contract value as published | £332,329 |
Where these figures come from
The barrier itself is a small part of the cost; access to the cavity (scaffold, removing and rebuilding cladding or linings) is most of it, so retrofit work is priced against an intrusive survey. The public contract value is from the East Suffolk Council award notice. Read November 2026. The passive fire protection cost guide explains what drives the price.
PF-07.3
What sound cavity barrier work covers
The parts of the job that decide whether it holds up in a fire.
Barrier positions on a drawing
Where each barrier goes: at floor lines, compartment walls, around windows and doors, and at spacing along long cavities.
The right product for the cavity
Barriers are tested for a cavity width and wall type. Open-state barriers that let air through behind cladding must suit the ventilation gap.
Continuous, no gaps
Joints butted or lapped and fixed, with no gaps at corners or around brackets and services.
Third-party certification
Whether the firm holds a third-party installer scheme for this work, and the certificate number so you can check it on the scheme register yourself.
Records you can keep
Photographs of every item before and after, a unique reference or label for each, and a schedule you can hand to the next fire risk assessor or put in the golden thread.
Making good
How the cladding, render or lining is put back, and who checks it is weathertight.
Found missing barriers behind cladding or above ceilings? Tell us the building and get it fixed.
Find a contractorPF-07.4
How cavity barrier work runs
Brief
Share the fire risk assessment, the fire strategy if there is one, floor plans and any earlier survey. Say what prompted the work and any deadline.
Intrusive survey
Openings made at sample points to see what is there and where barriers are missing.
Quote
A priced schedule against the survey, naming the tested product for each item and listing exclusions.
Works
Items are fixed in a planned order, with access agreed with residents or occupiers and hot works, asbestos and permits handled first.
Handover
Photographs, labels and certificates for every item, handed over for your records and the next fire risk assessment.
PF-07.5
Questions people ask
What is a cavity barrier?
A fire-resisting product, often mineral wool in a sleeve, a rigid board or an intumescent strip, fitted inside a cavity to close it and stop fire and smoke spreading through it.
What is the difference between a cavity barrier and fire stopping?
Fire stopping seals holes and gaps in a fire-resisting wall or floor. A cavity barrier closes a hidden void inside or between building elements. Both are part of keeping compartments intact.
Where are cavity barriers needed?
Approved Document B expects them at the edges of cavities, around openings, where a cavity meets a compartment wall or floor, and at intervals in long or large cavities. Exact positions depend on the building and the wall build-up.
What is an open-state cavity barrier?
A barrier that leaves a gap for ventilation and drainage behind cladding, and swells to close it in a fire. It must suit the size of the gap it was tested with.
Can cavity barriers be fitted without removing cladding?
Sometimes from inside the building, or through openings at specific points, but often the cladding or lining must come off. Your surveyor will say what access is needed.
How much do cavity barriers cost?
The barrier itself is cheap; access is the cost. Fitting barriers into a cavity during building work is a small item, while retrofitting behind cladding means scaffolding and rebuilding the wall. Expect a price only after a survey.
Find a cavity barrier contractor
Three short steps: where the building is, what the job is, and how to reach you. It takes about two minutes.
This is a free matching service, not a passive fire protection contractor. We find one who covers your area and name them to you first. Nobody contacts you until you say yes.
Thanks, your enquiry is in.
- Within one working day we will email or call to tell you which passive fire protection contractor covers your area.
- Your details reach that firm only if you say yes.
- They look at the site and give you a fixed price. No marketing from us, ever.