Fire Stopping: Penetration Seals, Cavity Barriers & Intumescent Products

Summary

Fire stopping is one of the most frequently overlooked and incorrectly installed elements of building work. Many tradespeople — particularly plumbers, electricians, and HVAC engineers — cut holes through walls and floors for services and leave them unsealed, or seal them with expanding foam (which is not a fire-stopping product). The consequence of inadequate fire stopping can be devastating: a fire that should have been contained to one room or compartment spreads rapidly through service penetrations, reducing escape time significantly.

Building Regs Approved Document B (Volume 1: Dwellings) sets out the fire-stopping requirements for domestic properties, including which elements are fire-resisting, what performance standard is required (measured in minutes of fire resistance), and what products are acceptable. For commercial and larger residential projects, Approved Document B Volume 2 applies.

The third-party certification of fire-stopping products is critical. Products should be tested to BS EN 1366-3 (linear seals) or BS EN 13501-2 (system classification) and carry CERTIFIRE or similar third-party approval. Installers should keep records of all fire-stopping work done — increasingly, fire-stopping surveys are part of building safety compliance for high-rise residential.

Key Facts

• Approved Document B — references clauses 8.2–8.5 for concealed spaces and fire stopping; Section 10 for cavity barriers
• Compartmentation — fire-resisting separation between different occupancies, or different floors in a building, to limit fire spread
• Penetration seal — a specific product or combination of materials that restores the fire resistance of a wall or floor where a service penetrates it
• Intumescent sealant — expands when heated (typically from ~150°C); fills the void left when a plastic pipe or cable melts; rated to specific FRL (fire resistance level) e.g., 30/30/30 (stability/integrity/insulation minutes)
• Intumescent collar/wrap — a mechanical device fitted around plastic pipes at the wall/floor; pipe melts and collar expands to close the opening; used where access to both sides of the wall is not possible
• Mineral wool pipe wrap — used with intumescent sealant for larger pipe sizes to restrict airflow
• Steel pipes — do not need intumescent treatment (they don't melt) but the annular gap around them still needs to be fire-stopped
• Cable trays and conduits — must be fire-stopped where they pass through compartment walls; intumescent pillows or blocks fill the conduit interior where plastic conduit melts
• Cavity barriers — must be installed at: the edges of cavities (at every floor and ceiling level), around openings in cavities (door/window frames), at the junction of cavity walls/partitions, and at maximum 8m horizontal intervals within roof spaces
• Cavity barrier materials — minimum 30-minute fire resistance; typically mineral wool, steel, or intumescent-based; must allow normal movement (e.g., in steel frame buildings)
• Fire-resistant expanding foam — standard polyurethane expanding foam (e.g., Polycell Great Stuff) is NOT fire-stopping; specific fire-rated foams exist (e.g., Hilti CF912, 3M Fire Barrier Foam) but must be used with appropriate certification
• Collars vs sealant — for plastic pipes ≤160mm diameter penetrating fire-resisting walls, intumescent collars or pipe wraps are used; sealant alone is only suitable for cables and rigid pipes ≤40mm
• EPS/PIR insulation — rigid insulation in walls must not bridge fire compartment boundaries; fire stops must be installed at compartment boundaries even within insulated cavities

Quick Reference Table

Detailed Guidance

Which Elements Require Fire Stopping

Not every wall and floor requires fire stopping. The requirement applies to fire-resisting elements — those that are part of the compartmentation strategy:

In dwellings (Approved Document B, Volume 1):

• Separating walls between dwellings (e.g., party walls between terraced or semi-detached houses) — typically 60 minutes fire resistance required
• Separating floors between flats — typically 60 minutes
• Floor between ground floor and first floor in a house with open-plan lower floor where the stair is not enclosed — may require upgrading
• Protected staircase enclosure — walls and ceilings forming the stair enclosure in a 3-storey house; typically 30-minute fire resistance

In commercial/residential (Volume 2):

• All compartment walls and floors (varies by building type and height)
• Protected corridors and lobbies in flats and HMOs

Where you're working through a non-fire-resisting partition (e.g., an internal stud wall within a single dwelling), standard sealing with acoustic/fire sealant is good practice but not technically required by Part B.

Intumescent Products — How to Specify

Always specify products by their certification, not just generic type. Key certifications:

• CERTIFIRE (by Warringtonfire) — most common UK third-party certification for fire stopping products
• UL (Underwriters Laboratories) — US standard, accepted in some situations
• BS EN 1366-3 tested — duct and pipe penetration test
• BS EN 13501-2 — fire classification of construction products

Key product families used on UK sites:

• Hilti: CFS-S ACR sealant, CFS-IC intumescent collar — widely specified in commercial
• Quelfire: QF360 collar, intumescent sealant — strong in residential market
• Rockwool: Rockcollar and ProRox pipe wrap
• 3M: fire barrier caulk, intumescent foam CP25WB+

Always read the system datasheet — the fire rating applies to a specific combination of products installed in a specific way (pipe size, wall thickness, sealant depth). You cannot mix components from different systems and retain the certification.

Cavity Barriers in Roof Spaces

Approved Document B requires cavity barriers in roof spaces at:

• Every compartment wall junction (at the point where the wall meets the roof)
• At the junction of the roof and every external wall
• At horizontal intervals not exceeding 8m in the plane of the ceiling
• At vertical intervals not exceeding 8m

In a simple semi-detached house, the party wall must continue through the roof space to the underside of the roof covering (or close the cavity there). This is a commonly missed requirement on extensions and loft conversions.

For timber-framed buildings, cavity barriers must be installed at all edges of cavities within the timber frame construction, at every floor level, and at roof/wall junctions.

Cavity barrier materials: must achieve 30 minutes integrity. Common options:

• 38mm timber (in a ventilated roof space, adequate as a barrier)
• 25mm mineral wool slab (e.g., Rockwool Flexi 1000 fixed with binders)
• Steel perforated strip at flank wall/roof junction
• Proprietary cavity closers with integral fire-stopping (Birtley, Cavity Trays Ltd)

Fire-Stopping on Older Properties

Pre-2000 construction rarely has correctly installed fire stopping at penetrations — pipes were commonly run through walls with no sealing at all, or with standard foam. When working on an older property where penetrations are exposed (e.g., during a rewire or replumbing), fire stopping should be retroactively installed.

This is also a client-care issue — if you're a plumber who discovers that the boiler flue passes through an unsealed party wall, advise the client in writing. You have a duty of care to flag significant fire safety deficiencies, even if it's outside your immediate scope of work.

Fire Dampers in Ductwork

Where ventilation ductwork passes through a fire-resisting compartment wall or floor, either:

• A fire damper is installed (automatically closes on actuation of a fusible link or detection system), or
• An intumescent ductwork wrap is applied around the duct for the length within the wall

Fire dampers must be accessible for maintenance and testing. They must be tested annually (BS 9999 recommends; HVCA guidance). Omitting fire dampers in ductwork through compartment walls is a significant life-safety deficiency.

Frequently Asked Questions

Is expanding foam acceptable for fire stopping?

Standard polyurethane expanding foam (the type in a can from a builders' merchant) is not classified as a fire-stopping product. It has very low fire resistance and will burn. Specific fire-rated intumescent foams exist (e.g., Hilti CF 912, 3M Fire Barrier 1000 NS) but these are much more expensive and have specific application requirements. Always use the correct certified product.

Do I need to fire-stop plastic pipe in a single-dwelling house?

Where the pipe passes through a compartment element (e.g., party wall, floor between separate flats), yes — intumescent collars are required. Within a single-dwelling (e.g., pipes running between floors of the same house where neither floor is a compartment boundary), fire stopping is not strictly required by Part B, though fire sealant to close the annular gap is good practice.

How do I access the back of a wall to install fire stopping?

For intumescent collars on plastic pipes, access to both sides is not needed — the collar is installed on one face and the wall penetration is sealed on the other with sealant only (or from the accessible side with an approved single-sided system). Always follow the product's certification sheet for single-sided vs double-sided installation — not all products are rated for single-sided use.

Who is responsible for fire stopping — the plumber, electrician, or builder?

Each trade is responsible for the penetrations they make. The plumber fire-stops pipe penetrations; the electrician fire-stops cable penetrations. The main contractor (if there is one) has an overarching responsibility to ensure all penetrations are closed. A building control inspection at the right stage should check this, but in practice many penetrations are hidden before inspection.

Regulations & Standards

• Building Regulations Approved Document B Volume 1 (dwellings) and Volume 2 (buildings other than dwellings) — fire compartmentation and fire stopping requirements

• BS 9999:2017 — code of practice for fire safety in the design, management, and use of buildings

• BS EN 1366-3 — fire resistance tests for service installations — penetration seals

• BS EN 13501-2 — fire classification of construction products and building elements

• PAS 79-1:2020 — fire risk assessments in buildings; relevant to fire stopping compliance checks

• Regulatory Reform (Fire Safety) Order 2005 — fire safety responsibilities for responsible persons in non-domestic/HMO premises

• GOV.UK — Approved Document B — compartmentation and fire stopping clauses

• Quelfire Technical Library — product certification and installation guides

• Hilti Fire Stopping — CERTIFIRE-certified system guides

• Rockwool Technical Documentation — cavity barriers and pipe wraps

• smoke alarms — smoke alarm positioning and interlink requirements

• escape routes — protected stairways and means of escape

• building control — building control inspection stages

• loft conversions — cavity barriers in roof spaces on loft conversions