CPD 10 2026: Fire safety and the building envelope

Learning objectives

• Recognise the role of the building envelope in limiting fire spread.
• Understand relevant building safety legislation.
• Know how the Building Safety Act’s gateway regime, golden thread and extended-liability provisions affect specification.

Combustibility and the building envelope

The fire performance of a wall is only as good as its weakest detail; the cladding panels, underlying insulation, cavity barriers and fixings that penetrate the build-up all interact to determine performance.

Combustible insulation materials are a source of fuel. When exposed to heat, combustible insulation degrades, releases smoke and other toxic gases and can contribute to fire growth. Some combustible insulation materials may not visibly burn as a solid but, under fire conditions, will decompose and release flammable gases that can ignite and contribute to fire growth. Understanding whether a product adds to fire load is therefore a key part of assessing its suitability for use in the building envelope.

Reaction to fire

“Reaction to fire” refers to how a material, product or system behaves when it is exposed to fire, including its contribution to ignition, flame spread, heat release and smoke production. The reaction to fire of construction products is now classified under the Euroclass system using BS EN 13501-1, which replaced the old British standard BS 476 reaction to fire tests in the statutory guidance document for the fire safety of buildings – Approved Document B (ADB) in England on 2 March 2025. (The parts of BS 476 that deal with fire resistance will also be withdrawn from ADB on 2 September 2029 in favour of BS EN 13501-2.)

Because fire safety is a devolved matter, the regulations covering external walls and combustible materials differ across England, Wales, Scotland and Northern Ireland – see more information below.

Table 1 below gives reaction to fire classifications of some insulation materials used in the building envelope for illustrative purposes. The suffixes s1, s2 (smoke formation) and d0, d1 (burning droplets) classify the material’s performance in real fire conditions.

It is important to note the following:

• Euroclass ratings apply to specific products and systems. The classification of materials such as PIR, phenolic foam and EPS depends on the individual product formulation, as well as details such as facings, and may vary between products.
• Classes B to F are considered combustible, with increasing levels of contribution to fire growth, smoke production and flaming droplets.
• Products that have not been tested can no longer be described as class F; they must instead be declared NPD (no performance determined), and are not permitted in applications where ADB specifies a minimum reaction to fire classification.

Under the UK Construction Products Regulation, thermal insulation product manufacturers must declare a product’s reaction to fire performance, in accordance with BS EN 13501-1, in its declaration of performance (DoP).

In England the functional legal requirement on the fire performance of external walls is found in requirement B4 of the Building Regulations, which states: “The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building.” Compliance is typically demonstrated by following the guidance in ADB; however, this guidance is not applicable in all building scenarios. Designers may follow ADB or demonstrate compliance through an alternative fire-engineered solution.

Post-Grenfell changes in England

Since 2018, in England regulation 7(2) of the Building Regulations 2010 has prohibited the use of combustible materials through the external walls and specified attachments of relevant buildings – residential, institutional and similar – with a storey 18m or more above ground level.

In 2022, an update to the Building Regulations 2010 extended the ban to hotels, hostels and boarding houses at the same threshold, and added guidance recommending the use of non-combustible materials achieving a Euroclass rating of A1 or A2-s1,d0, through the external walls of all residential buildings above 11m, although the statutory ban was not extended to buildings of this height.

One specific exception applies regardless of height or use: regulation 7(1A) prohibits relevant metal composite material – defined as panels with an unmodified polyethylene core of the type used on Grenfell Tower – from becoming part of an external wall or specified attachment of any building, regardless of height, where building work is undertaken. Beyond these specific prohibitions, requirement B4 applies to all buildings regardless of height, and designers are expected to consider the risk of external fire spread in every case.

Devolved nations

Fire safety requirements relating to external wall materials are devolved, and the three other UK nations have diverged from the English framework. Wales has tracked it most closely; Northern Ireland adopted the original 2018 English position but has not followed the subsequent extensions; Scotland has diverged most, with an 11m statutory ban in force since 2022 – the lowest threshold in the UK. The result is a four-nation patchwork, summarised in table 2, below.

Building envelope – balconies

Balconies can present a significant fire risk. A series of post-Grenfell incidents – such as the Samuel Garside House fire, Worcester Park fire and The Cube fire, all of which involved rapid balcony-to-balcony or balcony-to-cladding spread – have made the hazard impossible to ignore, and the British Standards Institution responded by publishing BS 8579:2020 Guide to the design of balconies and terraces.

Recent research, Fire safety: Balconies, spandrels and laminated glass (Building Safety Regulator, HSE), drew on a series of full-scale fire tests and confirmed that balcony fire spread can be influenced by factors such as moveable fire load (combustible items introduced by occupants, such as furniture and personal belongings), exposed timber decking, combustible balustrades and balcony arrangement, with timber decking identified as the dominant factor.

In England, balconies are statutorily defined as “specified attachments” under regulation 7(2) of the Building Regulations 2010, which pulls them inside the same statutory materials regime as the external wall itself. So, in relevant buildings, every component of the balcony – decking, balustrade, soffit, framing and fixings – must generally be non-combustible, unless covered by specific exemptions in regulation 7(3).

Cavity barriers and fire stopping

Alongside the use of non-combustible materials, fire stopping and cavity barriers are key components of envelope design. Cavity barriers restrict the spread of fire and smoke within concealed cavities such as rainscreen cladding systems or roof voids. Fire stopping maintains compartmentation wherever services penetrate walls and floors.

Cavity barriers are typically installed around window and door openings, at floor levels, at compartment walls and at the top of cavities. Correct placement is critical because ventilated facade systems can allow fire to spread rapidly through the chimney effect – hot gases rising and drawing air into the cavity, increasing fire spread rates. Many external wall fires originate from internal compartment fires breaking out through windows and impinging on the external wall, highlighting the importance of non-combustible materials and effective cavity barriers around openings.

Junctions and interfaces

The correct design of junctions and interfaces is also critical to envelope fire safety.

• Compartment line crossings – where compartment walls or floors meet the external wall, cavity barriers must maintain compartmentation across the rainscreen cavity. Horizontal barriers at floor lines and vertical barriers at wall lines are essential. These need to permit ventilation in normal conditions but seal the cavity in a fire.
  Vented intumescent cavity barriers, correctly sized and tested for the cavity depth, are the standard solution, and must be carefully co-ordinated with the cladding subframe and fixings.
• Window and door reveals – at every wall opening, cavity barriers must close the cavity at head, jambs and, where required, sill, so that fire and smoke cannot enter and spread within the cavity.
• Balcony interfaces – balcony attachments typically penetrate the external wall, and balcony soffits can fall within the ban on combustible materials through the external walls under regulation 7(2), depending on their construction and relationship to the facade. At these junctions, the key risk is the potential for fire to bypass the external wall via fixings, cavities or poorly detailed interfaces. Mitigation requires careful detailing: continuity of cavity barriers at the wall junction; fire stopping of anchor penetrations; and co-ordination of balcony structure, soffit and edge details with the external wall system. Consideration should also be given to how balcony use may influence fire exposure at the facade.
• Roof-to-wall and parapet details – where a flat roof meets the external wall, the roof edge is a critical fire-spread junction. The fire performance of this junction should not depend on BRoof(t4) alone, as external roof classifications do not provide a complete assessment of fire spread within the roof build-up, external wall system or roof-wall interface. The clearest and most robust means of reducing fire risk is to specify non-combustible materials classified as Euroclass A1 or A2-s1,d0 for significant roof and wall components, with correctly positioned cavity barriers, non-combustible parapet copings and flashings, and continuous fire stopping/ compartmentation at the junction. (Fire performance classifications for roofs are covered in CPD 04 2026: Fire safety for roofs and rooftop solar.) Combustible copings, flashings or trims can compromise this continuity. In practice, the parapet should be treated as part of the external wall, and non-combustible components should be specified throughout and cavity barrier continuity maintained.
• Service penetrations through the external wall – extract terminals, tumble dryer vents, ventilation grilles and small-bore pipes all penetrate the envelope and must be detailed so they do not compromise the fire performance of the wall or the cavity barrier system. Tested service-penetration seals, classified to BS EN 1366-3, are the appropriate solution.
• Curtain walling at floor edges – at every floor line, the perimeter gap between the curtain wall and the slab edge must be closed by a tested perimeter fire-stop system, not a generic cavity barrier. The system typically comprises a compressed stone wool batt, retainers and a smoke seal, designed to maintain its seal under the relative movement of slab and mullions, and tested as an assembly to BS EN 1366-4 or ASTM E2307. Substitution of components invalidates the test, and the test evidence underpins the fire performance of the system.

Good workmanship matters at junctions because they involve interfaces between trades, between products from different manufacturers, and between drawings produced at different design stages. Specifying tested system details, requiring photographic inspection records before junctions are concealed, and integrating those records into the golden thread are practical risk-reduction measures, and are increasingly expected by warranty providers and insurers as well as by the Building Safety Regulator (BSR).

Building Safety Regulator gateways

For higher-risk buildings – broadly those of at least 18m or seven storeys with two or more residential units – the Building Safety Regulator operates a three-stage gateway approval regime. For envelope work, the gateway regime means that “working it out on site” is no longer an option: each gateway is a hard stop, and what was approved is what has to be built.

• Gateway 1, at planning stage – requires a fire statement addressing how the proposals consider fire safety, including external wall fire performance, site access for firefighting, water supplies and the relationship to adjacent structures.
• Gateway 2, before construction – replaces the deposit of full plans with building control. The BSR must approve a complete application – including fire and structural strategies, the construction control plan and a fully developed design – before any building work on a higher-risk building can start. This is the point at which the external wall design must be complete, not developed on site: late value engineering and product substitution are precisely what gateway 2 is intended to prevent.
  
  The BSR will expect to see the external wall fire-spread approach, compartmentation drawings, and cavity barrier and perimeter fire stopping strategies; the full wall build-up with all products identified and supported by BS EN 13501-1 classifications, DoPs and relevant third-party certification; and competence declarations for those responsible for envelope design.
  
  The construction control plan then sets out how the as-built work will be kept aligned with the approved design, and how changes will be assessed, recorded and, where necessary, resubmitted to the BSR for approval before implementation.

• Gateway 3, at completion – a completion certificate cannot be issued, nor the building occupied, until the BSR is satisfied that what was built matches what was approved at gateway 2. For envelope work, this places particular weight on evidencing items that are no longer visible: cavity barriers, perimeter fire stops, breather membrane, insulation and fixings all need photographic and inspection records captured before the rainscreen is closed up, because, once it has been, this is often the only way to demonstrate what is actually there. Installer competence has to be evidenced not just by generic site competence cards but by third-party scheme certification – FIRAS, IFC and similar – for the operatives who installed cavity barriers and fire stopping. Product traceability runs alongside this: delivery notes, batch records and DoPs for the products actually installed matched against what was specified at gateway 2, so that any substitution can be traced and shown to have been properly reapproved. Every change made during construction must be reflected in the record – notifiable changes logged, major changes reapproved by the BSR – with envelope items such as cladding, insulation type and cavity barrier specification typically falling into the major category. This means that product swaps can no longer be treated as routine commercial decisions. The complete envelope record is then handed to the principal accountable person as part of the golden thread, in a form they can use to manage the building’s safety case in occupation.

The new building safety regime also includes mandatory occurrence reporting: dutyholders must report defined structural and fire-safety occurrences to the BSR through a digital reporting system. Envelope-related issues – such as the discovery of missing cavity barriers, non-conforming cladding or installation defects – can fall within this scope.

Delays to gateway 2 decisions have been a serious bottleneck for high-rise construction in England since the regime began in October 2023, with decisions stretching well beyond the 12-week statutory target. The Construction Products Association reported in October 2025 that average determination times had reached more than 30 weeks, with London worst affected.

In the 12 weeks to 1 May 2026, the BSR made 323 gateway 2 decisions, with a 71% approval rate. New-build innovation unit approvals had a median decision time of 22 weeks, while longer overall times continue to be driven principally by older and more complex remediation cases. Overall, approval rates are improving and complete, well-evidenced applications seem to be progressing more efficiently.

The golden thread

One of the most important new requirements brought in by the BSA is the golden thread: the requirement that the information needed to understand a building and to keep it safe is captured, kept up to date and made accessible throughout its life. It applies to HRBs and runs from the design and construction phases – where dutyholders must hand over a complete and accurate record at each gateway – into occupation, where the principal accountable person must maintain it and use it to manage building safety risks.

The information must be held digitally, in a structured and searchable form, and acts as the building’s single source of truth.

For envelope work, the golden thread is particularly demanding, because historically so much of what matters – the exact cladding panel installed, the insulation product and its reaction-to-fire classification, the cavity barrier type and locations, the perimeter fire-stop detail at each floor, the third-party certification of installers – has sat across multiple subcontractors, may have been substituted late in the programme, and may become invisible once the external wall cladding is closed up. The golden thread aims to prevent that information being lost by holding it in a clear, accessible and controlled form throughout the life of the building. This allows assessors, insurers, regulators or claimants to understand exactly what was specified, installed, substituted and evidenced. In the context of the Building Safety Act’s extended liability periods, including 15-year prospective and 30-year retrospective periods for certain claims, incomplete envelope records are not just a documentation problem; they are a long-term legal and commercial risk.

Extended responsibilities under the BSA

The BSA has clarified responsibilities by introducing dutyholder roles, and section 135 of the BSA made a significant change to construction liability, by amending the Defective Premises Act 1972 (DPA). The DPA gives leaseholders, freeholders and other interested parties a right to sue those who designed, built or arranged for the building of a dwelling that was not, when completed, fit for habitation. Claims can now be brought up to 30 years after completion for work completed before 28 June 2022, and up to 15 years after completion for work carried out from that date onwards.

The duty is also extended to subsequent works on existing dwellings rather than just the original build, encompassing refurbishment and recladding works that had previously sat in a legal grey area.

The BSA also tackled a recurring post-Grenfell problem whereby the developer or contractor responsible for a defective building was often a single-purpose vehicle that had been wound up or stripped of assets, leaving claimants with no one solvent to sue. New building liability orders, made by the High Court, allow liabilities to be transferred to an associated company in the same corporate group – a parent, a subsidiary, or another entity under common control.

Final thoughts

The fire performance of an envelope is a system property, only as good as the weakest detail in the build-up: poor workmanship, missing cavity barriers or substituted products can compromise performance even where compliant materials have been specified. Safe buildings depend on informed material choice, careful detailing and clear responsibilities throughout the project lifecycle – and, where appropriate, going beyond minimum compliance.

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