Cavity Barriers In Cladding

In modern construction, cavity barriers in cladding are fundamental safety components designed to prevent the rapid spread of fire and smoke through the concealed voids of an external wall system. When a rainscreen or façade is installed, a gap is left between the outer decorative panel and the structural inner wall to allow for ventilation and drainage. If a fire enters this void, the chimney effect can accelerate flame spread vertically and horizontally across the building face. Cavity barriers act as internal seals, closing these gaps when exposed to heat to contain the fire within its compartment of origin.

Key Takeaways#

• Cavity barriers are distinct from fire stops; they are specifically designed for the ventilated voids found in cladding systems.
• Intumescent strips allow for necessary airflow during normal conditions but expand rapidly under heat to seal the cavity.
• Identifying correct cavity barrier locations in external walls is a critical part of EWS1 assessments and safety audits.
• Regulatory compliance in the UK is governed by Approved Document B and the Building Safety Act, particularly for high-rise residential buildings.
• Common defects include incorrect orientation, gaps in the seal, and the use of unsuitable materials that do not meet the required fire rating.
• Professional cladding testing and inspection are essential to verify that hidden barriers are installed correctly and perform as intended.

What Are Cavity Barriers and Why Are They Critical?#

A cavity barrier is a piece of fire-stopping material that seals the gaps between different components of a building’s external wall. In a rainscreen cavity barrier application, these components must balance two contradictory roles: they must keep the cavity open for moisture management during the building's daily life, yet close it instantly in the event of a fire.

Without functional barriers, a fire that penetrates the cladding system can bypass the fire-rated floors and internal compartmentation of the building. The void behind the cladding acts as a flue, drawing flames upward at high velocity. This phenomenon has been a primary factor in several major high-rise fire incidents in the UK and globally.

Expert assessment of these components is vital. At CladdingTesting.com, we frequently find that while barriers are present, they are often installed with incorrect tolerances or gaps that render them ineffective. A barrier that fails to close the gap within the first few minutes of heat exposure offers little to no protection to the building’s occupants.

The Difference Between Cavity Barriers and Fire Stops#

It is a common misconception that these terms are interchangeable. A fire stop is typically a permanent, solid seal used to close an opening through a fire-separating element, such as where a pipe passes through a floor. They are designed to be airtight and static.

In contrast, cavity barriers in cladding are often "open-state." This means they feature an intumescent strip that leaves a 25mm to 50mm air gap to allow the façade to breathe. Only when temperatures reach approximately 150°C to 200°C does the material react, expanding to fill the void completely. Understanding this distinction is crucial for managing agents when reviewing cladding cavity barrier detail drawings.

Where Are Cavity Barriers Required in Cladding?#

Determining where are cavity barriers required in cladding is one of the most technical aspects of façade design and remediation. The requirements are dictated by the building’s height, use, and the specific materials used in the external wall buildup. In the UK, the primary guidance is found in Building Regulations Approved Document B.

Generally, barriers are required at the following interfaces:

• At every floor slab level: To prevent vertical fire spread between storeys.
• Around all openings: This includes windows, doors, and service penetrations where fire could jump from the interior to the external cavity.
• At compartment walls: To prevent horizontal spread between different flats or fire zones.
• At the top of the wall: To prevent fire from entering the roof space or parapet.
• At the corners of the building: To stop fire "wrapping" around the building elevations.

Cavity Barrier Locations in External Walls#

The exact cavity barrier locations in external walls must be mapped out during the design phase or identified during a cladding inspection. For high-rise residential buildings (over 11m or 18m depending on specific risk profiles), the precision of these locations is non-negotiable.

In a typical rainscreen system, horizontal barriers are placed at every floor level. These are usually open-state to allow drainage. Vertical barriers, however, are often "closed-state," meaning they are in constant contact with the cladding panel and the inner substrate. These vertical barriers create distinct "chimneys" that limit the horizontal movement of smoke and heat, effectively boxing in any potential fire.

The Technical Composition of Rainscreen Cavity Barriers#

Rainscreen cavity barriers are typically manufactured from non-combustible rock fibre mineral wool. This core provides the initial fire resistance and thermal insulation. The outer edge is then fitted with a high-expansion intumescent strip.

The performance of these barriers is rated by two metrics:
1. Integrity (E): How long the barrier remains physically intact and prevents the passage of flames.
2. Insulation (I): How long the barrier prevents the temperature on the "cool" side from rising above a certain threshold.

For most UK high-rise applications, a 30/15 or 60/30 rating is standard. This means the barrier provides 60 minutes of integrity and 30 minutes of insulation. Relying on the wrong rating for a specific building height is a common compliance failure we encounter during technical audits.

Materials and Longevity#

Because these barriers are located within the damp, ventilated environment of a rainscreen, they must be durable. Quality barriers are often wrapped in weather-resistant foil. If the foil is damaged during installation, the mineral wool can absorb moisture, leading to sagging and a total loss of fire-stopping capability over time.

Investigating Cladding Cavity Barrier Detail#

When CladdingTesting.com is instructed to assess a building, we look specifically at the cladding cavity barrier detail. This is not something that can be checked from the ground with binoculars. It requires invasive inspection—removing panels to see what is happening behind the "skin" of the building.

Common Installation Defects#

Even if the correct materials were specified, poor workmanship can compromise safety. Our teams frequently identify the following issues:

• Gaps at junctions: Where horizontal and vertical barriers meet, there are often small gaps. Fire and smoke are highly efficient at finding these "weak links."
• Wrong orientation: Barriers installed upside down or back-to-front, which prevents the intumescent strip from facing the cladding panel.
• Inadequate compression: Vertical barriers must be compressed slightly to form a tight seal. If they are too small for the cavity, they serve no purpose.
• Obstructions: Helping-hand brackets, rails, or cables passing through the barrier without proper sealing.
• Missing fixings: Barriers held in place by friction alone, which will likely fail and fall out of position during a fire.

The Role of Intumescent Strips#

The intumescent element is the "active" part of the fire safety system. In the UK climate, these strips must be able to withstand moisture and temperature fluctuations for decades. If an inferior product was used during construction, the chemical composition of the intumescent may degrade, meaning it will not expand when needed. Testing these components is a vital part of long-term building maintenance.

Regulatory Compliance: Where Are Cavity Barriers Required?#

The question of where are cavity barriers required cladding often arises during the sale or refinancing of a property. Under the Building Safety Act 2022 and the updated Fire Safety England Regulations, the "Accountable Person" must have a clear understanding of the external wall construction.

In the UK, barriers are mandatory in all cavities, regardless of height, but the stringency of the requirements increases with the building's height and occupancy type. For example, a care home or a high-rise block of flats requires a much more robust cavity barrier strategy than a low-rise warehouse. This is due to the increased "stay put" evacuation strategies typically used in residential blocks, which rely entirely on compartmentation.

PAS 9980 and EWS1 Requirements#

During a PAS 9980 assessment, a fire engineer will evaluate the risk of fire spread. If the cavity barrier locations in the external wall are incorrect or missing, the building will likely receive a "B2" rating on an EWS1 form, indicating that remediation is required. Our role at CladdingTesting.com is to provide the empirical data—through physical inspections—that allows engineers to make these life-safety decisions.