Why we can’t forget about cavity barriers
External cladding façade systems— both ventilated and non-ventilated types— are becoming an increasingly popular choice for building exteriors in many parts of India and beyond, thanks to their energy efficiency benefits and wide aesthetic possibilities. However, the air gap between the inner and outer finishes of these ‘double wall’ constructions presents a very serious fire risk if not carefully considered, especially in densely populated cities like Mumbai where high-rise residential buildings are becoming commonplace.
Whilst there is much focus on passive fire protection in curtain walling applications, and on material reaction to fire in cladding applications – cavity barriers for preventing the unseen spread of fire and smoke within the concealed space of both ventilated and unventilated external cladding systems are currently and frequently overlooked by designers, specifiers and authorities alike
Mind the gap
Ventilated façade systems typically consist of a backing wall or an inner concrete structural element, potentially an insulation layer, waterproofing layers, brackets, and then the external cladding finish, which can be made from a range of attractive materials from aluminium cassette panels to masonry slips. Separating the inner wall from the outer skin is an air gap. Also referred to as a cavity, this space allows any moisture that enters the façade system due to rainfall or humidity to (depending on the system design i.e. ventilated and drained or pressure equalised) either effectively drain or vent away, or limit water penetration, preventing condensation, corrosion and mould growth.
Additionally, due to the air pressure differential between the bottom and top of the cavity, this gap allows a continuous cycle of air to flow through the cavity from top to bottom. This helps to cool down the exterior of the building, supporting comfortable temperatures indoors without over-reliance on potentially costly mechanical air conditioning — making them a sensible choice for projects aiming for lower operational costs and higher sustainability credentials.
Non-ventilated cladding systems are similar in construction to ventilated façades in that they have a cavity; however, they are not engineered to expel moisture which can be particularly problematic with India’s rising relative humidity levels in both day and night.
However, regardless of the system type, in the event of a fire entering the external wall construction, this cavity can pose a real challenge to the building’s fire safety as it draws heat, smoke and flames up the building, affecting multiple floors and putting many lives at risk. Even the smallest fire within the cavity can quickly become ferocious as the confined space means any radiant heat is trapped and can only travel upwards, creating a hotter fire that can easily spread.
To prevent this, the cavity needs to be closed off using cavity barriers— blocks of fire-resistant material which seal the cavity in strategic locations, such as at junctions between walls and floors and around windows and doors, to subdivide the cavity into smaller voids creating compartments that contain the fire into an area of manageable risk until it can be fully extinguished. This prevents extensive fire spread, allowing occupants enough time to escape, fire and rescue services to perform their operations, and limiting property and asset damage.
Cavity barrier options
Standard cavity barriers are ‘full fill’ or ‘closed state’ products, meaning they are fitted directly between the internal and external layers to completely seal the cavity. This makes them ideal for vertical applications in both ventilated and non-ventilated systems as they can help maintain the air pressure within compartments, preventing the fire from spreading horizontally around the building.
They are also used in a horizontal orientation for non-ventilated systems, inhibiting fire and smoke from spreading vertically. Whilst ideal for dry climates, in high-humidity regions these closed-state systems can result in condensation forming within the cavity, leading to moisture buildup on the cavity barrier and dampness within the building envelope. Similarly, in ventilated façades, this orientation means that they would prevent the system from effectively ventilating or draining away water leading to the same problems.
To resolve this issue with ventilated systems, ‘open state’ cavity barriers were developed. These include an integral intumescent material that rapidly expands in reaction to high heat (around 130°C). This allows them to be fixed to the internal wall, leaving the ventilation gap open to allow for free vertical movement of air and moisture drainage day-to-day (sometimes referred to as the ‘cold state’). However, in the event of a fire, the intumescent expands in a matter of seconds until the void is fully closed and a robust fire seal is formed.
Cavity barrier requirements
Despite being fundamental to the safety of external cladding systems, currently, and surprisingly, there are no regulatory requirements for cavity barriers in India. However, many regions around the world do mandate their use, such as in the UK and the UAE, and there are an increasing number of building owners, investors and insurers from such places asking for cavity barriers to be installed on their Indian properties. Whilst it is likely that guidance will change in the future, architects and façade designers do not have to wait. Both testing and real-world fires demonstrate that cavity barriers can make a significant contribution to the overall passive fire safety of a building, whilst selecting cladding and cavity barrier systems that are appropriate for the climate conditions can help avert moisture-related issues.
Cavity barrier testing
However, to specify them correctly, it is important to understand what fire test standards to look out for. Standard full- fill cavity barriers can be tested to EN 1366-4 Fire resistance tests for service installations – Part 4: Linear joint seals. This standard determines the fire resistance of linear joint seals. It tests both horizontal and vertical applications and allows some movement in one direction before the test starts.
However, there is understandably some difficulty using the standardised tests to determine the performance of open-state cavity barriers, as the time taken for the gap to close would initially lead to a technical ‘fail’, even though as soon as the intumescent has activated, fire integrity and insulation are established. To help resolve this issue, the UK’s Association for Specialist Fire Protection (ASFP) produced a Technical Guidance Document, TGD 19. This outlines the test configurations and failure criteria for the testing of open-state cavity barriers and the pending prEN 1364-6 standard which is being developed specifically for cavity barriers. The test is based on the existing EN 1366-4 linear joint seal test (using the principles of EN 1363-1) but modified with upstands to better replicate the cavity construction and allows 5 minutes for the intumescent to close the gap.
Whilst these tests can determine standalone cavity barrier performance and therefore general suitability for the purpose it is marketed for, it is only through large-scale testing that we can fully understand how complete assemblies perform in their intended application and how they might perform in a real-life fire scenario. Such testing examines all the system components together, providing data on how the different products interact, along with the impact of details such as joints, gaps, and penetrations on the overall fire performance. Therefore, data from large-scale systems tests such as BS 8414-1 &2 and NFPA 285 that evaluate the performance of a complete façade assembly can be invaluable when considering cavity barriers.
It is important to note that, even with the inclusion of cavity barriers, systems can fail if the external cladding allows the fire to spread up the outside of the building causing the panels to move, break down or come away. Any malfunction with the cladding would likely leave a path for the flames to spread up and over the cavity barrier and up the cavity. Therefore, it is vital to always refer to the latest specialist advice for façade design and specification and to ensure high-quality workmanship at every stage.
Facades for the future
External cladding façade systems bring both a wealth of creative potential and clear thermal efficiency benefits to all kinds of buildings. Ensuring that they include cavity barriers that have not only been appropriately specified for the type of system they will be installed in, and that climate zone factors have been taken into consideration, but that their performance is backed by testing that reflects their end use can help to protect India’s built environment for generations to come.
