Figure 1 HILTI CFS-VB/NVB in rainscreen cladding application

Building façades play a crucial role in the overall safety and energy efficiency of structures. However, they also introduce new fire safety challenges, particularly in the cavities created by external cladding systems. Cavity barriers are an essential solution for mitigating fire risks in these spaces, ensuring that fire, smoke and hot gases cannot spread unchecked behind the façade.
 
Cavities in façade systems are typically designed to improve insulation and provide drainage and ventilation. However, in the event of a fire, these same cavities can act as hidden conduits for flames and hot gases, rapidly spreading vertically and horizontally across a building’s exterior. This phenomenon is known as the “chimney effect.” The Building Research Establishment (BRE) confirms that cavity fires climb rapidly and extend 5-10 times higher than flaming fires breaking out from a window, independent of the combustibility of surfaces of cavities*. Cavity fires can produce intense heat, causing structural damage to a building and produce toxic gases and smoke that spread at high velocities**. This can be extremely dangerous for occupants, making evacuation more difficult and increasing the risk of smoke inhalation.
 
Grenfell tower, London, UK exemplifies this. In June 2017, rapid fire spread from the 4th to the 23rd floor. The cited reason was the use of combustible exterior cladding, which ignited and combined with the gap behind the cladding, created a pathway for the fire to spread, and engulfed the entire building within an hour.
 
Rainscreen cladding systems
In the context of façades, cavity barriers are most commonly used in conjunction with rainscreen cladding systems. Rainscreen cladding is a non-load bearing construction which, unlike other types of facades, is not the actual envelope of the building but a protective covering layered over it. The façade primarily comprises an insulation layer placed flush to the surface of the structural wall, with a layer of cladding fixed offset to the insulation, maintaining the required air cavity for ventilation and drainage. The insulation stabilises the building’s internal air temperature from environmental fluctuations, while the exterior cladding panels – often referred to as rainscreen panels – protect the insulation against extreme weather conditions. These exterior panels can be made of materials like aluminum, ceramic, stone, composite materials etc.
 
Rainscreen cladding systems now provide the most prominent technique utilised to achieve global sustainable development goals. These systems are practical and effective in both new and existing buildings. Reports indicate a 20-50%*** reduction in building energy usage requirements when appropriate façade designs are implemented.
 

Figure 2 Components of rainscreen cladding

There are two primary types of cavity barriers used in façade applications:

1. Ventilated cavity barriers: These barriers are installed in applications where airflow is necessary for ventilation and preventing moisture drainage. During normal conditions, they allow air to circulate freely, maintaining the performance of the façade. However, in the event of a fire, the ventilated barrier is designed to close automatically, preventing the spread of flames and smoke.
2. Non-ventilated cavity barriers: These barriers are used where ventilation is not required and the cavity must be fully sealed. They are often installed in areas where fire compartmentalisation is critical, such as around windows, doors, and other building penetrations, to ensure that fire cannot spread through any gaps in the façade.

 
Testing standards of rainscreen cladding
To ensure safety from such incidents, appropriate fire tests as mentioned in different codes need to be performed. There are two wide categories of tests to choose from (depending on the country/region) such as:
 

1. Small-scale tests: These tests assess the integrity and insulation performance of a single barrier. For ventilated cavity barriers, performance is tested according to small-scale tests such as ASFP-TGD-19. For non-ventilated barriers, EN codes for linear joints are followed (EN1366-4). As per code, the cavity barriers are required to have an E rating of at least 30 minutes and an I rating of 15 minutes to be deemed successful.
2. Full-scale tests: Some countries mandate full-scale tests of the entire firestop assembly, including cladding, cavity barrier, substructure, mineral wool and anchors. This test provides a general pass/fail outcome.

 
HILTI Solution for passive firestopping of rainscreen facades
HILTI offers a range of cavity barrier solutions specifically designed for façade applications, ensuring that building envelopes are protected against the spread of fire. HILTI’s products are not only engineered for fire safety but are also designed for ease of installation, providing a comprehensive solution for modern rainscreen cladding systems.
 

Figure 3 CFS VB/NVB in horizontal/vertical position

• CFS-VB (Ventilated Cavity Barrier): HILTI’s CFS-VB is a ventilated cavity barrier that allows air and moisture to flow freely during normal operation, but seals off the cavity when exposed to fire. This barrier is made from mineral wool with an intumescent strip. The intumescent material reacts to heat by expanding, effectively closing the cavity and preventing the spread of fire and smoke. CFS-VB can expand up to 50mm and has a fire integrity rating of 60 to 120 minutes. It is primarily used for horizontal installations in ventilated rainscreen cladding systems and can be easily identified by its RED HILTI logo.
• CFS-NVB (Non-Ventilated Cavity Barrier): the CFS-NVB is HILTI’s non-ventilated solution, designed to fully seal off cavities where no airflow is required. This barrier is also made from mineral wool placed in a specific orientation to allow adequate compression against the cladding and is typically installed vertically, providing a solid barrier with no air gaps. It is commonly used around openings and penetrations in the façade where fire compartmentalization is crucial. Like the ventilated barrier, the non-ventilated version is easy to install and can be recognized by its BLACK HILTI logo.

Both types of cavity barriers are supplied with either stainless steel or galvanized brackets.
 
Unlike most brackets available in the market, which are flat and need manual labor for bending (and thus leaving scope for errors), HILTI offers new pre-bent brackets which act as a measuring stick for ease of installation. They can simply be slid into the back of the barrier for a firm fix, saving installers up to 40% on installation time compared to conventional solutions. No cutting or bending of brackets is required! Overall, this translates into easier, faster and higher quality installation. Furthermore, these brackets have been tested with HILTI’s direct fastening solutions on the building structure for an even higher productivity.
 
A common challenge when installing cavity barriers is their intersection with various construction elements such as façade rails, cassette panels or masonry brackets.
HILTI addresses this by offering a comprehensive fire testing database that validates its barriers in numerous installation scenarios. This provides the assurance that the products meet rigorous fire safety standards. To ensure peace of mind, HILTI validates the new cavity barrier solution through test certifications from Intertek.
 
 
Conclusion
Implementing effective fire safety measures in building facades is essential for preventing the rapid spread of fire and ensuring the safety of occupants. Selecting the right cavity barriers, utilising the correct installation for ventilated and non-ventilated facades and choosing well-designed cladding systems, is key to achieving these objectives. Do so will ensure the highest fire rating backed by a complete set of corresponding documentation and certifications.
 
For more information, please contact your local HILTI experts or go to HILTI's online shop and:
 
1.    For information on other additional attributes, check out the How To video
2.    For further information, you can check out other articles on this topic
3.    To check out the HILTIs online shop, visit HOL
 
References:
* Fire spread modes and performance of fire stops in vented façade constructions-overview and standardization of test methods “https://www.matec-conferences.org/articles/matecconf/pdf/2013/07/matecconf_isfsf13_02002.pdf”
**   A Systematic Review on Cavity Fires in Buildings: Flame Spread Characteristics, Fire Risks, and Safety Measures: https://mdpi-res.com/d_attachment/fire/fire-07-00012/article_deploy/fire-07-00012.pdf?version=1703760867
*** Light ventilated façade: design, fire protection sustainability “https://ask.HILTI.co.uk/webinar/light-ventilated-facade-design,-fire-protection-and-sustainability/lmnpHps”