Horizontal open state cavity barriers (OSCBs) are essential to the safety and performance of ventilated rainscreen facade systems. To ensure long-lasting protection throughout a building's lifecycle, choosing the correct fixing method is crucial.
In this guide, NBS manufacturer Siderise demonstrates why specifiers must choose positive mechanical retention when designing ventilated rainscreen systems for building projects…
For specifiers, ventilated rainscreen systems are an attractive cladding option that can improve the thermal performance, energy efficiency and durability of your project. They must, however, be supported by OSCBs, which play a crucial role in the safety and performance of ventilated rainscreen facade systems.
The fixing method you select for an OSCB is something that cannot be overlooked. In this article, Siderise examines OSCBs and fixing methods to reveal why specifiers should opt for positive mechanical retention for optimal longevity…
Key takeaways
This is an in-depth guide that explores fixing methods and OSCBs. We’d encourage all specifiers who work with building envelope systems to read the full article, but if you’re in a hurry, we’ve gathered some key points to give you a quick overview…
- OSCBs are critical safety components in ventilated rainscreen façades, enabling air/moisture flow in normal conditions and sealing cavities during fire.
- Fixing method directly impacts OSCB performance: poor fixings can lead to barrier failure under stress or fire.
- Positive mechanical retention, especially using full penetration brackets with bent tails, is the most reliable fixing method because it:
- Prevents lateral movement and sagging.
- Maintains fire integrity and air gap.
- Easier to inspect for correct installation
- Supported by guidance: A range of Approved Documents and CWCT guidance supports the use of mechanical fixing with full barrier penetration and bent-over tails for secure retention.
- Specifiers should demand tested systems that include fixings and have large-scale fire test data (e.g., BS 8414, NFPA 285).
What do OSCBs do?
ln the ‘cold state’, OSCBs allow for free-flowing air movement and moisture egress. In fire, the intumescent edge, activated by heat at circa 140°C, exfoliates to create a fire seal.
To provide this protection for the lifetime of the building, OSCBs must be securely and robustly fixed to the internal substrate using a retention method that prevents barrier displacement. This ensures they can maintain the specified air gap in the cold state and preserve their integrity during fire exposure.
As a specifier, you need to understand how fixing methods have a direct impact on the performance of your chosen system. Let’s take a closer look to help you make the right choice…
How does a fixing method impact the performance of open state cavity barriers?
Buildings and façades are regularly exposed to forces such as wind and settlement that can cause them to move, putting stress on fixings. In the event of fire, heat-induced air pressure and thermal expansion will add even more force.
If fixings lack sufficient tensile strength or retention capability, these forces can all potentially cause them to fail, and the OSCBs to completely or partially detach from the internal construction.
If the OSCBs fail, problems include:
- Possible thermal and moisture management issues due to fallen cavity fire barriers obstructing ventilation and drainage flow in the cavity.
- Compromised compartmentation allows fire and smoke to spread vertically through the unseen cavity due to gap formation between the substrate and the rear of the barrier. Severe OSCB misalignment that can lead to its intumescent layer not exfoliating within the limits of its stated air gap range, thus failing to form a complete seal against the opposing surface by not fully closing the width of the void.
To mitigate these risks, OSCBs must be selected with support brackets and fixings that are designed to keep them securely in position when subjected to differential movement or fire-induced stresses, and that allow for the intumescent to fully exfoliate when exposed to the heat of a fire.
What fixing approaches are typically used for OSCBs?
UK legislation does not prescribe specific requirements for the fixing method. However, specifiers can heed statutory and specialist guidance which advocates for barriers to be mechanically fixed in position, of which there are several common approaches:
- Direct through-fix to substrate using non-combustible fixings (small voids): usually stainless-steel countersunk head screws with washer sitting flush with intumescent face and screwed into the substrate at a sufficient depth.
- Mechanically fixed partial penetration brackets (large voids): usually stainless-steel spikes that penetrate the width of the barrier by typically 50-75% and fixed to the backing wall using non-combustible stainless-steel fixings, sometimes with pigtail screws piercing through the intumescent into the barrier.
- Mechanically fixed full penetration brackets with bent tails (large voids): as above without pigtail screws but with ‘tails’ piercing through the intumescent on the leading edge of the barrier that faces the open airspace of the cavity. These are bent flat to the outer face to positively retain the barrier and the intumescent.
A shared stance on mechanical fixing: Approved Documents B (England and Wales, separate editions), Technical Booklet E (Northern Ireland), and Technical Guidance Document B (Republic of Ireland)
All of these key documents require that cavity barriers are tightly fitted to a rigid construction and mechanically fixed in position. They go on to say that they should be fixed so that their performance is not rendered ineffective by failure of the cavity barrier fixings during a fire, building and external envelope movement, or the collapse of penetrating services or failure of abutting construction.
This guidance makes it clear that cavity barriers must be retained securely using suitable fixings, and without support from the envelope.
Reinforcing the requirements: Centre for Window and Cladding Technology (CWCT)
The above requirements are reinforced by CWCT in both its Technical Note 98 Fire performance of façades - Guide to the requirements of UK Building Regulations and Guidance on Built-up Walls publications.
It advocates more specifically for OSCBs used within ventilated façades to be fixed with brackets that penetrate the full thickness of the barrier, with tails bent over to positively retain the barrier in position.
For instance, Section 6.6.3.2.3. of Guidance on Built-up Walls states:
“To be effective, these should be securely fixed to the face of the back wall or, preferably, for cavity barriers on the line of compartment floors, the concrete slab edge. This will require a break in the thermal insulation in the rainscreen cavity... Steel spikes are used to fix the barriers, and these should penetrate the full thickness of the barrier with the tails turned over to positively hold the barrier in position. Failure to do this may allow the barrier to fall down the cavity, making it ineffective as a fire barrier and possibly blocking drainage.”
In section 4.24, it cites instances where this issue has been encountered, demonstrating that this is a very real potential problem:
“Cavity barriers should be positively fixed in position to ensure their effectiveness. Cases of barriers sliding off spikes and collecting at the foot of the cavity have been reported.”
Why choose positive full-penetration mechanical retention brackets over partial-penetration brackets?
Full-penetration brackets with retaining bent tails enable the OSCB to be firmly secured to the substrate, preventing lateral movement during its service life. This allows it to maintain contact with the backing wall, providing an effective cavity barrier.
They can also help support the structural strength of the OSCB by allowing its weight to be supported by the inner leaf, rather than relying solely on the tensile strength of the bracket.
Additionally, an equilibrium is established between the barrier’s outer face and the bent tails when the bracket is fixed to the substrate above the OSCB, which can enhance the structural strength of the OSCB and prevent sagging.
These bracket types also offer benefits during installation. As they do not rely on the external envelope for retention, they can be fitted at an earlier stage of the construction process. They are also much less likely to be dislodged or disrupted during the installation of the external envelope.
Crucially, it is also much easier for installers and inspectors to confirm if the OSCBs have been installed correctly. Some types of OSCBs require hand-screwed pigtail screws to be wound out until they touch the inside face of the cladding – after the cladding has been installed.
How to check the installation quality of full penetration mechanical retention brackets onsite
The CIOB-RIBA Guide to Managing Safety-Critical Elements in Building Construction stresses the importance of inspecting cavity barriers as safety-critical elements, stating that:
“[F]or all Elements that contribute towards fire and smoke compartmentation of a building - The compartmentation must be assumed to be part of a considered design, so there could be a significant risk of a serious injury or fatality if it was compromised by incorrect installation and there was a fire. Therefore, the compartmentation in its totality is a Safety-Critical Element.”
Checking cavity barriers can be complex if the retention method relies on an external facade being installed. An inspector would need to remove a section of the façade to check the fitting.
Full-penetration brackets with bent tails, however, are far easier to inspect as their visible tails eliminate the need for destructive inspections in build-ups where the thermal insulation abuts the OSCB from both above and below.
What testing and certification should specifiers consider?
Because the fixing method is intrinsically linked to the performance of an OSCB, specifiers should look for products that have been tested with their fixing methods.
This includes looking for products with a measured and defined design life, and that have been put through tests that replicate their intended application— demonstrating that they maintain integrity in real-world thermal and mechanical conditions.
The Association for Specialist Fire Protection (ASFP) Technical Guidance Document (TGD) 19 outlines the test configurations and failure criteria for testing Openable Smoke Compartment Barriers (OSCBs) and the recently published EN 1364-6 standard, developed specifically for cavity barriers.
Whilst this test 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 might perform in a real-life fire scenario. Therefore, data from systems tests such as BS 8414-1 & 2 and NFPA 285, which evaluate the performance of a complete façade assembly, can be invaluable.
Fixed for success with Siderise
Specifiers who want to uphold industry best practices and standards must choose properly designed OSCBs that use mechanically fixed, full-penetration brackets with bent tails that can withstand the rigors of environmental changes, serviceability movement, and thermal stress.
By specifying a system such as the Siderise Rainscreen Horizontal Open State Cavity Barrier, you can ensure that your building project remains adequately protected throughout its entire lifecycle.
Learn more about Siderise on their website or inspect their full range on NBS Source.
