Horizontal open state cavity barriers (OSCBs) are critical to the safety and performance of all kinds of ventilated façade systems, from masonry to rainscreen. In day-to-day use (sometimes referred to as the ‘cold state’), they allow for free-flowing air movement and moisture egress. In the event of a fire, the intumescent edge, activated by heat at circa 140°C, exfoliates to create a fire seal. However, to be able to provide this protection for the lifetime of the building, OSCBs are required to be appropriately and robustly fixed to the internal substrate with a retention method that prevents barrier displacement — allowing them to preserve the specified air gap in the cold state and maintain integrity during fire exposure.

In this Insights piece, we will explore why the fixing method of OSCBs is a critical factor that should not be overlooked during the design and specification process.

UK legislation does not give prescriptive requirements as to the fixing method. However, guidance, both statutory and specialist, all advocate for cavity barriers to be mechanically fixed in position. There are several common approaches to mechanically fixing cavity barriers:

1. 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 at a sufficient depth into substrate.

2. 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.

3. 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, or ‘counter folded’, to the outer face to positively retain the barrier and the intumescent.

Figure 1: For small voids, open state fire barriers tend to be fitted using a direct through fix to the substrate.

 

Figure 2: Side by side comparison of mechanically fixed full penetration brackets with bent tails (top) and mechanically fixed partial penetration brackets (bottom) in large voids (for illustration purposes only).

This statutory guidance is reinforced by advice from 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, which cover several façade types including curtain wall and rainscreen. It advocates more specifically for open state cavity barriers 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 that focuses on cavity barriers in rainscreen constructions 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 of the guidance, 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.”

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.”

Cavity barriers whose retention is reliant on the external facade being installed can present a challenge when it comes to inspection—requiring a section of the facade to be deconstructed to check how they have been fitted. Meanwhile, full penetration brackets with bent tails can simplify this process as their visible tails eliminate the need for destructive inspections in build ups where the thermal insulation abuts the open state cavity barrier from both above and below.

Figure 4: Installation of OSCB with positive retention bracket, demonstrating the visibility of the bent ends.

Siderise offers both in-person and remote inspections via our specifically developed Inspection App that allows users to capture images as each area / element is installed and update the data input fields using their smartphone or tablet. These can then be reviewed offsite by our technical team to ensure the installation meets recommendations and quickly identify any potential issues with the fixing methods. 

Contractors play a pivotal role in ensuring passive fire safety strategies are delivered in the way they have been designed. Therefore, it is vital that onsite teams are not only equipped with the right knowledge and skills to install these life-safety critical products correctly, but that they have the tools to both demonstrate and verify the quality of the finished works.

This is especially crucial on projects subject to Gateway 3, requiring those involved in the construction phase to evidence that they have carried out the works in line with the design proposals signed off by the Building Safety Regulator (BSR). One way to do this is by taking advantage of Siderise’s free site support and inspection services.

The Building Regulations requires buildings to be “designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.” Passive fire protection measures within the building envelope can facilitate this by containing the spread of fire, heat, and smoke, creating areas of manageable risk— known as compartments— using fire-resistant materials and gap-filling measures like cavity barriers and firestops. This inhibits the movement of flames, smoke and hot gases from one compartment to another, protecting escape routes and reducing the risk of fire-induced structural damage which could result in collapse.

It is essential these solutions are installed properly to work as intended. Failure to do so could result in gaps in the compartment, providing an easy path for fire to spread up and around the building. Additionally, passive fire protection solutions not installed in accordance with their manufacturer’s recommendations may not achieve a project’s fire safety design requirement. This is because the installation will not be reflective of the way the products or systems have been designed and tested to perform. 

Explore our passive fire safety products.

Ensuring accurate installation begins with equipping teams with the right knowledge and skills. As a general point, it is worth noting that, as installation contractors of passive fire protection systems are considered a ‘specialist trade, there are several independent schemes that can include training, third-party certification, and professional qualifications. Whilst voluntary, they are good practice and help to ensure a strong foundation of competency. These often include various assessments of both how projects are managed, how installations are carried out, and how they are overseen— in addition to annual audits and random site inspections. Depending on the schemes, they may also allow contractors to log their installations and issue certificates of conformity to the end client. 

However, for teams wanting to know more about the specific methods and approaches needed to install Siderise products effectively, we offer a range of free product-specific training and support opportunities. These have been developed to provide installers with invaluable insight into the specifics of our products and systems, as well as potential build challenges— allowing them to be addressed before the installation programme has progressed and preventing issues being ‘baked into’ the construction. 

This offering includes a combination of classroom learning and practical onsite training, covering everything from ‘toolbox talks’ that ensure installation instructions and method statements are clearly understood, to delivering hands-on training and benchmark installations to demonstrate best practice.

Furthermore, the recent launch of the Siderise Academy Online Learning Portal has made learning about our products more accessible with a range of on-demand courses, webinars, and CPD modules. Certificates of completion can be downloaded as evidence of training, supporting the need to increase the competency levels of individuals.

Also, being an ASFP Recognised Training Provider means our training material has been verified as technically accurate and relevant and can also be used within the ASFP Competency Pathway, leading to the ASFP Pass Mark.

Learn more about Siderise Training.

We can supply you with reports from our inspection services, as well as documentation about any advice contracting teams have received from our technical and site service engineers. These can be logged and shared with other project stakeholders and included in subsequent handover information, providing a clear digital record of exactly what’s in the building and how it was installed. This helps to maintain that vital ‘Golden Thread’ of product information, support Building Control and/or the Building Safety Regulator in ensuring the passive fire safety measures have been delivered correctly, and can be a useful reference point for any future modifications that may affect the building envelope.

Want to learn more about how we support contractors? Read our installation guidance page for more details. 

Acoustic separation in curtain-walled buildings can be a challenge for façade designers and engineers. Often constructed using low mass materials such as lightweight and hollow aluminum profiles and glazed panels, curtain wall façade systems are particularly susceptible to flanking sound transmission. In other words, any noise transferred between rooms by a path other than via the separating partition. Curtain walls provide several such noise paths. 

It is therefore important for designers and engineers to adopt well thought out acoustic strategies at the early stage of a project that considers the design of junctions between party walls and floors and the façade, as well as the transfer of noise through the façade itself. With proper consideration and good acoustic design, spaces abutting curtain walls can be acoustically comfortable and private. 

Before we explore the principal flanking sound transmission paths in curtain walling systems, it is important to clarify some of the key measurements used when talking about building acoustics 

All noise is measured in decibels (dB). However, decibel values alone do not give any indication as to the context of the noise being measured. For example, does a decibel value refer to the performance of a single element, or to an entire detail? Does it refer to airborne or structure borne noise? Does it inform us of the period over which the noise measured?  

To provide clarification at a glance, ‘acoustic metrics’ are used. These appear as a short string of text that accompanies the dB value, for example 45dB D_nT,w. Here’s a brief explanation of the metrics relevant to curtain wall acoustics: 

dB R_w - weighted sound reduction index 

This laboratory measured value identifies the airborne sound insulation performance of a single proprietary building element. It is used for internal or external walls, ceilings / floors, windows, doors or any other separating element. The higher the R_w value, the better the sound reduction performance of the element. 

Because dB R_w is a laboratory measured value, it does not take into account any potential site issues such as tolerance or workmanship, nor any flanking noise paths, which may be present onsite. This means that the R_w performance of an element in a laboratory setting is typically higher than what the element can achieve in the context of a full detail. 

dB D_nf,w – Weighted normalised flanking level difference 

This laboratory measured value represents the performance value of a flanking element only, for instance, a curtain wall façade, raised access floor or suspended ceiling. Any noise transferred between rooms through noise paths other than the separating partitions is flanking noise. 

dB D_nT,w  - Weighted standardised level difference  

This is a site measured value that represents the total airborne sound insulation between a given pair of rooms and accounts all noise paths, both direct noise through the separating partition and flanking noise through any other path.  

The D_nT,w test includes a correction to account for the reverberation time - or echo - in the rooms being tested. This allows acousticians to compare measured sound insulation results between different rooms and at different times, while accounting for the relative room size and any changes to furnishings applied when the room is in use, for example the presence of furniture and soft finishes like carpets and curtains. 

With these metrics in mind, we can take a closer look at the main five paths through which noise travels in curtain wall façade systems. The façade’s D_n,f,w performance is influenced by each of these, and the achievable performance would be limited by whichever of these noise paths is the weakest.  

Glazing 

Sound waves are incident on a pane of vision glass, causing the glass to vibrate. These vibrations travel through to adjacent rooms via the structure of the glass itself and re-radiate as noise. Noise that travels via solid materials in this manner is called structure borne noise. Glazing type, spans of glass and spandrel panels, the number of separating transoms and their stiffness, all influence how sound travels along this path. 

Transoms 

Most curtain walling systems use aluminium profiles. These framing elements are relatively lightweight and sound energy incident on them may easily break through them into adjacent spaces. The height and construction of spandrel panels as well as the transom design and number of separating transoms affect how sound travels along this path. 

The mullion joint  

Sound energy hits the mullion causing it to vibrate. These vibrations (structure borne noise) travel along the mullion and radiate into adjacent rooms. The design of the mullion joint is the most significant factor in how this noise path performs. 

Continuous mullions can be a significant limiting factor* on achievable performance. 

Mullion hollow 

If sound energy hits the mullion and passes through into the hollow profile, it can travel up and down the building before re-emerging in another space. The hollows of mullions create a hidden pathway for unwanted noise.  

Perimeter joint / Movement gap / Firestop zone 

The essential gaps at the joints where different building elements meet e.g. at the slab edge / façade interface, accommodate building movement and fire protection measures, but they can also present a path for flanking sound transmission. Carefully designing and treating these zones with high mass and isolating materials with fire-resistant properties is crucial for achieving good sound insulation as well as meeting the required fire resistance performance. 

*It’s important to note that the overall performance of a curtain wall system is only as good as its weakest link. This is known as the performance-limiting factor (PLF). Addressing the weakest sound transmission path is crucial for achieving substantial improvements in noise control. 

Investing in acoustic products with proven performance is critical. The standalone acoustic performance of materials and products are typically rated using dB _Rw. Whilst the exact performance requirements and product selection will depend on the project, choosing solutions which have been designed, developed and tested specifically for noise control in curtain wall applications can help ensure compatibility.  

At Siderise, we have developed a range of products that are specifically designed to address the acoustic issues associated with curtain wall systems, as well as ensuring their passive fire protection. These include: 

MI Mullion/Transom Acoustic Insert 

Inserting acoustically absorptive inserts within hollow mullions and transoms can reduce noise transfer through these internal cavities. Various solutions are available to suit different curtain wall configurations and noise control applications.  

MC Mullion Cover 

This is an external treatment that can be used to overclad curtain walling mullions where there are abutting partitions. This can substantially improve acoustic performance through the mullion. MC is also available as a fire-rated variant which can provide up to 2 hours EI fire resistance. 

As a decorative treatment, MC can be colour-matched to the façade. It is ideal for projects where acoustic infill upgrades to the mullion are not practical such as retrofits and situations where the internal layout is unknown during construction, or when fit-out or operation has already begun.  

FIP Acoustic Panel 

An acoustic panel developed to reduce noise transmission at the junction between internal partitions and curtain walls or window mullions. It provides a slim solution where high dB Rw performance is needed. 

Acoustic Barriers (CW-AB, CVB/C10) 

Installed beneath CW-FS Curtain Wall Firestops functioning as a perimeter fire seal at the edge of the floor slab, it can effectively reduce floor-to-floor sound transmission. Unlike traditional mass-barrier materials such as steel or plasterboard, our barriers can accommodate façade movement.