Why is building movement a vital consideration when specifying perimeter fire seals for curtain wall facades?

It is vital curtain wall facades are designed and constructed to withstand constant movement due to forces such as wind loading, and this must include their passive fire protection measures.

Managing movement 

Since the mid-1900s, buildings with glass external wall systems have become a common sight in cities and regions around the world, offering an ultra-modern aesthetic, floor-to-floor daylighting opportunities and excellent weatherproofing. Unlike more traditional wall constructions, curtain walls are not part of the building’s supporting structure but instead act as a vertical enclosure or envelope, separating and protecting the internal environment from the external conditions. This makes them highly susceptible to movement caused by forces acting upon their surface. This includes wind and rain loads, seismic sway, occupancy loads on the floor slab and thermal loads that cause the structure to expand and distort, especially in the event of a fire. Whilst various calculations and computer modelling can help designers to better understand the range of potential movement and work needed to mitigate it, it is impossible to design it out completely. 

Therefore, curtain-walled buildings are commonly built with a gap between the edge of the floor slab and the facade itself to allow for both vertical and lateral deflection. This is referred to as the movement gap or the expansion joint and the designed perimeter gap can vary considerably in void size. For example, this can be wider in certain cases such as where buildings are built in locations with high movement risk (e.g. frequent seismic activity) or include more dynamic, complex design elements (e.g. curved elevations). Whilst vital for allowing free movement without damage to the structure, this gap can present a challenge when it comes to ensuring passive fire protection.

Perimeter fire seals

If left unprotected, the movement gap can provide a clear path for flames, smoke and heat to be drawn upwards due to the chimney effect, enabling the fire to spread floor to floor. To prevent this, perimeter fire seals – also referred to as firestops – must be installed at each floor level to maintain the integrity of the compartment floor slab right up to the internal surface of the curtain wall and achieve compartmentation. 

However, as the gap width is in constant flux, it is important that the specified solution not only achieves the required fire resistance but that it can maintain a tight seal whilst withstanding constant inward and outward movement for the lifetime of the building. If the perimeter seal does not adequately recover from the compression cycles, then gaps could form between the curtain wall facade system and the structure, allowing the fire to spread through the building envelope and putting both lives and property at risk.

It is therefore recommended that curtain wall perimeter fire seals provide a 7.5% or greater movement capability at the movement joint, allowing both compression and flex. This is a very real requirement, with deflections sometimes being considerably larger in comparison with the void dimension being sealed. To do this, they need to be installed under compression. The importance of this is clearly explained by the Association for Specialist Fire Protection (ASFP) in its Red Book – Firestopping: Linear Joint Seals, Penetration Seals & Cavity Barriers (4th Edition):

‘The effectiveness of the fire stop will depend on the ability of the curtain walling/cladding system to maintain the compression fit for the duration of the required fire resistance period. Unless the system is installed pre-compressed and can move to maintain compression, premature failure of the fire-stopping may occur.’

Consequently, careful consideration needs to be given to how well a product can withstand these compressive forces over time, whilst retaining their ability to recover/flex. 

Importance of form and fit

There are several perimeter fire-seal options on the market, and their suitability for curtain wall applications is dependent on the compressibility of the product, and how well it can recover to allow for flex.

Traditional approaches required installers to cut and compress the non-combustible insulation material onsite, and seal it with a sprayed-on wet compound. They are often referred to as ‘wet systems’ and must be applied in dry conditions to prevent wash out. Once the compound is applied, the only way to verify if the right compression has been achieved is through destructive testing. This type of system also typically uses standard stone wool insulation with horizontal fibres. Whilst this makes the product stable, this orientation also makes it difficult to repeatedly compress without resulting in a breakdown of the bonding between the fibres. If this happens, the product will not recover and flex in line with the facade movement, resulting in gaps and distortions forming and causing the compartmentation to fail. 

However, innovative single part ‘dry systems’ have been developed that use vertical fibre orientation. They are engineered through a unique manufacturing process that involves cutting stone wool slabs into sections and turning the sections through 90 degrees until the fibres are vertically orientated. 

These sections are then laterally compressed under an automated factory process to remove any gaps between the sections and assist uniformity of product density. Whilst the inline compression is applied, foil facings are simultaneously heat fused to retain the ‘pre-compression’. This aluminium foil additionally provides weather protection without the need for wet seals. They are then further compressed by 10% when they are installed, wrinkling the foil and making it very easy to confirm if they have been installed correctly. 

Provision for movement in test standards

As with any fire-protection product, it is crucial that curtain wall perimeter fire seals are tested to standards which accurately reflect how they will be used and the conditions they will be subjected to. This ensures that their fire-resistance performance is truly accurate. As a result, both of the most commonly used test standards for perimeter fire seals around the world include some provision for movement. 

The EN 13830 product standard for curtain walling stipulates fire-resistance testing in accordance with EN 1364-4 and durability and cycling in accordance with ETAG 026 (now superseded by EAD 350141-00-1106). EN 1364-4: 2014 – Fire resistance tests for non-load bearing elements (Curtain walling – part configuration) is the European standard for curtain wall perimeter fire seals. It is a part configuration test that applies to both ‘Type A’ – fire-rated, and Type B – non-fire-rated, curtain wall systems. Non-fire-rated facades are typically made from aluminium and therefore when being tested, the critical spandrel zone requires protection to enable it to survive for the duration of the test. 

As the test construction is reflective of curtain wall construction, the perimeter fire seal will be subjected to movement caused by deflection of both the floor slab and the curtain wall facade due to the thermal load of the fire. However, EN 1364-4 also includes provision for long-term facade movement by referring to the European Technical Approval Guidelines 026-3, which is now superseded by European Assessment Document (EAD) 350141-00-1106. This is a harmonised technical specification for linear joint and gap seals developed by the European Organisation for Technical Assessment (EOTA) for cases where a product is not fully covered by harmonised European standards. EADs are the basis for issuing European Technical Assessments (ETAs). EAD 3501141-00-1106 requires that the perimeter seal is subjected to a minimum of 500 cycles between the minimum and maximum joint width, to simulate wind sway, seismic activity and thermal load at a rate designated by the test applicant. After this cycling, the construction is left to stabilize for 24 hours and cannot be altered before testing. The pre-cycled perimeter seal and overall system is then exposed to furnace testing of over 1,000°C at 20Pa positive pressure as per the EN 1364-4:2014 test standard.

If manufacturers are voluntarily applying a CE Mark, it is vital to understand whether they have tested in accordance with EAD 350141-00-1106 as only in these cases has true movement testing been applied.

EN 1364-4 has also been the recommended test standard by the Association for Specialist Fire Protection (ASFP) since 2014, as per the Advisory Note 7:

‘The ASFP recommends that horizontal linear gap seals used in association with curtain walls are only tested to BS EN 1364-4 and that test evidence obtained using BS EN 1366-4 cannot be used to support that end-use application.’

It is important to note that EN 1366-4 is a static concrete to concrete test and cannot be used to support curtain walling applications, the same is also true for the older BS 476-20 test standard.

ASTM E 2307-20 Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers is the American test standard for curtain wall firestops. This is widely referred to in regulations in the USA, UAE and various regions in the wider Middle East and Asia Pacific. It is designed to measure the performance of the perimeter fire seal only, assessing its ability to ‘maintain a seal to prevent fire spread during the deflection and deformation of the exterior wall assembly and floor assembly during the fire test.’ It uses intermediate scale multi-storey apparatus to expose the perimeter joint to fire, both from the room where the fire started and the exterior as the fire plume exits the room of fire origin through a window opening. 

Like EN 1364-4, it also includes provisions for pre-cycling, or ‘cold’ movement of the test specimen before the test begins, covering thermal load, wind sway, seismic movement and all these combined. The rates for cycling and minimum number of movement cycles are dependent on the type of movement required. 

Checking if these tests have been third-party certified is an important step for specifiers to ensure confidence in the performance and consistency of the products. Gaining certification is a rigorous process, usually involving reviewing product test data against appropriate standards and requirements, submitting product samples for analysis and as comparative samples. Factory visits and audits may also be carried out randomly, and certification will be withdrawn, and re-testing required if any significant changes are observed. 

Manufacturers of these products may also carry out their own accelerated age testing to further determine the robustness of their products. 

The importance of the ‘spandrel zone’

Both EN 1364-4 and ASTM E2307 are tests of the facade system. Therefore, it is vital that the complete tested and certified solution, including protection materials for the critical spandrel zone in the case of aluminium facades, is fully understood and detailed at the time of project design and specification. The perimeter seal, spandrel insulation and any additional fire-protection boards all operate together to deliver performance and can’t be used interchangeably or substituted without repeating the full curtain wall facade set of tests.

Fit for function

Curtain walls are a beautiful, functional facade option, but careful attention needs to be paid to ensuring appropriate passive fire protection. Specifying perimeter fire seals that have been engineered and third-party tested to ensure their performance reflects their intended use as part of a constantly moving curtain wall facade is vital to preventing fire spread and providing peace of mind for the lifetime of the building. 

For more information, go to www.siderise.com