Rethinking joint systems: Siderise breaks down fire performance at the slab edge
Iain Hoey
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Siderise explains the performance differences between one-part and two-part perimeter joint systems to help project teams assess the most robust option for curtain wall applications
Robust compartmentation, supported by an appropriate perimeter fire seal solution, is essential to the passive fire protection of glazed façade systems – and is legally mandated in many regions worldwide.
To prevent vertical fire spread, firestops installed at the slab edge extend the fire resistance of the compartment floor to the rear of the façade.
These solutions typically fall into two main types: one-part dry fit systems and two-part pack-and spray systems.
Two-part pack-and-spray systems
Traditional approaches to firestopping at the facade / floor slab interface require the installation to be carried out in two separate stages.
The first step involves manually compressing generic mineral wool safing insulation material (typically 25 and 33 % compression) before installing it within the movement gap at the perimeter construction joint.
The second step involves applying a wet compound such as mastic, spray ablative or fire-rated silicone over the top of the insulation to create a smoke- and weather-resistant seal once cured.
One-part dry-fit systems
These systems feature factory engineered pre-compressed stone wool Lamella insulation with foil facings applied to the cut surfaces of the fibres to retain the ‘built-in pre-compression’ firmly in place and assist with uniformity of product density.
Designed and tested to provide a robust fire and hot smoke seal in the perimeter void between the facade and compartment floor, without the need for a topical wet sealant, they are installed in a single-fix with further manual compression of 20% for a tight fit and to accommodate movement.
Unlike the wet seal of two-part systems, one-part dry-fit firestops can be ordered pre-cut to suit the cavity width and can be supplied as part of a complete system package from a single manufacturer with spandrel insulation, mullion and transom fire protection, plus ancillary components.
In addition, these parts are often tested together in multiple test scenarios, and third-party certified as an advanced perimeter fire containment system for a more holistic approach to passive fire protection in the spandrel zone.
Each of these approaches results in different characteristics when it comes to fire performance and resilience, installation accuracy and efficiency, and repeatability and inspection.
1. Firestop resilience, durability and fire performance
Non-combustible mineral wool firestops must be durable enough to maintain their form and fit, even when subjected to façade and floor slab movement caused by environmental loads.
The safing insulation component of two-part firestop systems typically comprises standard mineral wool insulation with horizontally oriented fibres, which run parallel to the floor slab when installed.
While this 90° or transverse alignment offers stability, continual exposure to building / façade movement makes the material more susceptible to degradation along its length, resulting in the fibre structure breaking down.
This means that the ability of the safing material to recover from repeated compression cycles in-service is reduced.
This can lead to the firestop no longer maintaining the compression fit, increasing the risk of a gap forming between the product and the facade over time.
Should this occur, the perimeter seal could fail prematurely, resulting in loss of compartmentation.
Furthermore, to ensure that two-part wet-seal systems comply with the criteria of any third-party approvals or listings published by the sealant manufacturer, the safing insulation product should always be the same as that used which was in the certified tested system.
Using substitution products risks undermining the specified fire resistance performance.
Meanwhile, one-part firestops with a stone wool Lamella composition, such as ours, comprise vertically oriented fibres that run perpendicular to the substrate when installed.
Products with a vertical fibre orientation are highly compressible and flexible laterally (across their width), which enables them to not only overcome challenges presented by curved and inclined facades, but accommodate the dynamic movement of the curtain wall or floor slab.
Due to the unique structure of Siderise Lamella firestops they are able to maintain their ability to recover throughout their design life, as demonstrated through extensive age and movement cycling testing.
This means that the seal is continuously maintained throughout the lifetime of the building and for the required fire resistance period in the event of a fire.
When exposed to fire, the foil facing is designed to delaminate from the stone wool Lamella core, allowing the built-in pre-compression to be released.
This enables the product to expand and maintain its compression fit and integrity, even under the greater thermal stresses caused by the fire load that leads to facade and floor slab deflection.
Both system types can be tested to UL 2079 for air leakage, determined as the L-rating, to simulate smoke movement through compartmentation in buildings.
Leakage testing can assist authorities in determining the suitability of firestopping systems for the protection of floor openings and smoke barriers for the purpose of restricting the movement of smoke in accordance with the NFPA (National Fire Protection Association) 101 Life Safety Code.
The difference between two-part spray and pack systems and one part dry-fit systems is that the latter does not rely on a wet seal to provide an adequate L rating
2. Application conditions, consistency and installation efficiency
The wet seal compound in two-part systems is sensitive to moisture and temperature dependent during application and curing.
It must be applied in dry conditions and controlled environments – which can be a challenge during hot seasons or locations with frequent rainfall– otherwise adhesion, curing, design life and product integrity may be adversely affected.
In most cases, the maximum application temperature is typically limited to 104°F (40°C) as high heat can cause rapid skinning or premature curing, making tooling difficult and reducing adhesion.
Similarly, exposure to rain, running or standing water, or high humidity can lead to washout or uneven curing.
Storage conditions for wet seal firestop compounds are just as important as application conditions because they directly affect product performance and shelf life.
Most manufacturers typically require sealants to be stored between 40 and 80°F (5 and 25 °C), which in turn requires onsite air-conditioned storage facilities in regions prone to inclement weather.
Improper storage can reduce workability if the sealant has thickened, shorten open time and curing consistency, and compromise the fire rating performance due to chemical degradation.
Since wet sealants rely on direct contact with the substrate, surfaces must be clean, dry, and free of dust, grease, oil or other contaminants.
Dust or debris creates a barrier that can weaken bonding and lead to gaps or peeling over time, while contaminated surfaces can cause voids or uneven curing.
Furthermore, the wet-film thickness must be applied consistently to achieve the required dry-film thickness as thickness variation due to human error can risk compromising the slab edge fire seal.
Wet spray firestop systems require specialist equipment such as pumps, mixers and nozzles.
Transporting and setting up heavy spray rigs on high-rise sites can be time-consuming while tight slab edge zones often lack room for bulky equipment, especially when façade works are concurrent.
In addition to having to wait for the right application conditions, the ‘curing’ and overspray clean up time also needs to be accounted for in the project schedule.
The flashing used to close off the spandrel zone from sight cannot be installed until after the installation has been approved, which means site operatives cannot immediately proceed with adjacent works, thus potentially holding up the build program.
This installer-driven and weather dependent approach can make it challenging to achieve a replicable level of quality across whole buildings of multiple stories, creating bottlenecks, especially when inspections are required before concealment.
In contrast, one-part dry-fit firestops are not weather or temperature sensitive and do not require any specialist application equipment and surface preparation of the substrate.
There are no curing times, wet or dry-film thickness complications or complex compression calculations.
Their straightforward installation approach, including a standardised compression percentage, can streamline the installation process and help reduce the risk of human error.
They also provide greater freedom for the installer in areas where access to the firestop zone is difficult after the facade is installed.
Dry-fit firestops can also be installed before the facade (including from the soffit).
3. Ease of inspection
With two-part systems, once the compound is applied, verifying the correct dry-film thickness, adhesion, and safing orientation and compression becomes difficult without destructive testing or a two-stage inspection.
Alternatively, when dry-fit, one part firestops are installed, visual inspections are sufficient.
The wrinkling of the foil makes it very easy to confirm if they have been installed correctly without disturbing the installation.
Their dry-fit application also makes it easier to judge bracket locations and distances, and tight jointing to ensure a continuation of the fire performance around the perimeter.
This can not only help to ensure a safer building but also reduce additional costs and lost time associated with remedial works.
Transparent protection
While traditional two-part firestop systems have been widely used in US construction, they often present challenges related to long-term durability, installation complexity, and inspection consistency.
In contrast, one-part firestop systems that have been engineered and tested to meet key standards such as ASTM E2307 and ASTM E2874 offer a reliable alternative.
By delivering uniform compression, accommodating façade movement, and enabling straightforward visual inspection, one-part dry-fix systems support code-compliant construction practices and help ensure consistent fire-resistance performance across every floor and project.
