The interface between the Steel Framing System (SFS) and the curtain wall is one of the most critical junctions in modern building envelope design. When executed correctly, it provides a seamless, high-performance façade that is both structurally sound and thermally efficient. However, failure to manage the design details at this connection point can cause serious, costly defects that can compromise the entire building's integrity. Understanding and mitigating the primary sources of failure is therefore essential for architects, engineers, and installers.
This article explores the three main challenges in SFS curtain walling design: structural movement, thermal bridging, and weather penetration. We will explain how to address these issues through collaborative design, specialised components, and a focus on continuity, ensuring a robust, compliant, and durable façade solution that stands the test of time.
The fundamental difficulty lies in connecting two distinct systems with different performance characteristics. The SFS systems form the building's structural infill, subject to live and dead loads from the primary structure, while the curtain walling systems provide the weather-resistant, glazed envelope. The connection must accommodate movement from the main frame while maintaining a continuous barrier against air, water, and thermal loss. Neglecting this dynamic relationship is a recipe for disaster.
A primary structure of steel or concrete will always deflect under load. This movement, though often minimal, can be significant enough to impose immense stress on a rigid curtain walling system if it’s not properly managed. If the SFS frame moves with the main structure, it can transfer these loads directly to the glazing and its aluminium framing. The consequences can be severe, ranging from cracked glass and warped frames to failed seals and compromised structural integrity. The key to preventing this is to isolate the curtain wall from the structural movement of the SFS. To achieve this isolation, designers must incorporate specialised components that allow vertical deflection, such as:
Slotted head tracks: these tracks allow the SFS studs to move vertically without transferring axial loads to or from the slab above. They are a fundamental part of accommodating slab deflection at the head of the SFS infill wall.
Deflection joints: purpose-designed joints within the SFS assembly and at the connection points to the curtain wall bracketry are crucial. They provide a controlled gap that can open and close as the structure moves, ensuring the load is not passed onto the façade system.
Without these elements, the SFS curtain walling junction becomes a rigid point of conflict. Every millimetre of building movement translates into a direct force on the glass and aluminium, increasing the risk of premature failure and rectification works.
A building's thermal performance is only as strong as its weakest link. ‘Thermal bridges’ are points in the building envelope where heat can travel more easily from inside to out (or vice versa), often occurring where materials with low thermal resistance interrupt the insulation layer. The junction between SFS systems and curtain walling systems is a notorious location for thermal bridging.
Metal-to-metal contact, particularly with steel brackets connecting the SFS to the aluminium curtain wall frame, creates a direct path for heat loss. This not only increases energy consumption and operational costs but can also lead to condensation problems.
To avoid this problem, you will need to create a continuous thermal break. This involves:
Thermally broken brackets: Using brackets made from materials with low thermal conductivity or incorporating thermal pads between the steel SFS and the aluminium curtain wall brackets. These components physically separate the internal and external metallic elements, stopping the flow of heat.
Continuous insulation: The insulation layer within the SFS wall must align perfectly with the thermal break in the curtain wall system. Any gaps or misalignments will create a cold bridge, undermining the performance of the entire wall assembly, so careful design and on-site quality control are essential to verify this continuity.
The façade is the building’s first line of defence against the elements, and so a failure to correctly detail the weatherproofing at the SFS and curtain wall interface could result in water ingress and air leakage. This poses the risk of internal damage, mould growth, and poor building pressurisation.
The challenge is to maintain a continuous air and vapour control layer (AVCL) from the internal SFS lining to the curtain wall system itself. Any break in this seal creates a path for moisture-laden air to penetrate the wall cavity, where it can condense on cold surfaces and cause significant damage. Achieving a truly weatherproof seal requires a multi-layered approach:
Vapour barrier continuity: The vapour barrier on the warm side of the SFS insulation must be seamlessly connected to the curtain wall frame. This is often achieved using specialised tapes, membranes, and adhesives that are tested for compatibility and long-term adhesion.
Air seal integrity: An uninterrupted air seal is equally important for both thermal performance and weather resistance. This seal prevents uncontrolled air movement through the façade, which can carry moisture and reduce the effectiveness of the insulation.
Drained and ventilated cavity: Proper detailing should ensure that any water that might penetrate the outer seals can drain away safely. The design of the connection must not block the drainage paths inherent in the curtain wall system.
Verifying the compatibility of all sealing components (e.g. tapes, membranes, and sealants) with each other and with the adjoining substrates is essential for long-term performance.
Preventing failures at the SFS curtain walling junction does not happen by chance. It is the result of a deliberate and collaborative process rooted in early design planning and a commitment to using tested, compatible systems. If you would like to find out more, please get in touch with one of the members of the team at Advanced Glass today, or [click here to download our free guide].
Image Source: Canva