One Exchange Square, London, UK

The project

The project team, led by Fletcher Priest Architects, who previously worked on 135 and 155 Bishopsgate, approached this retrofit project intending to honour the previous building and reuse as many materials as possible.

Most of the existing structure has been retained, including the steel frame, concrete slabs and foundations – components that typically incur the highest carbon cost for new constructions. Nearly 50% of the granite façade was also retained and refurbished in place, thermally upgraded, and new high-performance faҫade systems were installed.

By preserving these original materials, the project is designed to achieve 50% less embodied carbon than a typical office building of comparable size, surpassing the Greater London Authority’s carbon targets. Where new materials were necessary, they were carefully chosen to meet strict, environmentally considerate specifications. 

During the initial construction stage, Siderise were specified to supply the passive fire protection for the full façade, including both vertical and horizontal cavity barriers and firestops. When it became evident that the building needed further refurbishment, Siderise were approached again to supply acoustic products and further passive fire protection for difficult-to-reach cavity areas. Throughout this process, Siderise provided support via covered[RR1]  technical support, product installation training, benchmark constructions as well as inspections to support Multiplex. 

Products:

For the acoustic solution, Siderise Tissue Faced Stone Wool Acoustic Lining Slab (ALS) were fixed into place below soffits of the building behind aesthetic acoustic timber louvre panels to improve the overall sound absorption. These slabs have been specifically tested for acoustic absorption performance in large areas where reverberation noise can be problematic and contribute to noise transfer into other areas, such as shared office spaces. Made from a resin-bonded stone fibre to absorb sound and faced with a black non-woven glass fabric, this special formulation offers Class A sound absorption and meets the acoustic requirements needed for the project’s ambitions to achieve BREEAM Outstanding and WELL Platinum building certifications, both of which have onerous acoustic targets. 

CW-AB Acoustic Barriers were installed to further reduce noise transmission. Fitted as a mass barrier overlay above CW-FS Curtain Wall Firestop, the flexible membrane is designed to accommodate façade movement, unlike traditional mass-barrier materials such as steel or plasterboard. 

As part of the passive fire protection aspects of the project, Siderise CW/FS120 & CW/FS120X Firestop sealed the gap between compartment floors, walls and the external façade. The lamella insulation core comprises durable vertically oriented stone wool fibres laterally compressed under factory-controlled conditions, accommodating building movement and façade deflection, ensuring a tight seal for the lifetime of the building. 

Siderise TW Firestop was used to seal the tops of internal walls where gaps were present between the wall and soffit. This product has been tested to EN 1366-4 and can provide up to two hours of fire resistance in voids above compartment walls and partitions. It also contributes to occupier comfort by reducing ‘room-to-room’ sound transmission thanks to the unique Lamella construction of the stone wool insulation core, providing up to 23dB sound reduction, using a Weighted Sound Reduction Index (R_w) to give a true picture of its acoustic performance.

A number of Rainscreen products were also utilised for the project, namely RV & RH Cavity Barriers in varying specifications. RV Vertical Cavity Barrier is designed to completely seal the void between the external façade and internal structure, preventing the spread of fire. It can provide up to two hours of fire resistance and can compress and flex in line with natural building movement. RH Horizontal Cavity Barrier is an open state solution, allowing for adequate ventilation and drainage within the building envelope. The cavity barrier incorporates an integral strip of intumescent material along the leading edge which, in reaction to fire, expands rapidly until the gap is fully sealed, preventing fire spread for up to two hours. 

Open State Cassette Inserts were used to create a flat surface opposite Siderise Cavity Barriers, ensuring that the ventilation air gap is dimensioned correctly. The inserts are a small block of A2-s1, d0-rated stone wool with foil facings and an integral self-adhesive strip, making them easy to directly apply to the internal surface of cassette panels. 

The building also incorporated brickwork elements, which were supported by the Siderise EW Cavity Barrier and Firestop. Designed specifically for masonry walls, the Lamella core accommodates site tolerances and localised common imperfections in brickwork, and can be applied to wet brickwork without compromising the stability of the wall structure. 

Conclusion

With a wide range of Siderise products installed on One Exchange Square, it is an excellent example of how our passive fire protection and acoustic products can be used together to assist with achieving sustainability goals and building certifications. The project as a whole, also demonstrates how thoughtful retrofit strategies can deliver modern, high-performance workspaces while dramatically reducing environmental impact. By retaining much of the existing structure and selecting materials with high environmental, acoustic and fire performance, the project demonstrates that architectural ambition and carbon-conscious design can successfully coexist. 

View our full range of passive fire protection and acoustic products.