Thermally broken steel windows and doors manufactured by Clement

The existing commercial building at 22-23 Little Portland Street comprised ground, basement and four upper floors. Robson Warren Architects’ brief was to design a scheme that improved the office facilities and services, enabling the building to attract further tenants. Their proposals were required to take account of the environmental sustainability of the works and their impact on the property, which is situated in the East Marylebone Conservation Area.

Peter Warren from Robson Warren Architects said: “The building was entirely of post-war construction, with single glazed, steel-framed, W20 type casement windows. The subdivision of the windows and slender steel glazing bars was a significant contributor to the appearance, but the existing single glazing would prevent the achievement of the required improvements in environmental performance. Refurbishment of the existing frames was rejected on the grounds that they were too badly corroded, not thermally broken and provided practically no insulation value, acting as a cold bridge and source of condensation.

“Approval was sought from Westminster City Council to replace the existing windows with thermally broken, steel-framed, double glazed units to improve thermal and air tightness performance whilst replicating the existing appearance as closely as possible. Steel framing was chosen to allow the slenderest profile to be used while achieving the required performance improvements. Once planning permission was granted, proposals were sought from potential suppliers for both double and triple glazed windows and doors. After much consideration, slender Jansen Janisol cold rolled, mild steel sections which achieved the sight lines required were specified from Clement Windows. Their comprehensive system allowed us to incorporate open-in and open-out windows, French doors and lift and slide doors into the scheme.

Clement provided clear drawings to assist in the development of the design which were approved for manufacture. A request for a bespoke RAL colour for the frames was accommodated. The installation was completed to our satisfaction without any significant issues. We would not hesitate to return to Clement Windows for enquiries about future steel-framed window projects.”