Light house news: Richard MacCormac explains his approach to creating a new glass building next to a well-loved stone one.

The idea for a glass facade emerged from the necessity of replacing the slate roof which formed a mansard on the east side of Broadcasting House [by G. Val Myers, 1931] with a superstructure of equivalent weight, rather than the Portland Stone originally envisaged. The design of a light glazed attic led to the idea of a concave glass facade to terminate the axis of Regent Street, directly behind the spire of All Souls Langham Place--a kind of urban cyclorama. A polarity is set up between the assertive convex solidity of Broadcasting House and the receptive concavity and lightness of the suspended facade.

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Solar control is achieved by an outer glass screen 1200mm out from the building envelope which also has a high thermal performance. The screen has been the subject of an extensive development programme to achieve solar protection and to create the appropriate daytime and night-time backdrop to the church and relationship with the stone facade of Broadcasting House.

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The solar screen of laminated glass is etched in a pattern of vertical strips on the outer face and ceramic frit on the outward-facing surface of the inner laminate, with a grid of small squares on the same horizontal module as the etched strips. The ceramic frit is chromatically and tonally matched to the Portland Stone facade at the north-east corner of Broadcasting House. During daylight, the patterned areas of the screen will appear relatively opaque and comparable to the Portland Stone of Broadcasting House. At twilight, the facade behaves like a theatre scrim becoming relatively transparent as interior light levels exceed the level of daylight. The transparency can be reversed or varied by external floodlighting or by images projected on the facade.

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To understand and verify this concept, modelling exercises were carried out with the lighting designer and consultant Rogier van der Heide using his laboratory in Amsterdam. The first exercise measured the transmittance and transparency of a range of opacity in glass samples under various internal/external light conditions. Using the glass sample with the best balance between opacity under daylight conditions and transparency under night-time conditions, the second exercise involved more specific observations of the night-time visibility of floors, ceilings, stairs and furniture at various internal light levels. In particular, the model investigated how transparent the facade would be if light levels on working surfaces in the circulation and break-out areas were reduced to 180 lux so as to ascertain the impact of internal lighting behind All Souls. The third exercise included the spire of All Souls to test various lighting relationships between the two extremes of strongly illuminating the spire against a low level of facade lighting, and setting the spire in unlit silhouette against a high level of facade lighting. The broader context for these ideas includes the floodlighting of the south front of Broadcasting House and its symbolic Radio Mast. R.M.