Wolfson College, Oxford

Over the past 40 years, Wolfson’s campus of granite-faced concrete has become much loved, and fiercely defended, by staff and students. In 2011 it was listed, just as architect Berman Guedes Stretton was working up plans for the site’s first major extension: an auditorium, new entrance and academic wing.

That BGS’s design was barely affected by the listing says a lot for its engagement with the spirit of the campus. “We were acutely aware that it was a very important piece of architecture, and were surprised it wasn’t listed already,” says Marion Brereton, director at BGS.

The precast cladding on the academic wing uses the same Cornish Blue granite, with the aggregate placed by hand. The strong horizontal banding – an external expression of the floor slabs – is also reprised with structural precast beams in the same granite finish.

But this is no simple homage. BGS also had to address a number of problems with the original design, not least its propensity to leak heat through the cold bridges of its expressed structure. The college, alarmed at its rising energy bills, had already upgraded the insulation on the existing blocks and was determined that the extension would take a more energy-efficient approach.

The academic wing’s structural beams are therefore set back from the floor slab, highly insulated and reinforced with stainless steel – far less conductive than conventional reinforcement. Meanwhile, the auditorium is snugly wrapped in foamglass and insulated render. The overall U-value for the extension is an impressive 0.15W/m²K.

The horizontal banding posed another challenge, as it restricted BGS to the same floor-to-ceiling heights as the adjacent buildings. “We didn’t want a very obvious extension where you had to ramp up or ramp down,” says Brereton.

Structural engineer Price & Myers therefore had to devise a slab that kept depth to a minimum while supporting the long spans required in the open cafe area. Services were cast in, but the nearby library meant that some areas also needed a 38mm acoustic plaster. “We had to be very mindful that the ceiling level was coming down all the time,” says Brereton.

The extension incorporates a range of concrete techniques: a retarded, white-pebble finish for exterior columns; precast, acid-etched columns in the auditorium; a curved glassfibre-reinforced concrete (GRC) reception desk; and expanses of exposed in-situ structure, including the auditorium’s dramatic raking beams.

The exposed structure, which used a 40% GGBS mix, also plays a key role in the energy strategy, its thermal mass used in harness with night-time purging through the auditorium’s ventilation chimney. The chimney also helps to solve another of Wolfson’s longstanding problems.

Unusually for a listed building, BGS’s extension is at the front of the campus, which was an area where Phillip Powell felt he had failed, describing the entrance as “not very obvious or welcoming”. BGS’s chimney has been placed on an axis with the residential road leading up to the college. It is a powerful signal of Wolfson’s presence – Powell would surely feel that it finally has the entrance it deserves.

Featured in CQ Winter 2016 (pg 9)