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Clear thinking

Michael Wigginton has rethought every architect's favourite answer to the extension - the glass box

In the past, the conservatory for over-wintering exotic plants was the building type of which designers explored the potential of glass - think Chatsworth, Kew and Dublin Botanical Gardens. Today, on a domestic scale, it is the commonest form of extension.

A small collection of recent projects has used the conservatory as an essay in form and detail to examine the use of glass as a structural material. Architect Michael Wigginton, of the Designers Collaborative, with structural engineer Mark Whitby, has a different approach. 'To us there's a slight irrationality in an all-glass building - why make glass do all the structural work when metal does it so much better? And why compromise pane sizes and make the support of blinds and lighting impossible, ' he says.

Wigginton has had the opportunity to examine these questions in practice. The owners of Kelbrook House, a Grade II-listed terrace house in Amersham Old Town, Buckinghamshire, wanted a new conservatory to replace an old lean-to attached to the rear south wall of the house, and to give them a new kitchen and dining room. It was to be used in all seasons and weathers as an integrated part of the house.

The concept, as Wigginton explains, is 'a pure glass box slid between the garden walls, its precision and materiality separated from the skewed and bowed masonry around it'.

The conservatory is an 8.1 x 3.6 x 2.4m high cube, separated from the eighteenth century rear wall of the house (to allow for new foundations and a gutter), and with space at each end to allow for the angles of the garden walls, each about 2.5m high, and to accommodate the working end of the kitchen with its services and equipment.

The glass roof is supported by a steel structure set about 300mm away from the old walls to allow gutter and tolerances and to express the independence of the structure. The roof is as flat as possible, falling back 35mm in 3.5m - just enough to ensure that water actually reaches the gutter.

The steel structure, a series of parallel flange channel (PFC) columns and T-section beams, supports a roof of double-glazed panels, the edges of which lie precisely at the centre of the flanges of the T-section beams, where they are sealed with a silicone joint.

The south walls are of single-glazed 12mm toughened glass panels set in a floor channel and restrained at the top with stainless steel bolts. 'We went for a structure in which the elements had a 'furniture' quality of scale and finish, ' explains Wigginton. The crisp-edged channel columns and beams are galvanised and polyester powder-coated white; the floor is grey Pietra Serena limestone, incorporating silver anodised convector heating grilles; and the Bulthaup kitchen units are made of maple and grey laminate.

Being part of the house, the conservatory had to provide an acceptable quality of thermal and visual comfort in all seasons. The design was tested on the TA S computer modelling system to predict winter heat loads and maintain comfortable conditions in summer.

Heating is provided by an underfloor system to take the chill off the floor, and by a trough convector system, with a contribution from solar energy which can reach between 20 and 30 kW/h per day in December. To prevent overheating in summer, an automatically controlled motorised blind system is fitted to the webs of the beams, activated by a solar cell. The blinds reflect about 50 per cent of the radiation. Electrically controlled Venetian blinds protect the south-facing glass wall.

Together the white ceiling blinds and venetian wall blinds resolve another problem; they can be used to prevent the 'black hole' appearance of an all-glass room at night.

High air change rates are achieved in summer by opening the central pair of doors and the doors at both ends of the glazed wall. These are set in front of louvred panels to give security and keep the rain out.

As professor of architecture at the University of Plymouth, Wigginton has been developing his ideas about building and is about to publish Intelligent Skins with the Architectural Press. This thinking shows in the conservatory, which retains the visual simplicity of the glass box while using the structural strengths of steel to make it a more liveable space.

THE DESIGNERS COLLABORATIVE

The Designers Collaborative was formed in 1994 by five partners - Michael Wigginton, Robin Webster, Robin Spence, Dominic Reid and Christopher Glaister. The aim was to provide a basis for collaboration - at the time all the partners were working as sole practitioners. Two of the partners, Michael Wigginton and Robin Webster, are professors of architecture and the practice has great interest in the practical delivery of research through practice. In 1994 the practice won a European award for the 'zero' energy Zephyr project in Athens. In 1995 it produced a theoretical basis for the principles behind sustainable design for the chairman of the Earth Centre, Yorkshire. It is currently involved in several urban and rural regeneration projects.

CREDITS

ARCHITECT The Designers Collaborative: Michael Wigginton, Dominic Reid

STRUCTURAL ENGINEER Whitby Bird and Partners

QUANTITY SURVEYOR Davis Langdon & Everest

MAIN CONTRACTOR Bernard Waller

SUBCONTRACTORS AND SUPPLIERS steelwork Promet Technology; glazing Solaglas (South West); blinds Contravent Regal; underfloor heating ESWA; stone floor Diespeker Marble and Terrazzo; kitchen units Bulthaup; convector heating Kampmann; Lighting Erco, Reggiani; ironmongery Allgood

The steel structure consists of six pairs of 100 x 50mm PFC (parallel flange channel) columns which support the ends of six 102 x 125mm Tsection beams. The columns are set 1,620mm apart and are braced at the eaves by a series of 100 x 50mm PFC perimeter beams with welded end plates. The upper flange of each T-section beam rests on the flanges of a column; they are trimmed to align it with the flanges of the PFC beams. A 12mm diameter solid rod is slotted through all the structural elements - the web of the T-section, the column flanges and the end plates of the PFC beam - and bolted at its ends.

The top flanges of the T-section beams carry the glazing frames - 65 x 30mm inverted aluminium T-sections on 50 x 50mm aluminium box section spacers. The steelwork is set out to align the junctions of the 1,620 x 1,781mm double-glazed roof panels at the centre of the flanges, the edges of each panel resting on a leg of the inverted T-section. By pretapping the flanges to shop drawings provided by the glaziers, the necessary precision was achieved; the large rectangle of glass fitted exactly onto its steel frame.

Zero tolerance was achieved.

The south wall is single glazed with 12mm toughened glass; the panels are seated in a 35 x 55mm aluminium channel, the top of which is flush with the stone floor, and fixed back to the steelwork with stainless steel bolts. At the eaves the 12mm toughened upper sheet of the double-glazed roof panels is extended 58mm to meet the tops of the wall panels; the joint is sealed with a clear silicone joint.

Four glass doors, two at the centre and one at each end of the south wall, can be opened for ventilation.

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