Cool quality: an ingenious greenhouse for Alpine plants required a design which maximises light--and cold air.

The Royal Botanic Gardens in Kew, one of London's great wonders, had its origins in the seventeenth century, when a private family estate development included a noted orangery. A century later, two neighbouring estates were amalgamated, bringing together landscapes and buildings by Capability Brown and William Chambers, creating a marvel of vistas and architecture which was eventually made over to the nation in 1841--not least to formalise the botany research undertaken there. Significant buildings include various classical temples and the Pagoda by Chambers, and mid-nineteenth-century additions including the Palm House and Temperate House by Decimus Burton and William Turner. The new Alpine House, by the London architects Wilkinson Eyre, is a delightful and ingenious addition to this greenhouse history.

As part of a long-term strategy to complement its World Heritage Site status (formally granted in 2003), a competition to produce a site development plan was held, resulting in Wilkinson Eyre's winning proposal which is now being implemented. As part of the redevelopment of research facilities at the northern end of the site (by the same architect), a decision was taken to move the Alpine House, one of the least known of all Kew's pavilions because of its obscure location.

The new site is highly prominent, close to the Princess of Wales Pavilion in a surrounding of planting, trees and rockeries. The Alpine House, a small structure (144[m.sup.2] and 10m at its highest point), nevertheless makes a big impression. From a distance, it looks as though it might be the arch of a railway bridge, especially as you can see through it, the result of low-iron glass providing 90 per cent light transmission. In bright sunlight, internal fabric sails can be raised to protect the plants--this is not one of those glazed buildings where combating sunlight has been dealt with as an afterthought.

In Britain's temperate climate, the usual purpose of greenhouse design is to keep the environment warm. The requirement for alpine plants, however, is to give them plenty of light and, crucially, a constant flow of cool air. The architects had to provide a sheltered environment with suitable cooling, without resorting to air-conditioning and cumbersome plant, and retaining transparency as a key driver for the project.

The design process was aided by greenhouse engineer Green-Mark International; structural engineer Dewhurst MacFarlane; and service engineer Atelier Ten, who worked with Wilkinson Eyre to produce a response which is both pragmatic and visually striking. The height of the back-to-back painted steel twin arches is determined by the need for stack effect ventilation, created by opening vents at high level and low-level perimeter air inlets. Below ground, air is pushed through a labyrinth within a concrete double slab; its thermal mass cools the air, which is re-circulated round the perimeter and onto the plants via displacement nozzles and pipes. The silicone jointed glazing system, produced by Swiss firm Tuchschmid Constructa, uses laminated glass (two panes of 6mm toughened glass). The key feature is the structural glazing to the sloping side-walls, where patch fittings clamp on to tension rods which span between the upper and lower arches. The fittings support 2 X 1m panels at each corner. The advantages of this system over a conventional framed glazing alternative is the relative increase in light transmission allowed by the former (1.5 per cent obscurity) against 5 to 10 per cent with the latter.

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The architect has succeeded in creating what will be a new magnet for visitors, and has stayed true to the tradition of the iron and glass generation who combined creativity with economy of means.

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