The winners of the Green Building of the Year Award prove that environmentally sound, energy-efficient construction can also be highly practical and even beautiful. David Nicholson-Lord reports
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For a building that is one of the most innovative in Britain, the new hall of residence at Linacre College, Oxford, is remarkably discreet. Overlooking the Cherwell meadows, with a distant view of Robert Maxwell's former country retreat at Headington, it looks every inch the well-groomed collegiate newcomer - tiled roof, red-brick facades, a Dutch-style gable here, a Georgian window there. Only the nest-boxes built into the brickwork might give a passer-by a clue that he is looking at something out of the ordinary.

Inside, clues are a little more in evidence. There is a noticeable absence of long, straight corridors; students' rooms are grouped into little nests, opening off short, irregular passageways. There is an abundance of natural finishes - beechwood and stone floors, roughcast plaster walls, sisal carpets, linoleum. You will find very few synthetic materials, either; the pipework is recycled copper and the building is virtually plastic- free. Yet this preference for the natural at the expense of the man-made is no Arts-and-Crafts affectation; it is fundamental to the design philosophy.

Linacre's new hall, designed by ECD Architects, which last week won the Green Building of the Year award, sponsored by the Independent on Sunday and the Heating and Ventilating Contractors' Association, highlights one of the less appreciated aspects of green architecture. Generally, it is not flashy; it does not generate controversy among the cognoscenti and is not designed to boost egos, reputations and fees. This does not imply a lack of aesthetic quality, merely that other factors - habitability, harmony with the surroundings, frugality with natural resources - receive more attention. Is it possible for a building to be both pioneering yet sober and unobtrusive? Linacre shows that it is.

Most people, for example, think the energy costs of a building are limited to electricity and gas bills. But the power used to manufacture its components, and transport them to the building site - the so-called embodied energy - may be of equal significance. The embodied energy in a modern air-conditioned building can represent 20 years' worth of annual energy costs - by which time someone may have decided to knock it down and start again. Linacre's new hall is thought to be the first in Britain, to apply these lessons - lessons that will have to be absorbed more widely, since embodied energy may account for nearly a sixth of national energy consumption.

Embodied energy points in a direction that would have pleased William Morris. Natural materials do not merely look and feel better - producing no smells or "off-gassing", for instance. Because they involve less manufacturing, and, if they are locally available, less transport, they embody much less energy. Timber and slate are thus a far better environmental buy than plastic, non-ferrous metals and synthetic carpets (carpet tiles, according to ECD's Jes Mainwaring, are "incredibly expensive"). Recycling also cuts down on embodied energy; roof tiles from an old, demolished building at Linacre were saved and reused.

The second feature that makes the hall unique among buildings of its kind is a "grey water" recycling system, in which water from baths, sinks and showers is collected, stored, mixed with rainwater from the roof, and filtered before being used to flush the lavatories. When functioning normally, this will cut water consumption by 36 per cent - a useful feature in the coming age of water shortage. However, Linacre have paid the price of pioneering; the system is currently out of action. Its ultra-violet filters rely on clear water to function effectively and much of the effluent was too cloudy. One solution being considered is to divert the greyer water, from sinks, and to create a reed bed, at the entrance to the building, where it will be cleaned, purified and recycled.

Because of high insulation, a heavy building mass, which reduces temperature fluctuations, and "passive" solar design - study bedrooms look southwards, kitchens and utility rooms face north - the hall is an economical user of energy: between a quarter and a fifth less than a conventional building. Each year, however, it produces an estimated 42 tonnes of carbon dioxide, from gas and electricity use, to add to global warming. Early on, the college decided to "save" enough tropical forest to balance out these emissions - a square metre of rainforest absorbs about a kilogram of carbon dioxide each year - and spent pounds 10,000 adopting 40 acres of threatened eucalyptus forest in Tasmania.

Many other green features are equally hard to spot. Linacre sought advice from the Berkshire, Buckinghamshire and Oxfordshire wildlife trust on its new borders and shrubberies, planting species that encourage birds, butterflies and bees. Inside there are low-energy light bulbs, energy- efficient gas condenser boilers and sensors that switch off lights if nobody is present. Several innovations have produced unexpected problems. The light sensor system has proved capricious and prone to failure. Coconut matting stains easily - though lack of carpets generally makes cleaning easier. Painters had to be specially trained to apply organic and mineral paints. But one of the most central design features, the "passive" solar design and natural ventilation, works well; even in last year's hot weather, temperatures remained stable.

The new block, opened in 1994, was the first multi-occupancy residential building to be awarded the top rating of "excellent" in the Building Research Establishment's green assessment method. Remarkably, it has been able to package its greenery into a structure that sits happily in neo-Georgian surroundings. Yet to see it merely as an exercise in technology would be to miss the elements expressly designed to make it "liveable" - the avoidance of the spirit-crushing geometries that characterise much institutional architecture, for example. Mainwaring speaks of the "warm, tactile, spiritual" feel of natural materials; the organic ideas of Rudolf Steiner are a clear influence. Residents, mainly postgraduates, say it has a "Mediterranean" feel: one describes it as a "lovely place to live... the nicest residential college building in Oxford". And given the minimal extra cost of the green features, about 1 per cent of the pounds 2m total, the investment will have repaid itself by the end of this year, two years after opening.

The two buildings that were runners-up in this year's awards - the Inland Revenue Centre in Nottingham and the Queen's Building at Anglia Polytechnic University in Chelmsford - share several of Linacre's features, demonstrating the new consensus slowly emerging among designers about the principles underlying green architecture. Like Linacre, both avoid air-conditioning and place a strong emphasis on natural light and ventilation. They also attempt to integrate architecture, engineering and the environment in a way that would have been familiar to classical architecture or vernacular building traditions but that has been largely forgotten in the 20th century.

At the Inland Revenue building, designed by Michael Hopkins & Partners, a large complex of offices for nearly 2,000 people a few hundred yards from the city centre is split into six individual buildings, each punctuated by white-capped towers that look, from a distance, like elongated mushrooms. The towers serve both an aesthetic purpose - they cheer up the facade - and an environmental one: the air in them is heated by the sun, rises through the roof and draws currents of cool air through the building. The effect is similar to the draughts created by a chimney, except that it is computer-controlled: sensors measure levels of rain and wind, and computers decide how much to open the roof of the towers. The idea was imported from stage machinery at the Glyndebourne opera house. The towers also have a functional role; they contain the stairways.

By cutting out air-conditioning, the building not only saves enormously on energy costs, it saves on space, too. Nine-tenths of its area is useable, compared with three-quarters in a typical air-conditioned building. The designers also employed the technique of "thermal mass", another feature that would have been familiar to the builders of thick-walled Devon cottages and Mediterranean villas alike, as a means of husbanding heat and preventing big variations in temperature. Floors nearly a metre thick and weighing 26 tonnes were precast in concrete. Ceilings have a "wave-form", which traps the heat in pockets and allows the cooler air to flow through underneath.

Unlike the deep, dark office block that became standard in the Seventies and early Eighties, the Inland Revenue offices are relatively narrow - the buildings are 13 metres in width - and only four storeys high. They are plugged into the Nottingham combined heat and power system, a district heating network that derives its power from burning rubbish. And they have been built on reclaimed and contaminated land. Where the walls of an old coal-yard and diesel store once ran there is now a boundary of shrubby hedgerow, planted with native thorns and honeysuckle.

The Queen's Building in Chelmsford, designed by the architects of Linacre, ECD, and housing the university's library, is also on reclaimed land - it replaces an old ballbearing factory - and has its car park and lawns planted with a mixture of wild flowers and native trees. From the outside it has, deliberately, a look of the Victorian mills that once occupied the area, of which only one or two remain. The contrast with the inside is striking. Two atria - indoor courtyards - give a sense of light, space and airiness. According to one critic, they combine the "calmness of a library with the creative spirit of high technology".

Among the striking features of the atria, which stretch four storeys from floor to roof, are large white calico sails, white fabric panels and the "light shelves" under windows. These cut out direct sunlight but reflect light deeper into the interior of the building. But, like the stairway towers at the Inland Revenue centre, the atria also act as solar chimneys, driving the building's ventilation systems through vents and high-level windows that are opened automatically. Combined with a heavyweight concrete frame, providing thermal mass, and triple-glazed windows, the result has been described as a "classic example of passive, low-energy architecture". The building, instead of being designed as a "shell" in which heating and lighting systems are inserted almost as an afterthought, does much of the heating and lighting itself.

So will these innovations be more widely used in the future? Jes Mainwaring believes so. Green architecture, he says, is not a "hair-shirt and open- toed sandal philosophy pursued by a bunch of anarchists. It can and should be a mainstream activity." He believes the achievements at Linacre are "almost entirely replicable... This is not a high-tech building. It uses technologies that are widely available. Fundamentally it is a very simple building." !

The Green Building of the Year Award, now in its fifth year, is sponsored by the Independent on Sunday and the Heating and Ventilating Contractors' Association and supported by the Royal Institute of British Architects, the Building Research Establishment, the Energy Efficiency Office, the Chartered Institution of Building Services Engineers and the Building Services Research and Information Association.

This year's award was won for the new hall of residence at Linacre College, Oxford, by ECD Architects. The runners-up, both highly commended, were the Inland Revenue Centre at Nottingham, designed by Michael Hopkins and Partners, and the Queen's Building at Anglia Polytechnic University in Chelmsford, also designed by ECD Architects.