Green vault.

A new sports hall designed by Peter Hubner for a progressive German school near Darmstadt inventively exploits green principles to create a humane, welcoming building that has a minimal effect on its precious site.

The Odenwaldschule is one of a handful of private schools started in Germany in the early years of this century as a result of the progressive School Reform Movement.(1) Though partly influenced by the English public schools, their ethos was less militaristic and conformist, concerned instead to promote self-sufficiency and independence of spirit and to develop a sense of personal responsibility. Pupils occupy boarding houses as in the English system, but house groups are smaller, each with only 8-10 pupils.

Set in hill country south of Darmstadt, the school occupies a steep wooded hillside, dominated architecturally by its villa-like boarding houses. These were designed by Friedrich Metzendorf in the picturesque manner derived from the English Arts and Crafts and brought to Germany by Muthesius.(2) It was a difficult complex to add to, but Peter Hubner succeeded without resorting to pastiche with his ingenious and much-liked 'Baumhaus' classroom extension (AR September '96). He was therefore the natural choice as designer for the next phase: a sports hall.

The sloping site with its numerous trees and its strict nature conservation rules seemed at first to offer no plausible place for a large, flat-floored building such as a sports hall, but Hubner saw the possibility of setting it into the hill on the edge of a meadow and letting the sea of waving grass continue onto its roof. A curved profile would allow the ground surface to rise and fall again, while leaving the optimum space inside to accommodate the trajectory of a ball. Taking advantage of the sloping ground, the changing rooms could be slipped underneath, and a narrow two-storey foyer linking the levels could be added at the front, with its triangle section serving as a stable mounting for the bow-shaped roof ribs. With west-facing glazing, this foyer would provide a cheerful approach space and absorb solar heat on winter afternoons. The major snag was the demand that the sports hall be divisible in two with a central curtain, for it seemed impossible that such a thing be hung from the curve, remain moveable, and leave the volume unobstructed when furled. Hubner conceived the idea of housing the curtain within a straight box projecting up to appear on the curved profile like a backbone. Glazed, it could be visually delicate while introducing a shaft of daylight into the centre of the room. Furthermore, when provided with opening lights, it could become the driving element of a solar siphon for cooling in hot weather.

So began a servicing strategy that is truly green, since it saves on both building and running costs. It avoids complex mechanical plant that is expensive to install and maintain, and air-conditioners that burn energy against the sun and use noxious chemicals. Instead, it balances out the given conditions with a series of openable barriers. On hot summer days, the glazed box on the roof heats contained air so that it rises through open vents, drawing more behind it. Suction pulls cool air in from the sides at floor level, which arrives via an underground duct big enough to walk through. Air intake is at the centre of the building on the uphill side, so air is sucked through the buried passage first from back to front and then out to the sides. Exposed to this enormous area of wall, the air is cooled by the surrounding earth, so by the time it arrives in the hall, its temperature has typically fallen by six or seven degrees. The glazed foyer heats up, but not excessively, as it has high opening vents, external shades against high sun, and some protection from seasonal climbing plants. Dropping blinds between foyer and hall protect the hall from direct solar radiation and from absorbing the foyer's heat.

It is on winter afternoons that the foyer really starts to work for its living, both as a buffer for the hall and as a solar collector. When it reaches the required temperature a photocell-powered fan cuts in to deliver its hot air to the hall. The blinds are up, so there is also direct transmission of the low sun. The glazed south wall has also taken the low sun's heat directly: this is prevented in summer by external blinds. On sunless winter days the hot water heating system in the floor slab brings energy from the school's central boiler, but it is still helped by the thermal siphon, now working very gently. The underground duct warms the incoming air with the accumulated heat from the surrounding earth, raising it typically from -10 to +5 degrees.

This description covers the extreme conditions, but for much of the year the temperature lies between, and the building can be adjusted to gain or lose heat by opening or closing its vents and blinds. Although it is by no means self-sufficient in energy terms, the energy input is entirely top-up heating, therefore far less than a comparable building without its high-insulation, solar wall and underground heat storage. It is also significant that apart from the solar water heating panels for the showers set on the south facade, the energy measures are not add-ohs but integral to the basic design.

The parts of the building belonging to the ground are in concrete, the parts above in timber and glass. The roof deck is of thick plywood covered by insulation, membrane and turf, giving a natural surface at once protective and insulating. It overhangs the ends of the building just enough for shelter and shade, and to produce a definitive edge. The structure is laminated timber, perhaps the most renewable and least energy-intensive choice, but a material in its most technically developed state. It is left naked to age naturally like the rest of the timber construction, for protective coatings harmful to insects may in the long run be harmful also to us.

Hubner's triumph at Oberhambach is that despite its undoubted green inspiration, the large building makes the gentlest impact on its precious context, and is a joy to use. The bright welcoming foyer is a good foil to the calm gentle hall and the daylighting seems generous. Despite being set into the hill, it seems very open because of the glass perimeter on three sides. Sport is not compulsory, and it is left to the pupils themselves to co-operate and organize teams. Since the new building was opened sport has become more popular, and the building has become in addition to its main purpose a popular social centre and meeting place. The highest praise came from one of the teachers, who admitted that he had formerly been very sceptical of the idea that architecture could change people's lives.Now that he has the evidence before him, he has to grant that it can and does.