Walled green city.

Intended to spearhead the revitalisation of a run-down district in Nuremberg, this mixed-use complex combines a commercial brief with ecological and energy-saving principles.

Prisma, a complex of housing, kindergarten, offices and shops, is only a stone's throw from the medieval city walls of Nuremberg. There are interesting similarities. Energy-saving, passive ecological building principles (with a few modifications), match those of the tenth century; thick masonry walls, breathing renders, structural and insulating timber cladding. The medieval moat, which was a rain water collector serving as a defensive barrier and water source, becomes today's climate modifier, an oxygenated pond with plants and fish.

Joachim Eble, one of Germany's leading ecologically conscious architects, built a prototypical commercial property, called the Oko-haus, in Frankfurt am Main in 1993 (AR February 1993). Prisma is bigger, comprising 61 flats, 32 office units, nine shops, cafe, kindergarten and a 15 000sq m greenhouse lobby. The Nuremberg city council, who owned the site, decided it should be a catalyst for the revitalisation of the surrounding run-down district. Something forward looking and special was required. The Karlsruher Insurance Company were only able to develop the site on the understanding that they build a mixed-use, socially balanced complex according to ecological and energy-saving principles.

The site, at an important traffic junction, is at the centre of a mixed shopping area, close to a bus and underground station. Prisma consists of three blocks around a landscaped inner courtyard partly covered by the greenhouse foyer. Along the busy Rothenburger Strasse, shops with six floors of offices above face the street. A second block, with shops facing into the greenhouse and four floors of offices topped by two floors of flats, lies at an angle to Rothenburger Strasse and has no street facade. Between these two six-storey buildings the entrance into the green foyer juts out on to the pavement as a pointed glazed corner. On the quieter southern boundary, a five-storey block of flats with balconies overlooking the open landscaped courtyard closes the site. The kindergarten for 50 children is a quadrant plan on three levels with roof terraces and steps leading down into the courtyard garden which also serves as a play space.

Like Nuremberg's medieval walled city, Prisma shows protective sheer walls to the outside world, and creates a sheltered inner realm. The green foyer entrance is the equivalent of a drawbridge in a fortified city, only opened to let in citizens, trade and commerce. However in stark contrast to medieval Nuremberg, the modern ecological model is filled with light and greenery.

Including site, fees and financing, Prisma cost 75 million DM, around 550 DM per cubic m cheaper than the Frankfurt Oko-haus, but only because of the building industry slump. There are no solar panels. District heating, supplying steam at 90-95 degrees Celsius, from a nearby plant was considered more economical and electricity for power and lighting is supplied by the local utility company. Within the current cautious economic climate, Prisma is seen as an investment risk which will not bring in a profit for some years, so the budget has been closely monitored. For instance, the capital cost of extra plumbing outweighed the advantage of rain-water flushed lavatories and at one stage the glasshouse foyer was in danger of being deleted. This was shown to be a false saving when the cost of upgrading the elevations facing into the glasshouse (which would then have to be weather proofed), was taken into account.

Compared to a similar-sized conventional German building, Prisma's energy-saving construction and passive systems save over 8000 litres of oil fuel per heating period. Over 10 years, it is estimated that this will save 25 000 tons of carbon dioxide emission. Microclimate modification has been achieved by increased insulation, using the structure's heat storage capacity, appropriate orientation, shading, natural ventilation, botanical regulation of humidity, air oxygenation and cleaning, without the capital cost and servicing associated with air-conditioning equipment. The south-facing green foyer has a centrally or individually regulated hydraulic mechanism to open its glass roof (by php glastec) and operate roller blinds for summer ventilation and shading. Plants were chosen for their oxygen producing and air filtering qualities.

The 490mm insulating blockwork walls are limestone cement rendered with mineral coloured paints. Soundproof ventilation slits incorporated in opening frames allow for fresh air without necessarily having to open windows. Rain water collected in the courtyard garden pond flows into a basement cistern which feeds the greenhouse pond, six air cooling water walls which are features in the foyer, and the landscape irrigation system on roofs, balconies and terraces.

The green idyll should be an everyday experience, but ecologically based briefs are still the exception. Much depends on the client being able to persuade enough commercial tenants to share in the dream. Whereas the Oko-haus was initiated by ecologically committed clients, Prisma must convince a more sceptical lay public.

Architect Joachim Eble Architects, Tubingen, Germany

Associate architects Hunger Lorenz Ocklenburg Schaffner/Harms Reinebeck Timm Weigel

Landscape architect Adler & Olesch

Structural engineers Schneck & Schaal/Buro Strobel und Konstruktives Bauen

Services engineer Speiss-Jarasch

Electrical services engineer Burghar

Photographs All by Gunther Schmidt except 1 and the one shown on p25 which were by Ralph Richter