This week Specifier looks at all things sustainable, including the cost of solar hot water systems and some great green products. We kick off with Freiburg’s eco-community, including the new Sonnenschiff development that could teach the UK housebuilding sector a thing or two …

The glass facades Freiburg’s Sonnenschiff development include triple glazing – one of many sustainable features

The glass facades Freiburg’s Sonnenschiff development include triple glazing – one of many sustainable features

It’s the grass between the tramlines that makes all the difference. In British cities, the gap between the tramlines is covered in blacktop, but in the South German city of Freiburg, tramlines are almost entirely concealed by a carpet of green. The grass is not only there to make the streets look better; it is part of the sustainable urban drainage system and, more importantly, it halves the noise from the trundling trams, which means a much quieter life for nearby residents.

You can see these kind of details everywhere in Freiburg, and they have helped to make the city a popular destination for a special type of tourist – the one with an interest in sustainable architecture and placemaking. Freiburg has become Germany’s eco-capital over the course of the past 20 years, and now about 35% of residents don’t own a car, new homes routinely incorporate photovoltaic panels and even the Best Western hotel’s bedrooms have low-energy minibars.

Staff from PRP Architects and a handful of their clients made the pilgrimage to Freiburg this autumn to see what lessons the city held for UK housebuilding. They were wowed by what they saw. “It wasn’t the individual innovations that caught our eye, it was the whole package of measures and how they are put together that was impressive,” says Andrew Mellor, associate with PRP, who sits on the company’s in-house sustainability steering group. What stood out in Mellor’s mind was the grass in the tramlines; the Café Velo, which combines bicycle parking and rental with repair services, a café and a public transport information centre; and the fact that solar technology is not simply parked on rooftops but is often integrated into building design, with solar hot-water tubes serving as balcony panels, for instance.

The architects’ whistlestop tour took in the city’s best-known house, the Heliotrop – a building that revolves to make the best use of solar energy – and the Solarsiedlung (Solar Community), which contains the latest addition to Freiburg’s solar architecture – the mixed use Sonnenschiff (Sun ship) development. These and other buildings in the city are designed by German eco-architect Rolf Disch, who lives in the Heliotrop himself.

The Heliotrop is the city’s green icon, but it is hardly a model that could be widely applied in the UK. Built about a decade ago, the house produces roughly five times more energy than it uses. However, it did cost the equivalent of *2.2m (£1.5m) to build, largely because of its revolving technology - its 54 m2 rooftop photovoltaic array revolves to catch the sun’s rays, while the house itself rotates in the opposite direction to maximise solar shading.

The new Sonnenschiff development is perhaps a more appropriate model for the UK, with its high-density mix of uses wrapped in an environmentally conscious building (see “What’s so sustainable about Sonnenschiff?”, above). The 125 m long mixed-use building combines 1160 m 2 of retail space at ground floor, with 3600 m2 in two floors of office space. Above that there is 1160 m2 of living space in the form of eight 112-300 m2 houses. Car use is by no means prohibited, as the scheme incorporates 112 underground parking spaces, plus 28 on ground level.

Like all homes built in Freiburg, the Sonnenschiff houses conform to one of three energy-efficiency standards, all of which are higher than German norms (see “Energy standards for housing in Freiburg”, above). These standards are imposed by the local authority, which owns much of the development land in Freiburg. The standards are also, crucially, endorsed by the local community in a willingness to pay a premium of up to 3% for a sustainable home. Freiburg locals were converted to the benefits of buying green by the prospect of having a nuclear power station built in their midst. They won a battle to halt the project, leaving a shortfall in the region’s energy supply pipeline and generating a demand for environmentally friendly alternatives. Now the city produces about 65% of its energy from photovoltaics and combined heat and power, in the process saving 28 GJ of energy and 2100 tonnes of carbon dioxide a year.

As for Mellor, he came away from Freiburg with a conviction that it is people, rather than technology, that make sustainable communities happen. “We learned that it is not impossible to implement sustainable measures, but that it does need the support of the local authority and residents. The key to Freiburg is that the local people wanted this and the local authority had to do it,” he says. With the UK entering its own debate about the need for nuclear power, could homebuyers and local authorities here one day learn the same lesson?

Energy standards for housing in Freiburg

1 Holistic standard

  • energy use of 65 kWh/m2/year (a typical UK house now uses 200 kWh/m2/year)
  • 250 mm insulation in walls
  • 350 mm insulation in roofs
  • high-performance double-glazing

2 Passive House standard

  • energy use of 15 kWh/m2/year
  • 400 mm insulation in walls
  • 450 mm insulation in roofs
  • high-performance triple-glazing
  • use of solar gain

3 Energy Plus standard

  • Passive House standard or better for house fabric
  • photovoltaic array producing surplus electricity

What’s so sustainable about the Sonnenschiff?

  • It mixes houses and commercial, putting the former on top of the latter. The homes are protected from wind and from street noise by a 3m high glazed screen, and have their own 쳫y gardenn top of the commercial space.
  • Glass facades, windows and doors have triple-glazing.
  • The housing elements are designed to Passive House standard (see above), maximising the use of the sunenergy and preventing heat loss with 400 mm insulation in walls and 450 mm insulation in roofs. They also have a photovoltaic array to bring them up to Energy Plus standard.
  • Elevations are made from timber panels, timber-metal composite panels and vacuum sandwich panels. The latter are 40 mm deep, but achieve the same U-value as a 400 mm thickness of insulation. These were used on the retail and office elements of the building (about half of the building) and cost *200,000 (£134,500) more than conventional panels would have done overall, but provided increased net lettable floor space.
  • There is no mechanical air-conditioning. Instead, the scheme relies on passive cross-ventilation and night-time ventilation. Brightly coloured composite panel screens with side grilles project some 400 mm from the main building. Behind them are 줯orshat can be opened at night to allow cool air to enter the building through the side grilles, without fear of burglary.
  • Roofs are topped with about 1000 m2 of solar cells, providing 135 kWp of electricity.
  • Further energy supply comes from the community combined heat and power plant, fuelled 80% by woodchip from the Black Forest and 20% by gas.
  • Pipes buried in the earth preheat or cool air to between 8 and 12°C before it enters the building. This method reduces the requirement for heating in winter and cooling in summer.