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Growth in the green house

Structures Sustainable buildings

Britain's first ever large-scale carbonneutral housing scheme is taking shape in south London. Mark Hansford finds out how new housing needs can be met without degrading the environment.

Environmentally friendly housing is nothing new in the UK. All around the country there are many examples of small, oneoff projects using sound sustainable principles. However, a scheme in Beddington, south London, is about to launch energy efficient and environmentally sound housing in the UK into the big league.

Beddington Zero Energy Development - BedZed - will be Britain's first large scale high density housing scheme to integrate the latest thinking on sustainable development through the whole life cycle.

Some 82 homes and 1,600m 2ofcommercial space will be squeezed on to the 1.4ha site of a former sewage works. Power will be produced onsite by a combined heat and power unit fuelled entirely by waste trees, providing 350,000kWh per year.

All wastewater will be treated in a 'living machine' to river water standards before recycling back into toilets and irrigation systems. All structural timber and 90% of structural steelwork is reused material. Roads and pavements use the latest in 'porous paving'. There is even space for eco-friendly photovoltaic cells.

But the key to the scheme is in applying building physics concepts such as thermal analysis, passive cooling and ventilation and energy grading, ideas all in common use in the commercial office industry, explains services engineer Arup project director Chris Twinn.

'The basic principles are the same, ' he says. 'We store solar heat gain as energy to offset the heat losses of the house. It puzzles me why some people cover their buildings with photovoltaic panels in the name of energy efficiency and still only achieve a saving of 10% on total energy demand.

'Not every kilowatt is equal, ' Twinn continues. 'It is 100 times cheaper to get a kilowatt through a window than through a photovoltaic cell. The trick is to match building energy needs with energy availability, yet this principle is largely ignored by the housing industry whose answer to energy efficiency is to flog heat pumps at the home buyer's expense.'

Everything at BedZed is designed with the building physics concept firmly in mind.

The six, three-storey terraces have south facing living spaces, with full-length windows to maximise the solar gain. This, combined with triple glazing, 300mm thick rock wool insulation, concrete floors for thermal mass damping, and a heat exchanger on a wind-driven ventilation system that recovers between 50% and 70% of heat from outgoing air, has - in theory - completely eliminated the need for further heat input.

'The key point is that the internal temperature does not drop below 19infinityC at any time, even if it is below zero outside, ' says Twinn. 'We have a heating cylinder for hot water, passive ventilation and a heat recovery system. But it is critical that we do not allow cold air to get into the building.'

And it is cost effective too, explains Twinn. 'It is a step change in design. If you can eliminate a heat cost then you can justify 300mm insulation and triple glazing. But it does mean you have to put a lot of effort in at the start of the project, which is when clients don't want to pay you.'

With solar power providing all the building's heating needs, power requirements are significantly reduced and, vitally, vary little throughout the year. This makes the constant output of an on-site biofuel combined heat and power plant not just environmentally sustainable but cost effective too.

At BedZed, the biofuel is chipped urban tree waste, a material that local authorities have in abundance and are desperate to dispose of in a way that avoids costly landfill taxes.

Supplied free of charge, the tree waste provides hot water and electricity to the low energy lighting and appliances used in the development. By avoiding fossil fuels, BedZed can claim to have zero net carbon emissions, making it truly 'carbonneutral'.

After two years of planning, work began on site in May 2000, and completed on target for the December 2001 deadline. Of the one, two, three and four bedroom flats and houses, 30% are for local authority-nominated affordable rent and 30% for shared ownership. The rest are for outright private sale at market value and are selling fast. The 60% reduction in conventional energy demand and a 90% reduction in heat demand makes an attractive prospect for buyers.

While design and construction time has been significantly longer than would be expected for a normal 82 home development, this will improve as the team moves along the learning curve, claims Twinn.

What is really critical is the fact that in terms of overall structural cost BedZed is little different to a conventional structure, explains Jonathan Deans, construction manager and Gardiner & Theobald partner.

'The build cost for BedZed works out at ú950/m 2 (-1350/m 2).For similar high density residential schemes the construction cost will vary from -925/m 2to-1,700/m 2depending on location and the type of units for sale.'

'We have shown that sustainable construction does not cost more and is a market driven solution, ' says Deans. 'It also shows that sustainable construction based on the units designed for BedZed offers a better quality of lifestyle, significantly lower running costs, and a real contribution to reducing global warming.'

Sustainable innovations As well as the building physics, BedZed can claim several other sustainability successes, not least a water strategy that will cut demand by a third.

Up to 150m 3of rainwater is collected and stored in underground tanks and pumped as grey water for flushing toilets and watering gardens. A living technology machine comprising a system of septic tanks and biological reed bed filters treats up to 50m 3/day of effluent and recycles it back into the grey water tanks.

Photovoltaic cells, initially discounted on the grounds of cost, have been added at a late stage thanks to a grant from the European Union. Mounted on the roof of each terrace the PV cells will provide enough power for a pool of 40 cars, available to residents for local journeys. 'An electric car may only have the range of 100km, but it is a fact that in excess of 98% of all car journeys are less than 40km, ' explains Twinn. 'And you can run a car on PV for just 1p/km.'

Sustainability even extends to the materials used in construction. Some 90% of the 120t of steel used on site was recovered from scrap before it went into the energyintensive recycling process, explains structural engineer Ellis & Moore project director Lachlan McDonald.

Structural capacity of the steel sections was calculated from historic tables, with compromises made on size, wastage and visual inspection.

A 10% saving was made on the raw material cost.

Re-using structural timber proved much more difficult, says McDonald.

'Quality was very difficult to guarantee and each batch had to be stress graded. In both re-used steel and timber there is much good quality material available. If the suppliers knew what they were selling, it could be sold as new.'

Who s who Client: Peabody Trust Environmental consultant:

BioRegional Architect: Bill Dunster Architects Structural engineer: Ellis & Moore Services engineer: Arup Construction manager: Gardiner & Theobold

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