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Bill Dunster doesn’t quite fit into the stereotype of architects being the dilettantes of the building industry. Far from flitting from one architectural interest to another, he single-mindedly ploughs a single furrow – truly sustainable zero-carbon residential and live-work buildings. So it comes as no surprise that Bill Dunster Architects has chosen to follow up its highly publicised BedZed development, completed in south London three years ago, with more of the same.

That said, the practice is carrying Dunster’s evangelical mission into two new areas. The first is the deliberately humdrum world of everyday housing construction. The goal here is to make the construction of homes designed to low-carbon “Zed standards” achievable at reasonable cost by ordinary building contractors and housebuilders. As Bill Dunster puts it: “We are trying to share our expertise with the wider industry.” The second area is more exotic: the practice has just landed two sizable low-energy projects in China.

Social sustainability

A four-storey block of 12 key-worker flats for rent has been built on thermally efficient BedZed principles on a typical infill site in south London. The flats are heated and ventilated using solar, wind and renewable biomass energies, and if Dunster’s calculations are accurate, it is reckoned that a household’s total energy bill will be about £75 a year.

The block has been developed by Presentation Housing Association using a Housing Corporation grant. It is part of the regeneration of St Matthews council estate in Lambeth, south London, and its design is the first fruit of a joint venture between the zero-carbon fundamentalism of Bill Dunster Associates and PRP Architect, the UK’s largest mainstream housing practice. Both firms were novated to the design-and-build contractor Mullalley Construction.

The two architects strove to make the detailing and construction as simple, robust and buildable as possible. As Dunster says: “This project shows how any housing association can adopt the Zed standards and reduce demand for heat and power to the point where renewable energy starts to make sense.” Stephen Harris, Dunster’s project architect, describes the scheme as “the distillation of BedZed detailing”, which he says was needed because of the project’s wide spread of design responsibilities and the Housing Corporation’s cost guidelines.

The resemblance to BedZed is instantly apparent from the street, where all the flats are fronted on the south side by generous conservatories or sun-rooms. Although these are not habitable spaces in the strict residential definition, the sun rooms are 2 m deep which is enough to accommodate a table and chairs. They are also heated for free by the sun, which means they can be used when the weather is too cool for the adjoining balconies. In an independent valuation, property consultant Savills reckons these lifestyle features add an extra £10,000 to each flat’s asking price on the open market, and that is over and above what it would be E E worth if it were conventional internal habitable space.

The roof of St Matthews is put to a more practical, and indeed innovative use than at BedZed. Instead of BedZed’s Babylonian-style roof gardens, St Matthews’ roof is entirely covered in water-powered solar collectors. The innovation here is that the solar-heated water is used for domestic hot water and space heating.

In summer, the solar-heated water rises to 60°C and supplies all the flats’ domestic hot water. In winter, water reaching a more modest 25-30°C is circulated in plastic pipes within the floor screeds to provide space heating – particularly important in the north-facing bedrooms.

In a special accumulator hot-water tank serving all 12 flats, water is stratified according to temperature, so that hot domestic water and tepid space heating water can be drawn from different levels of the same source. Top-up heating and hot water for the whole building is provided by a single boiler heated by burning wood-pellets and connected to the hot-water tank.

Another prominent feature of BedZed that is missing at St Matthews are the colourful rooftop cowls that suck fresh air through the building. The flat sizes change on each floor and Lambeth council planners required the block to step back. To Harris’ irritation these requirements ruled out natural ventilation flues, which cannot cope with more than two offsets over their height. Instead, the building is mechanically ventilated by electric fans.

Just as in BedZed, the building fabric was designed to provide a massive heat sink combined with high thermal insulation and good weathertightness. The external walls include an 150 mm inner leaf of massive concrete blocks and a 300 m-wide cavity filled with expanded polystyrene, along with a conventional two-coat plaster internal finish to provide an extra level of airtightness.

The upper floors and roof are made from 200 mm-thick hollow concrete planks, the undersides of which are left exposed to absorb heat; and the roof has a 300 mm topping of insulation. The building’s U-values are identical to those of BedZed, with external walls, roof and ground floor all coming in at just 0.1 W/K/m2, and windows, which are double or triple-glazed, have a value 1.0 W/K/m2. This is a great improvement on Part L of the current building regulations, which gives U-values of 0.35 W/K/m2 for walls, 0.25 W/K/m2 for flat roofs and 2 W/K/m2 for windows.

The extra-wide wall cavities called for special consideration. They required special two-part stainless-steel wall ties that were clipped together on site. “Installing the windows was a particular challenge,” says Harris. “Normally, the builder runs a PVCu cavity barrier all round the window opening to seal it against penetrating rainwater and then fixes the window over it. But this wouldn’t work over a 300 mm cavity. On BedZed, the window openings were not sealed, and rainwater ran down the outer sides of the cavities.

“So, at St Matthews, we laid an extra-wide cavity tray over the top of the opening and ran it down both sides to deflect the rainwater. Then we fixed the timber window to outer brick face and strapped it back with metal ties to the inner face. And finally, we sealed the cavity all round in timber boarding.”

Generous south-facing sun rooms are both a lifestyle bonus for the key-worker flats at St Matthews and a source of solar gain

Another improvement on BedZed detailing came with the roof covering. This consists of a proprietary standing-seam metal roof system, which can be easily clipped to the concrete roof planks through 300 mm of rockwool insulation. The standing seam roofing can take either a sedum-planted green roof or solar collectors or photovoltaics.

The unit construction cost of the project at £1597/m2 does not look particularly cheap, whether compared with £1473/m2 at BedZed (at 2002 prices) or with current average for social housing construction, of about £1500/m2. “Most of the complications of the building come from factors other than sustainability,” says Harris. “A steel structure was needed because the top floor steps back.”

Several other low-carbon social housing developments designed by Bill Dunster Architects are in the pipeline. In Leicester there is a five-storey block of 185 flats and maisonettes, mainly for large families. And scattered round the country are 84 dwellings based on the RuralZed concept of prefabricated components of laminated timber beams and concrete lining panels.

Less successful, though, is the architect’s attempt to interest contractors in ZedinaBox, a package deal that brings together a ready-made supply chain of low-carbon components.