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Creating a green house

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An adventurous project by Peter Cuming in North London to create an environmentally friendly house - involving greywater, photovoltaic cells, rabbits and steel, the greenest material of all - is coming to fruition

An idle moment with a British Steel annual report provided a mini 'Road to Damascus' experience for Peter Cuming. It pointed out that steel is the most recycled construction material and, impressively, that it is the most recycled material of all. Says Cuming: 'That made steel a pretty green material.'

With Stephanie Mills, his partner in Urban Designers, Cuming just happened to be in the middle of designing a 'green' house for a slightly unlikely site in London's Kentish Town. He had already worked out that the concrete on the site could be crushed locally and used as aggregate in the foundations. But there was also 40 tonnes or so of steel in the form of a gantry and abandoned materials from the old works which led Cuming to decide that steel of some sort should be used in some way in the new building.

The little development now going up in Kentish Town contains three compact flats with a fourth, destined for Cuming and Mills on the third, fourth and fifth floors of the small tower which is the main feature of the scheme. Next to it is a three-storey glazed entrance atrium with access stairs, and next to that a single-storey flat with a part glazed and part grassed roof. On the other side of the tower is a small housing association residential building.

The main structure is a steel frame with concrete floors. The structural tasks of the steel frame are to support the load of the 150mm-thick Bison precast planks which are grouted to provide a diaphragm action and serve as a primary support for SureBuild infill framing. British Steel's SureBuild steel-frame housing system is designed as a rival to timber frame and as a clear rival to block work. Its virtues are speed and precision. Cuming was particularly attracted to the notion that he could eliminate many of the traditional specialist wet trades and that, for example, windows could be plugged in at the last moment in the knowledge that they would fit exactly. Here SureBuild is being used not so much as a whole-house frame but more as an infill wall frame and as the support structure for internal partitions. The external walls are made up of a tea-cosy of layers of insulation plus a rainscreen devised by the Austrian firm Heraklith.

A 25mm stucco skin is laid over a thick sandwich of 100mm mineral fibre between two thin boards of long wood shavings in a matrix of magnesite. The outer skins look (and probably are) rather like a very thin version of what we used to call woodwool. This 100mm Heraklith board is attached to the SureBuild framing using fixings involving thermal washers. The 75mm SureBuild frame thickness is effectively the cavity zone in the wall. In Cuming's scheme it is filled with a flax-based board called Heraflax. The flax is, virtuously, grown on ec set-aside land. On the inner face of the SureBuild frame is a Heraklith board, this time with a 50mm insulation core to which plasterboard can be fixed. The overall U-value of the wall is an unsurprisingly very low 0.2W/m2degreesC.

Other parts of the building are heavily insulated as well. The ground floor is laid over the crushed remains of the original workshop slab with a layer of 75mm of non-toxic polystyrene insulation in between. The roofs have 150mm of insulation, and the roof of the ground-floor flat has an additional 250mm of soil and turf. Windows have been specified as low- E double glazing with an inert gas between the glass layers. All this produces U-values way, way below new Building Regulations limits.

All this came about because, as all green designers have to, Cuming and Mills took a view about the external insulation. One possibility was to use the externally insulated mass of the walls as a jacketed heat sink. 'I originally thought that a high-thermal-mass house needed enormously thick walls,' says Cuming. But after discussions on the green network he learned of the alternative approaches in which the walls are highly insulated with air-tight elements enclosing the warm air inside. A certain amount of thermal capacity is desirable. But there are other ways of storing heat. Current research indicates that it is only the lower 75mm of concrete floor slab that is effective in thermal performance, a depth which can be used to control the internal environment. Cuming used 150mm concrete floor slabs for the basement and a steel decking system for the rest of the house.

Cuming originally had the idea of a basement, for the tree roots and storage, including water, but a seven month planning delay and price hike in the meantime meant that the basement idea had to go. What he now has is a 10,000-litre pool in the entrance atrium which holds rainwater and light greywater from baths and showers. This water is pumped to a large header tank in a big wedge-shaped space above the fifth floor and used for flushing lavatories. The local water company, Thames Water, estimates that each flat will use 120,000 litres annually, a third of that for flushing lavatories. The annual cash saving for the four flats will actually be not much more than £160 annually, and the water saving around 160,000 litres. It is necessary to provide a back-up water source for lavatory cisterns. Although anti-siphonage valves to prevent contamination of the household supply are on the market, they are a lot more expensive than installing two separate cheap cisterns, one for greywater, one for pure water-company water.

The glazed atrium enclosing the open stairs and giving access to the two ground-floor flats and the flats on the first and second floors serves as a thermal buffer, collecting solar energy during the day and trapping escaping heat at night. The warm greywater in the pool will aid in warming the atrium environment. Above the atrium an 11m2 bank of evacuated solar water-heating tubes will provide around 80 per cent of the total hot-water requirements for the building. This kind of solar-collection system is more durable and efficient than standard flat-plate collectors: tubes can collect the same amount of energy from only 70 per cent of the area of flat panels. The remaining 20 per cent of hot-water needs are met by small, efficient gas condensing boilers.

It is also planned to install 60 photovoltaic (pv) panels on the roof of the tower. As they are used as the outer waterproof layer of the south- facing pitched roof, the budget can offset the cost of tiles or slates. Each module has a nominal peak output of 35W but the conversion efficiency over a year is actually quite low. Cuming's now thinks that they don't offer such a good return. And there are distribution problems: how, for example, do you decide which flats are supplied with free electricity during peak loads and how do you work out the electronics to do it according to the rules you have devised? Only two houses in the uk currently deploy pv cells and although there is a theoretical possibility of feeding surplus electricity into the National Grid the the electricity companies in the uk are not interested in paying much for it and there is a high potential cost in installing special metering gear. (Apparently in the us the household meter is simply reversed when power is flowing from the house into the electrical grid.) Nothing so simple here.

Cuming had originally persuaded London Electricity to pay for the pv cells. Then the public utility was taken over by a us company and the new owners cited such things as windfall taxes as reasons for withdrawing. There are other pv systems: Redland imports one from Germany which has the area of four standard Redland tiles and can be added to as the householder can afford it. At the time of writing Redland was engaged in takeover talks. Recently it seemed that Cuming would be able to get the panels for cost price, but with the financial problems of selling back to the National Grid the pv cell array remains in doubt, although the roof is still designed to take it.

Despite vague government pronouncements approving the idea of doing things the green way, you have to be as totally dedicated to the idea as Cuming and Mills to actually do it. The cells saga is just one of the issues Cuming has faced.

It is just as well that Cuming, trained as a geologist, is a member of the Royal Town Planning Institute and a former planning inspector. The site was designated for industrial use but Cuming knew that even notoriously ideological Camden had begun to sense the unlikelihood of industrial jobs returning to the area and had anyway recently allowed the building of the housing association scheme on one corner of the site. He also knew that the borough had produced a formal planning guidance note on the desirability of building greenly.

The scheme he proposed had all the right green credentials: grass roofs, photovoltaic cells, internal heat sinks, insulated rapid-warm-up external walls, recirculation of waste water, and so on, plus the recycling of steel structures and gantries on site and the reuse of existing bricks. Cuming had actually considered burning bricks using the London clay just below the surface but that would have been a smoky business, especially for neighbours.

Living rooms face south with bed and bathrooms generally on the north. Cuming also offered the planning gain of the removal of an ugly air raid shelter on an adjoining property and planting outside his site. Camden predictably refused permission in spring 1996, citing among its objections the intrusion on nearby neighbours' windows when Cuming cut the first- floor-level grass roof at one end. Cuming offered to install rabbits. There were objections to the reflections from the glass of the photovoltaic cells in the direction of a future sports area, although the cells are designed to sit at an angle facing the sky. His appeal was allowed at the beginning of December last year. Crazier bits of planning refusal were discussed in the local press and one correspondent solemnly urged Cuming not to use rabbits on the grass roof because they would certainly, poor bunnies, be picked off by local eagles.

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