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What is sustainability?

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Sustainability in buildings, we all agree, is a good thing, but what exactly does it comprise? This was the issue that Allford Hall Monaghan Morris had to address after winning a competition to design a sustainable school at Great Notley in Essex (aj 16.10.97). It received little help from the client - deliberately. 'All the way through we never said what we thought we meant by sustainability,' says Gordon Powell, Essex's energy manager and technical adviser on the project.

ahmm's Simon Allford believes that 'sustainability is like plumbing or air-conditioning - it's a general attitude you apply to a building. It doesn't create its own architecture but it informs the way the architecture is produced'.

The design team has now produced a stage D report, and submitted an application for planning permission. The main ideas embodied in the scheme are:

efficient use of space - the form of the school is roughly triangular, a shape not too far removed from the ideal but impractical circle which would give the minimum perimeter for the maximum contained area. But as well as containing the area within an economic perimeter, the design also manages to save on the overall space requirement. The total area of the school is less than that specified in the DfE cost model for schools, yet the designers have managed not only to fit in all the required elements but to create an 'extra' space, the triangular covered courtyard which comes off the main hall.

orientation - services engineer Atelier Ten ran a dynamic-simulation program as the initial proposals for the school were being worked out. They found that the school, as a building occupied between 08.00 and 16.00, was fairly insensitive to its orientation shifting between south-east and south-west. 'It was much more important to insulate it properly,' says Bellew. The team therefore picked the current orientation, with the classrooms facing south-west, because it suits its position on the site, and because of access requirements.

sustainable materials - the team has been auditing various materials and making the 'greenest' choices where possible, although all decisions have been tempered by the need to stay within a standard Essex school budget. This has induced a degree of pragmatism. For instance, the team is looking at a boarded finish to the exterior, having considered plywood initially, but is using plywood for the roof construction. Although this is not the most environmentally friendly of materials, because of the glues involved in its manufacture, it will allow a lightweight and economical roof to be built. The design proposes the use of breathing walls, which both do away with the need for an environmentally unfriendly membrane and have important implications for the ventilation of the building. Cables and possibly the pipework will be pvc-free.

The other obviously green element of the building is the roof - almost literally so. This will be planted not with grass but with sedums in a special system by Erisco Bauder which is used widely in Germany (and which ahmm is also using on its Walsall bus station). The advantages of this system are that it is relatively shallow - only about 100mm deep - and hence lightweight. Unlike a grass roof, it does not need mowing or cutting. In fact, once established after three years it needs very little maintenance at all. Its other advantages are that it protects the roof membrane from ultraviolet degradation, it provides significant thermal insulation, and it can temporarily absorb water from sudden storms, hence limiting the amount of guttering that is needed.

simplicity of services - there is agreement from all concerned that sustainable design does not mean using a lot of gizmos. 'It is the low- tech stuff that is the clever stuff,' says Powell. Bellew's philosophy is: 'Keep it simple. When you make things that require a lot of understanding of building physics, the users will probably get it wrong'.

The building will be naturally ventilated. Originally the triangular courtyard at the centre of the plan was to be open, to assist with ventilation but when they switched to breathing walls the team found that the extract from the wcs, plus air brought in through opening doors, would be sufficient. The option of mechanical ventilation with heat recovery was rejected because the short length of the school day meant the cost of putting in fans could not be justified. Bellew thinks that it is right that cost constraints (the same as for a standard school) limit the options. 'There is no point in developing a non-replicable model,' he said. 'If everything costs extra, people won't do it.'

'touching the ground lightly' - this has become almost a catchphrase of the project. Whatever the lifetime of the building, seen in the broadest view it will be relatively short. Since it is going on a greenfield site, the aim is to produce as little disruption as possible. This is reflected in the design of the foundations, the way the building is positioned on the site to minimise earthmoving, and the relatively compact area which will suffer disturbance.

flexibility - particularly with a limited budget, one can actually have too much flexibility. Space-frame construction would have allowed almost infinite flexibility in terms of rearranging the internal space, but at an unacceptable cost.

construction methods - the concept of 'touching the ground lightly' would be negated if the contractor were then to wreck the whole field in the construction process. ahmm will be specifying construction techniques and supervising the process.

This building is, as it should be, very much a response to a particular site, yet Essex is also hoping to learn some broader lessons from it about designing and building sustainable schools. 'In many ways,' says Powell, 'it is the process that is important; the thought behind the building.'

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