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SpaceCraft Architects' sustainable design for a primary school at Harrietsham, Kent, won a three-way pitch to Kent County Council.

As Sutherland Lyall discovers, the building is naturally ventilated, protected from winds by remodelled parts of the surrounding topography, deploys recycled insulation and has sedum growing on several of its roofs.

THE DESIGN The plan of this single-year entry school has a nice simplicity.

There are four teaching rooms for the four junior years and two for infants and reception children. These are arranged in pairs side by side in a row that faces south and are connected at the back by a long passage - known as the exhibition street. At the west end of this street is the almost square block of the barrel-vaulted school hall. The sloping topography means that this sedum-clad roof virtually merges into the ground behind. Next to the hall, also on the north side of the exhibition street, is a flat-roofed block containing the school office, library, staff rooms and other services rooms. The row of classrooms has a standing-seam roof with a concave curve, the overhang of which curves out at one end some metres from the building to form a shelter. This roof is broken near the middle by a flat-roofed section over the entrance that runs between junior and infant classrooms. The two age groups have to have separate playground space and the entrance path to the building is the dividing line. This front block is clad in horizontal boarding while the two blocks behind have white rendered walls, which underlines their different functions.

Project architect Thomas Böhringer says the building is more or less consistent with the concept design that won the practice the job: 'The roof is less curved and we have moved the building up the hill so that it is more integrated into the existing landscape and dug it in a little to minimise access problems.'

CONTRACT This was a standard local authority JCT 98 with quantities in which the architect is the contract administrator and design-team leader with responsibility for getting the brief and the design information together. Böhringer says: 'We worked as a team with regular meetings, presenting the scheme to the client.' The consultants were all members of the team that pitched for the project, so that there were none of the occasional difficulties architects face when clients tell them which consultants they must work with. Kent runs a framework agreement system in which approved professionals and contractors and subcontractors go on to a preferred list for a period of four years.

Lead by project director Laura Irving, the architect used update 42 of the electronic version of NBS, NBS Building. The practice used Microsoft Windows and version 8 of Microstation, the CAD application it has been using for a decade.

KEEPING ON TRACK The quantity surveyor, Gardiner and Theobald, advised during the design process and there were the inevitable additions and omissions to keep the design on budget. Böhringer says: 'We had originally had timber decking at the back of the school for spillout space from the library. But we had to specify cheaper tarmac.

We specified products that met our design criteria and were sustainable and the client bought into them - as long as they met the budget.' He said he would have liked to use Kentish oak for the classroom block elevations but it was decided that a Brazilian hardwood was more economical. And there were unexpected issues, such as the need to install a newt-proof fence to prevent the return of crested newts which had been moved from the site to preserve their lives. Apparently the newts quite liked their new setting and didn't bother to try returning. Some interesting Mesolithic flints discovered during the work will be displayed at the school.

BUILDING SKIN The classroom block has untreated timber rain-screen cladding.

It is a certified Brazilian hardwood called Louro, supplied by Penarth-based Ecotimber. Böhringer says: 'It gives the same effect as oak, starting off brown and weathering to a silver colour. It's a good lesson for the kids about how things get older and change.' The timber is cut to the specified lengths in Brazil and comes in a shiplap profile - the surface is all in the same plane: individual planks have a rebate at the top front and bottom back which slot together vertically and are surface-fixed with two screws.

Across the front of the main block, the elevations have a repeating pattern of solid timber panels alternating with glazed voids. The timber panel is edged with an aluminium angle.

The full-height glazed opening has two doors flanked by similarly dimensioned fixed glass, with transom panels above. The horizontal division created by the door head continues across the panels.

Above the line in each panel the shiplap changes to timber strips 57mm deep, to match the depth of the shiplap, and 19mm high with a 13mm gap between each strip. Böhringer says: 'The whole approach was to have natural ventilation. In winter you have to provide quite a lot of it. To manage that we had to introduce Passivent ventilators which have acoustic baffling. The A20 and M20 roads are not far away so it's quite noisy. So we have these big louvres with acoustic buffers behind the slats.' The glazing across the front elevation and elsewhere on elevations is grey powder-coated aluminium by Technal. Böhringer says: 'We wanted standard aluminium thermally broken windows for the elevations. So it was Technal.' The canted clerestory windows high along the north wall of each classroom and the row of roof lights to the exhibition street behind are a Vitral system, which Böhringer's team chose because Vitral opening lights look the same as fixed lights. Every third or fourth panel in these windows can be opened.

GREEN ROOFS Although the final building will nestle into a protecting circle of hillside, the swooping roofs are the structure's most visible features.

The sedum system on the short barrel vault of the school hall and the flat roof of the rear services block is by Bauder. Böhringer says: 'We looked at some alternatives such as Alumasc. But for this scheme Bauder seemed the obvious choice because it is very simple:

insulation with a vapour barrier and a growing mat on which you roll out the sedum blankets and finish off with angle trims. The roof is a variety of mosses and plants - it won't appear as a grass roof.' The main swooping roof is clad in a separate version of the Kalzip package from Corus, called Falzinc. This is a light aluminium sheet with a weathered zinc surface, and is laid here in 600mm-wide strips with standing seams of minimum height.

This is fixed to a German Endele insulation system specially made for standing-seam roofs. It has timber slats integrated into its surface at 300mm centres, to which the standing-seam clips are fixed.

The insulation sits on top of a vapour-control barrier over the two 12mm plywood layers of the structural diaphragm.

This is fixed to TrusJoint timber purlins at 600mm centres spanning between the I beams (pre-curved by Dollcast) that run front to back across the main teaching block. These are supported on steel columns buried in the external and internal walls.

The Gyptone plaster ceiling board, some of which had to be perforated to attenuate noise in each classroom, is fixed to a curved aluminium C-section MF system from British Gypsum.

The architect opted, on grounds of cost, greenness and performance, for Warmcel insulation. Böhringer says: 'It's excellent.

It is recycled newspaper blown into the 200mm cavity of the timber stud wall construction.' When the wall is clad both sides, he explains, 'they drill a hole and blow in the insulation to fill up the whole cavity. It has an extremely good U-value.' For the two blocks at the rear the architects have chosen StoTherm render because 'it is a slightly flexible surface; it is through-coloured and you don't need construction joints. Damage can be cut out and patched up. It's got neat crisp edges as well.'

ON THE INSIDE Scheduled for completion in the middle of next year, the building will have a mix of flooring, from vinyl to carpet to timber - the latter an oak veneer on engineering board. Heating, except in the services areas, is an OSMA underfloor system with pipes laid in grooves in the 70mm-thick insulation board. This sits on a supporting structure of block and beam - chosen because of unstable foundation conditions. The architect's original idea was to have mesh reinforcement in the screed on top of the insulation, but the main contractor, Barwick Construction, proposed an 80mm concrete topping with fibre XT mesh reinforcement. Böhringer says: 'Barwick is on the ball and came up with changes that are positive and very helpful.' It was chosen via the Kent preferred-list system and a competitive tender process.

For once door handles are not D Line but Scott Bevan's Radius: 'Very industrial and not too expensive, ' Böhringer says.

Finishing touches will include new, local trees like apple and hazelnut: 'We want to give something back.'

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