A school in the shade of the plane-trees

The Jean Carrière nursery school in Nimes (southern France) is located in the Haute Magaille neighbourhood, south-east of the city centre, in an area of low-rise housing. The former Platanettes school, which was in prefabricated buildings, was demolished to make way for a modern scheme, with a commitment by the client to ambitious environmental objectives, particularly adapted to the Mediterranean climate.

The plot is characterized by its landscaped setting. Existing large plane trees form a remarkable framework of greenery, create a warm atmosphere, and act as natural climate regulators. The proposed scheme preserves most of them and associates them actively with the architecture.

The second major factor concerns the habitability of the site, which is subject to a major constraint related to drainage. The street that runs alongside the scheme is a storm water run-off corridor. The flood risk prevention regulations put in place by the City of Nimes limit occupation of the ground floor and require the classrooms to be placed on the first upper floor. This arrangement, which is quite atypical for a nursery school, is turned to advantage by the architects, who propose a building that is suspended in the foliage of the plane trees. From the classrooms, the children have a unusual pleasant view of the school’s natural setting. 

Simple organisation

In order to reduce the mass and impact of the scheme, the building is broken down into four volumes, which seem to slide between the trees. These volumes, occupied by the classrooms and the leisure area, rest on a series of solid stone walls arranged on the east-west grid lines, and on the north-south lines of slender metal columns. This heavy-light contrast is one of the distinguishing features of the scheme.

The cross-shaped plan allows simple, clearly visible operation. The circulation areas are not just ordinary corridors. They are treated like habitable areas, widely open to the landscape, with views in all four directions. The partitions between the classrooms and the circulation areas are partly glazed, to allow through views from one external wall to the other.

The internal atmosphere is similar to an open-plan office area, as opposed to the conventional type of corridor confined between enclosed classrooms.

The ground floor contains the canteen, the multi-purpose room, and the ancillary and service areas. These areas are divided into two self-contained blocks, to preserve the openness and transparency of the whole. The unconstructed areas in the west are used for the schoolyard and the covered playground area, while the east side is used for parking. The covered playground area is extended by raised timber decking, which is itself extended by a lawn play area. At the south-west corner, a storm water retention pond forms a wet garden, inaccessible to the children, which physically represents the site’s liability to flooding.

On the upper floor, the five classrooms face east. The offices, rest areas, computer rooms, library and leisure area are arranged along the east elevation. The building services plant rooms occupy the central strip.

The main elevation facing onto the public space is the south gable. Its limited size is compensated by two large cantilevers, one on each side of the entrance, which is itself framed by two solid stone walls. Together with a raised forecourt, it presents the visible face of the facility on the street, with the school entrance clearly expressed. After going through the glazed entrance lobby, there are two possible routes, one horizontally to the schoolyard and the canteen, the other via stairs to the classrooms on the upper floor. The colours and lighting of these two routes are given deliberately playful treatment.

Bioclimatic design

The scheme’s environmental objective is one of the requirements stipulated in the brief, but it also corresponds to the architects’ habitual general motivations. The Jean Carrière school meets the low energy consumption building criteria, and it has BBC-Effinergie certification (with consumption lower than 40 kWh of primary energy per m2 per year). It has also received the “label Or” (Gold Label) certification of the Bâtiments Durables Méditerranéens association for sustainable building.

The architecture’s “passive” behaviour is fundamental for adapting to the Nimes climate, and the scheme’s architectural style is mainly a result of this bioclimatic concern. 

In particular, one may mention the protection of all exposed façades (with double solar protection: fixed horizontal protection and adjustable vertical protection), appropriate management of the construction’s thermal inertia capacities (sufficient for night-time summer cooling, but not too much to allow the building’s good response for intermittent use), extra insulation, particularly for the roof (with a ventilation layer under the built-up roofing membrane, then 43 centimetres of accumulated insulation consisting of cellulose wadding and wood fibre), natural ventilation of external wall membranes, good airtightness (1.14 m3/h.m2 measured by the “blower door” test during and at the end of construction work), bioclimatic use of existing trees in the west and south, and the performance of the timber-framed building envelopes. A ground-source heat pump uses a groundwater heat exchanger to provide heating in cold weather and cooling in warm weather (with cooling provided directly by the groundwater heat exchanger), with a low temperature heating/cooling floor.

During periods when the building is used, ventilation of typical areas is provided by single-flow forced-draught fans, with back-up heating and cooling provided by a reversible heat pump. Air is extracted by roof exhaust fans For collective areas (canteen, multi-purpose room), a double-flow air handling unit renews air and provides back-up for heating and cooling from the reversible heat pump.

In addition, during periods when this system is not used, additional natural ventilation is provided at night-time, evaluated by dynamic thermal simulations using unfavourable scenarios (as regards weather and uses of the building). It is put into operation according to the external temperature. Its impact on summer comfort was found to be sufficient to justify the use of a complete system with roof exhaust fans and motor-driven intake grilles, controlled by the central building management system and equipped with anti-intrusion systems. For health reasons, the ventilation system changes more air than required by applicable standards (25 m3/h instead of 15 m3/h per person).

Sanitary hot water is heated by evacuated-tube solar panels with evacuated-tube heat-pipe solar collectors that include a temperature limiter (set to 160°C) provided by a form memory spring, to avoid any problems of overheating in the summer. The building was tested and checked during its construction, and its performance will be monitored for two years after its handover.

A Meccano set of construction elements

A certain number of factors justify the use of light dry construction, including in the Mediterranean climate. The main advantages concern the adaptability and reversibility of the constructions, the use of industrial technology and prefabrication, the speed and cleanness of construction work, the less laborious construction work, the greater construction precision, the limitation of the ecological footprint of the constructions, and the better carbon footprint (In comparison, the carbon footprint of the Jean Carrière school determined by Sintéo is 280 kg CO2 / m2, whereas the mean given by the Ademe energy management agency for a conventionally-constructed low-consumption building is 436.) In addition, the building has a better response for intermittent use (particularly in the case of a school).

The choice of light construction, that is, with low thermal inertia, required a specific strategy for responding to the requirements of the Nimes climate. The building is mainly constructed in timber (structural frameworks of elevations, floors and roofs). In addition, a primary structure in industrial structural sections is used for the cantilevers. With these industrial structural section, it is possible to have relatively slender pilotis “pile-columns” on the ground floor, and also to comply with the height requirement stipulated by the local planning regulations (a timber primary structure would have had very negative spin-off effects in the habitable areas, which already have minimum heights).

The insulation materials are from plant sources (cellulose wadding and wood fibre). The external walls are cladded in solid timber and in Trespa bakelized panels. To provide variable thermal capacity and incorporate a system for cooling and heating via the floors, “heavy” concrete top layers are cast in situ (as opposed to a solution with a dry top layer that could have been envisaged).

Traditional masonry construction is still used for two things: the concrete foundations and substructure, and the solid stone structural shell walls on the ground floor. Overall, the scheme applies a principle of the combination of three categories of materials: wood and products from plant sources, steel, and stone and concrete masonry. Every material is used where it is the most effective: wood for building envelopes and minor floor structures, steel for large spans, cantilevers and columns, and masonry for elements that ensure structural stability. This constructional combination is a distinguishing feature of the Jean Carrière nursery school and, more generally, of all schemes designed by the school’s architects.

Greenery, an indispensable element of the scheme

As we have seen, the existing plane trees on the site play an essential part in the scheme. Apart from acting a natural “air conditioner”, they form a distinctive feature, with greenery as the school’s external setting. In this scheme, which makes reference to the idea of a tree-house, the landscaping has a special role.

Since the site is liable to flooding, the buildings have to be raised by around one metre and an inaccessible retention pond has to be excavated in the plot.

Above all, the landscaping scheme aims to satisfy the resulting topographical requirements, while keeping the existing trees and preserving a maximum surface area of schoolyard in this cramped plot. A part of the schoolyard has raised timber decking which is built around the plane trees. The schoolyard also has play facilities, a lawn part and a strip of vegetable garden. Informal hedges are planted in the remaining spaces with a range of Mediterranean plants that resist drought and the strong winds that blow in the area.

To etablish some architectural complicities

The building seems to float a few metres above the ground, like a vessel hovering at the height of the roofs of the surrounding houses. This atypical arrangement could have resulted in a distance between the facility and its environment, or in a denial of the context. But this is not the case. It was necessary to avoid any direct architectural complicity with the neo-traditional low-rise housing that surrounds the project. 

The facility had to be placed clearly within a contemporary perspective, since the client had no doubt on this issue. In the end, the height imposed by the flood risk prevention plan establishes a positive dialogue between the school and its context. The scheme creates and occupies its own specific geographical layer which is added quietly to the existing cityscape, without any direct conflict.

This scheme has an indirect relationship with the Nemausus ⁽¹⁾ apartment complex designed by Jean Nouvel in the late 1980s, near the project site. While still looking out of place in the context of Nimes, Nouvel’s scheme is now a part of the city’s history, and it acts as an anchoring point of heritage for the city’s southern neighbourhoods, which do not have any other real personality. Jean Nouvel’s apartment blocks are themselves on pilotis pile-columns, with large metal louvered sun-shields overhanging and extending the facades, and they use metal eggcrate grilles as the architectural motif. This vocabulary is reinterpreted for the Jean Carrière school, but replacing the galvanised metal industrial character with a warmer, more mixed aesthetic. The eggcrate grilles and the metal structure are white powder-coated; they are combined with wooden facade panels and floors, and solid stone structural walls. The association of these three materials in matching tones creates a very soft, gracious presence in the southern sun.

(1) Nemausus is a social housing complex built in Nimes between 1985 and 1987, designed by the architects Jean Nouvel and Jean-Marc Ibos.