This hard-stone elementary school is located in Mulhouse in the Alsace region (north-eastern France), near the border with Germany. It is a listed building, since it was built in XXVIII sec as a spinning factory at a time which Mulhouse had an active textile industry and became a school in 1870. It has recently been retrofitted with a project balanced between low energy consumption and heritage preservation, with the availability of a constraint budget.
The building has a heated area of 1925 m² on three floors. Before the renovation, the building presented several problems related to structural safety, humidity accumulation and presence of mould. Therefore, coupled to the energy renovation, a deep restoration has been implemented.
Particular attention has been devoted to the ventilation, in order to limit the humidity and mould generation and to ensure the indoor air quality of the building. In fact, the building is currently naturally ventilated through the window openings, that was not sufficient for keeping the building in a good state.
The renovation is part of a wider program for improving the general conditions of the school buildings in Mulhouse. Further than the restoration of the building, structural safety and health, the owner set ambitious targets in terms of acoustic and daylight quality of the spaces and energy saving.
The project management team used building information modeling (BIM), preventing conflits between technical and architectural decisions and ensuring the communication with every stakeholders, including the project owner.
Concerning the building envelope, the original exterior lime coating was progressively replaced with of cement coating in the eighties. To put things back the way they were before and prevent more moisture-related damages, cement coating was removed and replaced with lime coating. In the interior, plaster coating was also removed and 8 cm of mineral wool and a humidity-variable vapour-retarder were installed and recovered with plasterboards. A technical space between the vapour-retarder and plasterboards was designed in order to pass the electrical network but also to prevent the piercing of the vapour-retarder. Biobased insulation was first chosen, but mineral wool was finally used because it was cheaper. A dew point analysis (see picture) was performed for walls to validate this choice. The insulation, made by mineral wool, and the air gap brought the thermal transmittance from a measured value of 3 W/(m2 K) to 0.34 W/(m2 K)
Some windows had been already replaced in 2010 with double glazed ones. Therefore, the windows have been partially restored, while new windows have been installed to substitute the most damaged and old ones. The new windows have wooden frame and double-glazed, with a U-value of 1.4 W/(m2 K).
The attic floor was insulated by the interior with 26 cm of mineral wool, leading to a U-value of 0.12 W/(m2 K). A vapour-retarder was installed between the insulation and the ceiling cladding.
The heating system is supplied by two condensing gas boilers of 224 kW each, that cover 200 % of the heating demand. This is a specific requirement of the city of Mulhouse, in case of failure of one of the boilers. An underfloor heating was installed in the entire ground floor, while the intermediate floors are heated with new radiators. Temperature is being controlled in each classroom by the city's technical services thanks to a building management system (BMS) that monitors and controls also the building energy consumption. Moreover, a mechanical ventilation system has been installed in order to comply with the local regulation on air change rates.
The overall energy performance is estimated to pass from 214 kWh/m2 years towards 69 kWh/m2 year, leading to a significant energy saving corresponding to the class B of the French energy performance certificate. The retrofitted school obtained the French low energy building ("Bâtiment basse consommation") label.
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