House B at Saint-Gély-du-Fesc
Eco-design: lightweight structure, high-performance envelope, rational layout
Prefabrication and speed of construction
Built for a family of four - a couple and two children - "House B" is the result of precise work on its location and the choice of a lightweight construction system combining a light gauge high yield strength steel structure with various insulating, partitioning and cladding materials. The combination of these various parameters enables attainment of very good low consumption energy performance for an architect-designed house at a very competitive cost.
As the architect says: "The house is currently undergoing Promotelec certification and should obtain a BBC-Effinergie seal. At the design stage, the house had been selected following a call for proposals issued by the French Environment and Energy Management Agency (ADEME) and the Languedoc-Roussillon region. As such, it is equipped with metrology tools to assess its thermal performance over time.
House B is located in Saint-Gély-du-Fesc, a town situated about ten kilometres north-west of Montpellier. In order to exploit the benefits of the climate and to control its uncomfortable effects, the architect opted for a bioclimatic architecture, playing on the layout of volumes, the openings and the orientation of the rooms. This design is accompanied by the choice of high-performance building materials. The installation of solar panels on the roof for water heating completes the systems selected to reduce fossil fuel consumption.
With a net internal area of 205 m² including net dwelling area of 170 m², it is located in the upper portion of a gently sloping, south-facing 900 m² plot. The living space comprises a primary ground-floor volume located perpendicular to the slope. It incorporates two bedrooms plus a bathroom and a toilet and the communal rooms. All these rooms have direct access to a wide terrace bordering the house over the full length of its south facade. The upper floor, accommodating the parents' bedroom, a bathroom, a toilet and an office on a mezzanine floor above the hallway, constitutes a second volume. Smaller, it is arranged perpendicular to the ground floor and incorporates a 2.40-metre overhang on the south side. A third, single-storey volume houses the garage; it is built at right angles to the house adjacent to the entrance located on the north façade.
Play of light and shade
The bedrooms and communal living spaces face south whereas “serving” spaces are north-facing. On the ground floor, the lounge, dining room and kitchen constitute a non-partitioned series of spaces differentiated by their concatenation and their natural lighting. The living area is relatively enclosed with two large, fully-glazed doors on the south and west facades, compared with the kitchen that opens fully onto the terrace with a 6-metre wide glazed opening. The kitchen and its continuation onto the terrace are protected from the summer sun by the first floor overhang that also serves to frame the landscape: an ideal setting for preparing meals. There are sliding shutters with adjustable slats in front of every opening, sheltering the house from excessive direct sunlight. By virtue of the openings on all four facades, ventilation occurs naturally - in addition to the mandatory mechanical ventilation - and contributes to the comfort of the house.
An efficient hybrid structure
The choice of a steel structure designed using Styltech® building systems harmonises with the house’s architecture and economy. Built of "Multibeam" light-gauge galvanised high yield strength steel sections whose zigzag web increases their rigidity, the structure is erected on lighter concrete foundations than would be the case with a conventional solution.
The profiles are located in accordance with a standard 60 cm grid corresponding to the dimensions of the other construction components used for internal and external wall cladding such as partition panels. Here, the spans reach up to 7 metres between supporting columns. The correlation between the grid and the dimensions of the cladding panels enables the latter to be screwed directly onto the structure without any need for a subframe. The galvanising of the steel protects it against corrosion and the application of plasterboard to the steel structure provides the required fire resistance.
Internally, the plasterboards are therefore screwed onto steel studding between which mineral wool is inserted. Externally, the cladding covers the insulation, which is fixed to OSB (Oriented Strand Board) panels, which are screwed directly onto the sections. The principle of external insulation systematically employed with this type of construction system avoids thermal bridges and therefore provides better performance. In addition, the services are built into the walls and ceilings.
"This structural system allows me to design the envelope very freely. I have a greater richness of expression than with a traditional construction solution," states the architect. Thus, he chose to highlight the first floor volume with a slatted red cedar cladding that contrasts with the coating on the ground floor facades.
The sections are joined primarily by means of self-drilling screws and occasionally by bolting when the strength and stability requirements are greater. In the case of the glazed opening in the kitchen for example, the 6-metre gap in the grid and the cantilevered volume above it entail greater load distributions on each side. Moreover, some parts of the roofs are planted while those above the children's bedrooms and the garage floors are load-bearing floors capable of supporting future extensions. Here too, particular strength is required, which was taken into account at the design stage. Thus, the floor joists and roof structures as well as some vertical elements are constructed using double sections and an intermediate tubular column - the only visible evidence of the steel structural - completes the system.
An industrial process and controlled costs for economic architecture
All of the construction components are the product of routine industrial processes. This represents both a very substantial timesaving for the construction and significant control of costs.
The computerised design of the structure makes it possible to address all specific points with great reliability even before the start of work on site. So, the sections are prefabricated and delivered to site with an assembly drawing. Due to the lightness of the components and the simplicity of assembly, operations are performed manually - a large hoist is not required - by a small number of workers - up to five in the case of this house - in a very short time. In the eight months of construction, less than one month was required to carry out the complete assembly of the structure.
Lastly, from an environmental point of view, this type of dry building site – with the exception of the concrete foundations - causes little inconvenience to the neighbourhood, produces little waste to be removed and results in relatively comfortable working conditions for the workforce. And steel is 100% recyclable on infinity.
Built from standard components, the house is the product of a distinctive architecture with its volumes and generous openings that enable it to benefit from the sun and the light.
Text: Eve Jouannais
Architect: Frédéric Jauvion