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Visionary aims and traditional construction

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Alan Tovey of Tecnicom explains how concrete is playing an important role in the design and construction of some non-traditional new homes

Designed by the renowned 'green' architects Brenda and Robert Vale, the Hockerton Housing Project near Nottingham is the uk's first earth- sheltered, self-sufficient, ecological housing development. The project was initiated by Nick Martin, who built the Vales' own autonomous house and who has a strong interest in renewable energy and low-impact development.

The five terraced homes at Hockerton are currently being built on 10ha overlooking a lake. They are being built as single-storey with a modular layout of three to four bedrooms with living areas that open out to a spacious conservatory, which is designed to optimise fuel efficiency and allow daylight into the home. Because they are earth-sheltered, the homes have low visual impact and high thermal efficiency, and are designed to be among the most energy-efficient, purpose-built homes in Europe. They also capture the project's objective to develop a holistic approach to day-to-day living that includes the growing of organic foods, farming of fish and water-margin plants and enhancement of the local environment for wildlife. This includes an increase in biomass to counter the effects of construction so that over a period of time the net co2 emissions will be negative. The project also aims to be self-sufficient in energy and water.

The houses are deliberately simple, repetitive structures that combine high insulation with enhanced passive solar heating using off-the-shelf components. A particular feature of the design is the use of the structure's thermal mass and structural strength to support the roof's soil cover. Concrete, with its inherent thermal mass and low maintenance, was the obvious choice. The concrete floors, walls and roofs of the homes store and release heat energy over a long period of time and so help maintain a stable internal temperature which is calculated to remain at 10 to 20degreesC all year round. Complementing the use of the building's thermal mass with high levels of insulation means that the space-heating requirements for each home are significantly reduced.

Construction began with the excavation of the slope on which the houses are being built. The removed earth will form part of the lake construction and landscaping. A continuous concrete sub-slab was then laid, on to which 300mm of structural insulation was placed, which, together with the perimeter insulation, provided the permanent formwork for the top slab. This was then heavily reinforced to spread the imposed loads to form the raft sandwich foundation.

The retaining walls of the rear and the sides are built with high-density concrete blocks in two leaves with a reinforced concrete infill. This was then tanked and 300mm of insulation added on the cold side. The front brick and block walls have a fully filled 150mm cavity.

Simple blockwork crosswalls form 3m-wide bays which, although identical for simplicity of construction, can be partitioned to suit individual family requirements. To avoid thermal bridging, double concrete inner and brick arch outer lintels were used with plywood box linings to openings rather than brick rebates. All windows are triple-glazed to achieve a thermal performance of 1W/mK. The conservatory has Low-E double-glazed units which are expected to achieve 2W/mK.

The walls support an innovative use of precast-concrete flooring units. Rather than being underfoot, the units provide the roof. Due to their size, the inverted 'T' beams of the flooring system were mechanically lifted into position before the blocks were laid in between to complete the roofs. The flooring system was chosen over other traditional roofing structures for its ability to support the loads generated by the concrete and the 400mm soil cover.

The soil covering is an intrinsic element of the design. Not only does it make the homes invisible from adjacent roads and public access, it also plays a major role in the heating and cooling of the homes. Soil temperature at a depth of 400mm lags air temperature by three months. This means that the sun's energy stored in September is radiating heat in December. Meanwhile, soil temperatures in spring and early summer will have a cooling effect.

Although the objectives behind the Hockerton Housing Project are visionary, the method of construction is traditional. The project clearly demonstrates the potential benefits of concrete for domestic construction.

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