A revolutionary Swedish system of prefabricated housebuilding promises to slash construction costs — starting with an easy-to-assemble domestic basement.
Can you imagine building a domestic basement in two days? A basement with electricity conduits already installed and window and door frames fitted? Swedish company Thermonex claims that its prefabricated basement system makes this possible, and is about to launch it in the UK.

The system could revolutionise the way houses are built in this country. “In two days, we can build the basement and the ground floor,” says David Aulton, managing director of the company’s Bolton-based UK operation. “In three days, we can build a whole house.”

Using the prefabricated panels could save money as well as time. Thermonex has yet to build a house in the UK, but Aulton says that, based on its experience using the system in the Republic of Ireland, the savings could be substantial. “For a large detached house, a complete basement system would cost around £28 000 compared with £50 000 for traditional construction.” For a semidetached property, the basement system comes in at about £15 000.

In the UK, the system will initially be targeted at domestic homes as a cost-effective way of increasing floor area without adding to plot size. But, in Sweden, where the system originated, it is already being adapted for walls, floor and even roof elements in commercial and industrial buildings. Aulton expects that once the system has established a foothold in the UK domestic market, it will be launched as a commercial building system here, too.

The system has a series of prefabricated, highly insulated concrete wall panels at its core. These are formed from a 60 mm thick inner and outer skin of lightweight concrete and reinforcing mesh, between which a 200 mm layer of expanded polystyrene insulation is sandwiched. The panels are custom made in the factory in any size up to 12 × 2.4 m.

Making the panels to order means that electric conduit and junction boxes, and door and window frames can be cast into the panels during the manufacturing process. “The basement can be ready for second-fix electrics and carpentry within 48 hours,” says Aulton.

Assembling a basement is a simple operation. Once the ground excavation is complete, the foundations and basement floor are constructed using traditional methods but to a specification produced by Thermonex. In reality, the only difference is detailing the slab’s edge and adding a power-floated finish to the concrete. Once the foundation slab has cured, the basement walls can be erected.

The custom-made walls are delivered by lorry to site, where they are craned into position and assembled by a two-man team. Before the panels are erected, steel dowels are screwed into sockets cast into their base. Corresponding holes are then drilled into the foundations to receive the dowels and locate the wall panels.

Temporary props are used to support the walls until a corner plate is fitted linking the top corners of adjacent panels to secure the construction. Once in place, the locating dowels are grouted in. Grout is also poured into the joint where the base of the walls meets the floor slab to prevent moisture entering the basement. The system is complete and structurally sound once the 250 mm insulated panels that form the basement’s roof are added. These panels also form the ground floor of the house.

Very large basements need internal cross-walls for added strength. These are constructed from 135 mm thick precast panels that subdivide the basement into smaller rooms, creating an egg box-like structure. The walls can be finished by painting or tiling the inner surface.

A typical domestic basement can be built from just four large wall panels. Aulton points out that this means there are only four joints to waterproof. A typical joint between panels consists of a rebate detail with a 10 mm Rockwool quilt insert, a composite rubber gasket to seal the outside and a mastic sealant on the inside. Larger basements can be created by joining a series of panels together.

As with a traditional basement system, a gravel infill and land drain system needs to be installed around the outside to keep ground water at bay and to ensure that ground water pressure is not a problem.

The panels are made from a new type of structural grade, lightweight, waterproof concrete developed in Sweden, called X-Concrete. The aggregate they contain is a mix of expanded clay beads in different sizes up to 10 mm diameter, and expanded polystyrene pellets up to 2 mm in diameter. An organic additive anchors the polystyrene pellets in the mix and stops them rising to the surface.

The porous structure and lightweight aggregates give X-Concrete good insulating properties. With a U-value of between 0.27 and 0.44 W/m2ºC, depending on the strength of the concrete, it compares favourably with standard concrete, which has a value of only 1.7 W/m2ºC.

Aside from its U-value, Aulton enthuses about the concrete’s resistance to water penetration, which is kept to between 2 and 10 mm compared with 50 mm for standard concrete. Aulton claims that this makes it more durable. X-Concrete is also said to perform well in a fire because of its insulating properties and because the aggregate is more easily compressed, which, says Aulton, “makes it capable of withstanding thermal shock and so prevents the risk of explosion when water is sprayed on it in a fire”.

In Sweden, the panels are already being used with a structural frame to form large warehouses and for overcladding and renovation work. Thermonex is opening a manufacturing plant in the Republic of Ireland in the spring. “When the system takes off in the UK, the company will open a manufacturing facility here, too,” says Aulton.