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Insulating concrete formwork - plastic-foam blocks filled with concrete - is a fast-track, highly insulated walling technology that is being used for a house at Harrow Weald, just outside London.

Insulated Lost Formwork, Permanent Insulating Formwork - pick a name. We will go with ICF - Insulating Concrete Formwork - since that's the name adopted by the UK trade association, the Insulating Concrete Formwork Association (ICFA) 1. It is the technology of building with hollow insulatingfoam blocks, subsequently filled with structural concrete. There is little yet built in the UK, though it has been used in Germany and the Netherlands for over 30 years, and also in North America.

The highly engineered blocks lend themselves to straightforward building and rapid weathertight enclosure, making them popular too for self build, though training is needed. Typically, they are used for one-off houses; there are examples too of commercial buildings, even multi-storey cinemas.

There are a number of proprietary systems, varying in detail. While often used for single-storey buildings, three storeys are usually possible without reinforcement. Basements subject to lateral pressures will need reinforcement. Plaster and render finishes can be applied direct to the expanded polystyrene (EPS) foam blocks, or other conventional claddings such as weatherboarding, tile hanging or brick slips. A brick skin can be supported off the foundations or off corbelling (see Working Detail, pages 54-55). As well as standard wall blocks, there will be various specials such as corners, closures for openings and blocks for forming lintels. Concrete fill of the hollow walls with ready-mix can be done a floor at a time.

Typically, four skilled workers can build and concrete-fill the walls of a three-bed bungalow in a day. Some vendors also have proprietary flooring systems. Services can be surface mounted or chased into the foam, or pipes and conduits cast into the concrete.

White Magnolias (the clients are in the horticulture business) is a house in Harrow Weald, north-west of London, designed by architect and family friend Valerie Gyoker of Gyoker Designs. It is three storeys including a basement, 346m 2 in total.

The layout is organised around a central atrium with a pyramidal glazed roof, providing space for business entertaining as well as improving daylight penetration.

Built using the Styro Stone ICF system 2, its outer walls were built up with insulation/cavity/insulation blocks. These are 155 x 140 x 55mm thick blocks for external walls above ground, the 155mm used as the outer leaf. For basement walls and partitions, 55 x 140 x 55mm blocks were used. Basic blocks are 1,000mm long and 250mm deep. They are scored for cutting down by 25mm increments in length, though a design module of 250mm is more cost effective. Specials are available, such as 90º and 45º corners and cavity closers. The foam leaves have plastic strip ties between - better in fire and acoustic performance than the earlier block designs, where the EPS extended rod-like between leaves.

Wall U-values for filled but unclad walls are 0.25W/m 2K for the smaller blocks and approaching 0.13W/m 2K for the larger blocks.

The ground-floor construction is a Styro Stone system (a beam-and-foam-block system with in situ concrete topping), providing some thermal separation of the basement spaces.

Upper floors were built more simply, with timber joists.

To make the walls, blocks were stacked, with proprietary trestles alongside, helping with access, vertical alignment and stability during concrete filling. Filled a floor at a time, the whole house eventually took around 35 trucks of structural-grade ready-mix. Here, the external cladding is brick slips. Internally, the wall lining is Fermacell board, allowing ready support of fixtures and fittings.

Like other modern methods of construction, the final surface appearance is what the architect and client chooses - the walling technology is concealed. The benefits lie in the simplicity and speed of construction and in environmental performance in terms of insulation and thermal stability.

1. www. icfinfo. org. uk 2. www. styrostone. co. uk, tel 00871 789 76 78. Styro Stone has BBA (Agrèment) certification and a Building Regulations type approval for England and Wales under LANTAC (Local Authority National Type Approval Confederation - certificate SA-164-12-7021) and is seeking the same for Scotland under STAS (the Scottish Type Approval System)

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