Insulating concrete formwork for low-rise housing

ICF is a quick, cost- effective method for constructing watertight, highly insulated building envelopes. Could it be a solution to the government’s housing challenge?

The government has committed to building 1.5 million new homes over the lifetime of this parliament. This flagship policy will inevitably focus not just on what we build, but also on how we build it, continuing the drive for modern methods of construction (MMC) to improve efficiency, quality and speed of construction.

In the context of skills shortages, the need to maximise resources and the sheer amount of construction ahead of us, the case for rationalised factory production is compelling. But low-rise family housing also poses a number of challenges for MMC providers. Residential developments need to cater to different markets and usually require a variety of designs specific to local needs, environmental factors and the complexities of a site.

In response, some MMCs incorporate elements of traditional construction, allowing greater design flexibility without compromising build rates. One such method is insulating concrete formwork (ICF), which currently accounts for 2-5% of the UK housing market. ICF systems comprise hollow blocks or panels made of an insulating material. These are produced in a factory before being delivered to site, where they are stacked to form the shape of the building’s outer wall.

The outer and inner leaves of panels act as a mould for the concrete, which is pumped into the cavity to form the structural element of the wall. If walls have large openings or significant loads applied to them, reinforcement can be added before the concrete is poured. The blocks or panels remain in place as a permanent part of the wall assembly, and cladding and internal finishes can be applied directly to the insulating material. This allows contractors to quickly achieve a watertight, airtight and highly insulated envelope.

Types of ICF

A wide range of ICF systems are available, but there are two main categories: blocks and panels. In the UK, blocks are usually made of expanded polystyrene (EPS) or woodcrete, a rigid material made from wood fibres bonded with cement. For external walls, woodcrete blocks usually contain a layer of rigid insulation behind the outer face.

All block systems are composed of two leaves of insulation joined together by insulation or a matrix of steel or plastic ties. The ties can also be used to support reinforcing steel, and provide anchor points for the fixing of finishing materials. Blocks are designed with interlocking surfaces, allowing them to be stacked quickly on site. Unlike blocks, panels don’t have to be moulded, which means they can be made from extruded polystyrene (XPS), which has a closed-cell structure.

Panel systems are typically connected with metal or plastic rails, requiring a greater degree of assembly on site, but offer the flexibility of being able to change the depth of the wall assembly.
Panels and blocks are light, and therefore easy to handle without mechanical assistance – one reason why the system is often favoured by self-builders.

Concrete specification

The consistency of the concrete is important – too dry and it won’t flow around the connectors, too wet and it may place too much pressure on the formwork. Widely available designated concretes may be used to achieve the necessary properties – these should be discussed with the designer and concrete supplier. Typically a slump of S2 or S3 and a maximum aggregate size of 10mm is recommended by manufacturers.

Mix designs may vary for different applications, and you should check first with your supplier. The concrete is typically placed by pump, and then vibrated to expel air and increase density. If reinforcement is used, self-compacting concrete should be specified to avoid the need for vibration. A proprietary support system may be required around openings, corners and cut units to provide additional stability. This is removed once the concrete has set. The formwork may need to be temporarily supported externally during pouring, depending on the height of the pour and concrete consistency.

The permanent formwork offers an additional benefit because the concrete will cure quicker as heat is retained by the surrounding insulation. This is particularly useful for concrete containing high levels of GGBS cement replacement that is being poured in winter. ICF therefore potentially facilitates the use of lower carbon concretes.

Thermal efficiency

Reliable thermal performance is one of the key advantages of ICF. Studies have shown that gaps of as little as 3mm between insulation boards can undermine U-values by up to 0.2W/m2K. For ICF, continuous insulation is one of the system’s defining characteristics: if there are any gaps, it will very clearly fail as formwork. There are numerous examples of ICF being used to construct to Passivhaus thermal standards.

As a fully integrated, enclosed system, ICF offers very little opportunity for thermal bridging from connecting ties or elements such as lintels and cills. The fact that the concrete sets in direct contact with the insulation also reduces risk of thermal by-pass and enhances airtightness, preventing any air movement in the assembly. Inherent airtightness and virtual elimination of cold bridges also limit the risk of condensation and mould.

Fire performance

ICF systems typically offer between two and four hours of fire resistance, with greater resistance achieved by walls with wider concrete cores. They should be tested to consider both reaction and resistance to fire in accordance with the relevant EAD standard.

Concrete does not burn, does not emit toxic fumes when exposed to fire, and has a slow rate of heat transfer, ensuring the structural integrity of the building. Polystyrene, however, is a flammable material so EPS and XPS insulation is treated with a flame-retardant additive to resist ignition for longer. In the event of a fire, the concrete core will help to contain the fire within the room and prevent it spreading. For woodcrete, the mineralisation process that occurs during manufacture makes the blocks burn-resistant, and can give them a 90-minute fire rating.

Design considerations

ICF systems can be adapted to a variety of designs, to maintain architectural quality on larger developments with multiple housing types. At Wilkinson’s Brook in Dublin, for example, architect Proctor & Matthews originally designed the 69-home scheme for either traditional construction or MMC. Working with developer Glenveagh Homes to assess the potential of different panellised systems, the team found that ICF could be used on all seven housing variants on the masterplan while achieving the architectural vision.

Designers are aided by a wide variety of standard forms, including straight planks or panels, and right-angled, 45° and curved blocks. If needed, these can be cut using standard construction tools. The systems can work with any foundation – including strip, concrete raft and insulated raft – and most common floor and roof types, such as wooden joists, concrete beams and block, and hollowcore planks.

External walls need protection to provide weather resistance, and can be faced with various materials, including render, brick skin, timber and stone. Internally, plasterboard is the most common finish, fixed directly to the walls. The formwork can be chased to accommodate wiring and pipework.

Further technicial information is available from the Insulating Concrete Formwork Association (ICFA)

Photo BecoWallform