Insulating Concrete Formwork System

An insulating concrete formwork wall begins as a dry-stacked shell. Hollow units of insulating material are laid up course by course and held at a set separation by internal ties, forming a continuous cavity that runs around the building. Reinforcement is placed within that cavity, and concrete is then introduced to fill it. The units are never stripped away.

What distinguishes the assembly is that one component changes its job partway through the build. Before the fill, the insulating units are formwork: they resist the pressure of wet concrete and define the wall's line and plumb. Afterwards they carry no structural duty at all and become the permanent thermal layer on both faces. Nothing is removed, so the finished wall has insulation inboard and outboard of a core that was cast in place.

The nearest sibling is the structural insulated panel system, and the boundary is testable on site. In insulating formwork the insulation is the mould and the cast core is the structure, so the structure comes into being where it stands and the face material can be cut back locally without touching it. In a structural insulated panel the facings and the bonded core act together as the element itself, so cutting through a facing removes the composite action the panel depends on. Ask which part would still be standing if the insulation were stripped away.

The ties are the hinge of the whole assembly. During the fill they are what stops the faces spreading, so the layout follows from the unit and from what the fill demands of it rather than from the fixings a room will later want. Afterwards those same positions become the map of where a handrail, a kitchen run or a cladding rail can be attached, because the faces have no holding power. That is why the layout has to be established and recorded before the fill: the finished building is planned to it, and a fixing decided late has nowhere to go except into the core.

The core behaves as a wall only if it is continuous. Reinforcement, formers around openings and any embedded item all occupy the same cavity as the concrete, and where they crowd together the fill has to travel past them. A congested zone that does not fill completely leaves a void that is invisible the moment the faces are closed, which is why what goes into the cavity, and where, is a design decision rather than a site improvisation.

Because insulation sits on both faces, the core is enclosed from the moment it is placed. Anything that has to be cast into it, whether a bearing, a strap or a sleeve, must be positioned beforehand, since afterwards it can be reached only by cutting a face away. Drying and moisture movement in the core also happen inside an insulated envelope, and what that means for a finish applied early is a matter for the designer and the manufacturer's documentation.

At the base course the faces, the barrier arrangement and the core all change at once. The line of insulation has to be carried past that junction if the assembly is not to be interrupted where it meets the substructure, and the core has to be tied to what is beneath. This is where the continuity the rest of the wall achieves almost incidentally is most readily lost.