Insulating Concrete Forms (ICFs) are a formwork for concrete that stays in place as permanent building insulation for energy-efficient, cast-in-place, reinforced concrete walls, floors, and roofs.
The forms are interlocking modular units that are dry-stacked (without mortar) and filled with concrete. The forms lock together somewhat like Lego bricks and serve to create a form for the structural walls or floors of a building. Concrete is pumped into the cavity to form the structural element of the walls. Usually reinforcing steel (rebar) is added before concrete placement to give the concrete flexural strength, similar to bridges and high-rise buildings made of concrete (see Reinforced concrete). The forms are filled with concrete in 1–4 foot “lifts” to reduce the risk of blowouts like with other concrete formwork. After the concrete has cured, or firmed up, the forms are left in place permanently.