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 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 resulting walls flexural strength, similar to bridges and high-rise buildings made of concrete (see Reinforced concrete).

After the concrete has cured, or firmed up, the forms are left in place permanently for the following reasons:

• Thermal and acoustic insulation
• Fire protection
• Space to run electrical conduit and plumbing
• Backing for gypsum boards on the interior and stucco, brick, or other siding on the exterior

Initial Cost
ICF material cost ranges from about $1.75 per square foot to about $3.50 per square foot. Cost of installation labor, reinforcement, bracing, and concrete placement will be additional.  On average, ICF homes cost about two to five percent more than wood-framed construction. However, contractors installing ICFs for the first time, because of training required and a learning curve, may find that total installed cost is from five to seven percent.