ICF's (Insulated Concrete Forms) are basically forms for poured concrete walls that stay in place for a permanent part of the wall assembly.

The forms, made of foam insulation, are either pre-formed interlocking blocks or separate panels connected with plastic ties. The left-in-place forms not only provide a continuous insulation and sound barrier, but also a backing for drywall on the inside, and stucco, lap siding, or brick on the outside.