Insulating concrete form or insulated concrete form (ICF) is a system of formwork for reinforced concrete usually made with a rigidthermal insulation that stays in place as a permanent interior and exterior substrate for walls, floors, and roofs. The forms are interlocking modular units that are dry-stacked (without mortar) and filled with concrete. The units lock together somewhat like Lego bricks and create a form for the structural walls or floors of a building. ICF construction has become commonplace for both low rise commercial and high performance residential construction as more stringent energy efficiency and natural disaster resistant building codes are adopted. ICFs may be used with frost protected shallow foundations (FPSF).