Abstract: A modular wall system for a building structure that includes a plurality of pre-fabricated wall sections that are attachable to each other to form the foundation for the building structure. Each wall section forms the entire length of a wall of the foundation. The wall sections include a plurality of steel studs having a track attached thereto at the top and bottom, at least one structural insulated panel attached to the studs, and at least one waterproofing sheet attached to the outer surface of the structural insulated panels to allow the wall sections to be used below-grade.