Abstract: An insulated structural panel is made from expanded polystyrene (EPS) panel and a plurality of interior and exterior structural members that are affixed to one another by screws is provided. When assembled, the interior and exterior structural members are flush with the EPS panel. A tongue at one end of the EPS panel engages a grove at another end of another EPS panel, therein permitting interlocking two or more EPS panels. The interior panel is C-shaped and is slid into a C-shaped passage in the EPS panel formed by hot-wire cutting the EPS panel. The exterior structural member is positioned within a channel formed opposite the passage. A top plate connects a plurality of interior structural members. A seam covering member is affixed to the one end of the EPS panel and is flush with the EPS panel. A thermal break is provided between the interior and exterior structural members.