This paper presents a new type of Composites Structural Insulated Panels (CSIPs) for structural wall applications. The proposed composite panel is made of low cost thermoplastic orthotropic glass/polypropylene (glass-PP) laminate as a facesheet and Expanded Polystyrene Foam (EPS) as a core with very high facesheet/core moduli ratio. The proposed CSIP walls are intended to overcome problems of traditional Structural Insulated Panels (SIPs) such as termite attack, mold buildups and poor penetration resistance against wind borne debris. This paper investigates the behavior of CSIPs under concentric and eccentric loading. CSIPs specimens failed by global buckling mode in which no debonding was observed. The eccentric specimens failed at load 35% lower than that of the concentric ones. Global buckling formulas for concentric and eccentric loading were presented and validated using the experimental results and were in a good agreement. An equivalent stiffness formula was also developed for sandwich wall under in-plane loading considering the shear deformations effect of the core. Design study for CSIP walls is also presented to help in designing this new type of composite panels.