Structural sandwich composite panels are extensively used for modular panelized construction. This paper includes the structural characterization of the innovative composite structural insulated panels (CSIPs) developed for the exterior walls of a modularized structure. The facesheets of the CSIPs consist of E-glass fibers impregnated with polypropylene matrix, while the core consists of expanded polystyrene foam. The overall panel dimensions including the thickness of the facesheets and the core were obtained using sandwich composite design theories. Uniaxial compressive loading and high velocity impact (HVI) loading are the most important loading criteria for an exterior wall of a structure. The proposed CSIPs were tested for a compressive load of 13.13 kN/m (900 plf) in this study. The deflection at this load was ∼3 mm which was within deflection limits of span/150 as prescribed in ACI 318-05 building design code. The proposed CSIP wall panels were also tested under HVI for the loads prescribed by Federal Emergency Management Agency for design of emergency shelters in hurricane prone areas. Based on the HVI testing, CSIPs were able to stop a blunt object projectile traveling at 135 m/s proving its suitability against HVI type of loading. © 2010 ASCE.