Degradation of structural material due to floodwater is one of the major damaging effects during flooding events. As a result, thousands of homes at the coastal states, especially those that are constructed from wood-based materials, have been destroyed after each severe hurricane. This paper evaluates the performance a new type of Composite Structural Insulated Panels (CSIPs) after exposure to flood water. The proposed composite panel is made of low cost orthotropic thermoplastic glass/polypropylene (glass-PP) laminate as a facesheet and Expanded Polystyrene Foam (EPS) as a core. The proposed CSIP panel is intended to overcome problems of the traditional Structural Insulated Panels (SIPs) especially that of strength degradation. An extensive experimental program was conducted to investigate the flexural strength degradation of CSIPs after exposure to floodwater for different periods of time. The residual flexural strength following full submergence of CSIPs in simulated floodwater was evaluated using a four-point loading test. Further, experimental results were validated using new analytical models and were observed to be in good agreement. Insignificant strength degradation was recorded by CSIPs and therefore they demonstrated the potential for their use in sustainable building structures against flood hazard.