Degradation of structural material because of floodwater is one of the major damaging effects during flooding events. As a result, thousands of homes in 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 panel (CSIP) after exposure to floodwater. The proposed composite panel is made of low-cost orthotropic thermoplastic glass/polypropylene laminate as the facesheet and expanded polystyrene foam as the core. The proposed CSIP panel is intended to overcome problems of traditional structural insulated panels, especially those resulting from strength degradation. An extensive experimental program was conducted to investigate the flexural strength degradation of CSIPs after exposure to floodwater for various periods of time. The residual flexural strength following full submergence of CSIPs in simulated floodwater was evaluated using a four-point loading test. Furthermore, the 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 hazards. © 2013 American Society of Civil Engineers.