A new type of Composites Structural Insulated Panels (CSIPs) is presented in this paper. These panels are proposed for structural floor and wall applications. The developed composite panels are made of low cost orthotropic thermoplastic glass/polypropylene (glass-PP) laminate as facesheets and Expanded Polystyrene Foam (EPS) as a core. CSIPs have a very high facesheet/core moduli ratio. The common mode of failure of these panels is facesheet/core debonding. Accordingly, this investigation presents models for interfacial tensile stress and critical wrinkling in-plane stress associated with debonding of CSIPs. The facesheet in compression was modeled as a beam on a Winkler foundation. The proposed models were validated using full scale experimental testing for CSIPs floor and wall panels. Both types of panels failed by facesheet/debonding with natural half-wavelength approximately equal to the core thickness.