A structural panel includes a shell having spaced first and second fiber reinforced polyester (FRP) exterior panels and separate frame members adjacent edges of the FRP panels, and at least one polymeric sheet disposed between the FRP exterior panels and bonded to an adjacent exterior panel. The polymeric sheet is made of a thermoplastic material and has a plurality of openings through its thickness spaced apart by flat wall portions of the polymeric sheet. At least one blast- or ballistic-resistant core layer is disposed adjacent the at least one polymeric sheet. A curable and hardenable foam insulation material fills substantially all the space between the polymeric sheets, frame members, and blast- or ballistic-resistant core layer in the shell interior.