Composite sandwich panels composed of a balsa core and glass fiber-reinforced polymer (GFRP) face sheets may offer attractive applications for building slabs and bridge decks. Such composite panels may contain air-filled gaps (caused by initial defects or damage) or through-thickness cracks and different types of metallic inserts (used for facilities and piping systems) in the balsa core. The fire performance of sandwich panels can be reduced by the local heating effects inside such air gaps and inserts. The thermomechanical response of GFRP-balsa sandwich panels comprising through-thickness air gaps or steel inserts, subjected to combined mechanical load and one-sided ISO 834 fire, were thus investigated by fire experiments and uncoupled thermal and mechanical numerical analysis. The results demonstrated that the shear resistance reduction of the core is not critical, while the moment resistance reduction can lead to panel failure. The propagation of a transverse tensile crack in the balsa core produced a similar moment resistance reduction to an air gap or steel insert. The air gaps and steel inserts charred the inner core zones and heated the cold face sheet above, leading to face wrinkling. The effects of air gaps and steel inserts remained local. Depending on their location in a panel, they may thus not exhibit significant effects on the whole structure. If they are placed on critical load paths however, e.g. in the vicinity of supports, they may cause collapse of the whole structure. Since the extension of transverse tensile cracks is not limited, they may also lead to an overall collapse.