A critical consideration for the serviceability of composite sandwich panels is their thermal behaviour during fire incidents. This research aims to observe the thermal performance and investigate the insulation failure of the lightweight concrete sandwich panels (LCSPs) in non-load bearing wall systems. Six standard one-sided coupling fire tests on LCSPs in accordance with Australian standard AS 1530.4 were conducted via an electrical furnace; measured by thermocouples and a thermal camera, to assess the insulation capacity and their behaviour during fire events. The results indicated that the sandwich panels have insulation capacity for 75 to 110 minutes depending on the thickness and density. The bowing of the panels due to the expansion of the exposed steel shield and the consequent de-bonding and cracking of concrete was one of the primary reasons of insulation failure. Additionally, this bowing led to the opening of the joints between the panels, which could allow the heat flows towards the unexposed surface. Moreover, the propagation of accelerated drying shrinkage cracks in the restrained concrete core was another reason for the failure. Lastly, the results suggested the benefits of increasing the thickness and density on thermal performance and insulation failure of the composite sandwich panels.