The use of prefabricated elements is widely adopted in the construction sector to reduce the necessary on-site construction works and time, to maximise efficiency in material use and to mitigate on-site fabrication and assembly errors. The established use of such methodologies necessitates ongoing investigation and improvement of their performance both under normal loading conditions and when exposed to high temperatures. The present research proposes and assesses an innovative precast concrete wall composite section, incorporating insulation layers of varying thickness and position in relation to the fire source. Considering the combustibility of the incorporated insulation layer, sheds new light on the impact of the insulating material properties on the thermal performance of such innovative wall assemblies when exposed to fire. Quantifiable indicators are established, and qualitative conclusions are drawn, to aid with appropriate specification of materials and assemblies. Wall samples are subjected to a standard temperature–time curve as per EN1991-1-2 and their thermal and structural performance is reviewed and commented upon. It is concluded that the positioning of rockwool insulation against the fire front can reduce the temperature of the protected/non-exposed to fire side of the wall by up to 60 °C. The thickness and position of the insulation also lead up to a staggering 520 °C difference in the steel frame temperature when protected with 10 cm of rockwool compared to the same thickness of insulation installed on the non-exposed side of the assembly. A comparison between the performance of the surface-mounted expanded polystyrene to rockwool shows a temperature difference of up to 155 °C on the non-exposed boundary, for the same insulation thickness and positioning. The analysis is carried out through an experimentally validated 3D finite element model developed on COMSOL Multiphysics®.