In the present research work, an attempt was made to develop a casting shell mould that would enable the manufacturing of thin-walled investment castings from nano-bainitic steel. Nano-bainitic steels have unique properties, offering a very favourable strength-to-plasticity ratio. Thus, the use of nano-bainitic steel enables the production of lighter components with reduced wall thickness. However, the manufacturing of thin-walled castings is often accompanied by the formation of hot-tear defects. One of the reasons for the formation of hot tears is the high mechanical strength of the shell mould. The aim of the present work was to investigate the possibility of reducing the shell mould strength while simultaneously maintaining its other functional properties. To achieve this objective, different types of pore-forming materials were added to shell moulds based on fused silica. Metallic and organic materials were selected for shell mould property modification, including brass micro-powder, wood pellets, spelt husk, buckwheat husk, sliced barley, and oat flakes. All modifiers were introduced into the shell mould as stucco materials. Analysis of the mechanical test results, such as flexural strength tests, Weibull modulus and Young’s modulus determination, as well as porosity and permeability measurements, showed that all the investigated materials effectively reduced the mechanical strength of the shell moulds. The flexural strength was reduced by 58–62%. It was also observed that the hot permeability of the modified shell moulds did not change significantly, except in the case of brass micro-powder, which completely blocked gas flow inside the mould. Based on the results of the research, thin-walled demonstration castings were successfully manufactured.