In accordance with current sustainable development objectives, it is intended to implement innovative and sustainable solutions at economic, environmental, and functional levels, boosting the construction sector to increasingly contribute to society. Expanded cork is an alternative to sand aggregate in improving the thermal insulating properties of renders. In addition, other materials can be incorporated as aggregates in an attempt to contribute to both the improvement of the physical characteristics that the render must fulfil as well as the improvement of thermal behaviour (if possible). In this sense, bio-based materials generated by the aquaculture sector, such as oyster shells, can contribute as a bio-based insulation material for thermal renders. In this study, thermal natural hydraulic lime mortars were produced from a mixture of expanded cork (EC) insulating aggregate and oyster shells (OSs). The percentages of replacing EC by OSs were 20, 30, 40, and 50%. The tests were carried out in fresh and hardened states. The studied mortars presented a thermal conductivity and compressive strength of 0.151 W/(m·K) and 0.63 MPa, respectively. The most interesting performance between thermal conductivity and compressive strength was for the composite with 50% of each bio-based material. The potential of incorporating oyster shells as a bio-based insulation material could contribute to a sustainable blue circular economy.