Thermocill is an energy savings product that is designed for installation under the window board and above the radiator in a room. It is made from recycled plastic materials and can be retrofitted to existing homes as well as new builds. In its operation, the product tends to direct the natural convection from the radiator to create a wall of warm air immediately in front of the internal side of the glazed window. In this project, the performance of the Thermocill was investigated based on the previously designed experimental condition. The main aim of the CFD simulation was to assess Thermocill’s effectiveness in a typical bedroom consisting of a radiator, a double-glazed window and a door. Several high-fidelity and transient simulations were conducted to reproduce the experimental conditions and to provide an insight into the details of Thermocill’s operation, particularly near the window. Subsequently, a design optimisation on several geometrical aspects of Thermocill were carried out. The CFD simulations successfully reproduced the experimental data and led to major changes in Thermocill’s design. Products such as Thermocill play an important role in decarbonisation of heating in homes in the UK, which is one of the priorities of the UK government in achieving net-zero emissions.