We developed the concept for a thermal validation tool for the semiconductor industry: a thermally insulated room that maintains an exceptionally stable environment (within a few millikelvin per hour) to verify the performance of a lithography machine module. Using a model, we compared multiple architectures and performed sensitivity analyses to understand how parameters such as wall and cavity thickness, airflow/flow variation, and control settings influence thermal stability. The simulation results guided key design trade-offs, identified dominant heat paths, and de-risked the concept before hardware, supporting robust mechanical design and precision system engineering. Outcome: a validated design that meets the milliKelvin-per-hour stability target and a clear specification baseline for detailed engineering and build, developed in cooperation with ZEST Innovate.