The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) Mission will send an approximately 1-meter diameter probe into Venus to take detailed measurements of the Venus atmosphere and surface to answer questions about terrestrial planet formation and evolution, including the role (if any) of oceans. This probe, called the Descent Sphere, contains five science instruments and avionics that, as a system, must survive and operate thru the extremely challenging environment of the Venus atmosphere with pressures and temperatures reaching 90 bars and 465°C respectively. To mitigate the influence of this environment, the Descent Sphere design utilizes passive thermal control methods that include a 1-inch thick, high temperature MLI, and a 2-inch thick mat insulation to minimize the internal radiative and convective heat leaks from the very hot outer shell. To characterize the performance of the MLI and mat insulations, an engineering development unit (EDU) Descent Sphere was fabricated and tested at an industrial furnace where the predicted transient temperatures were simulated (up to 465°C), data collected, and subsequently a thermal model was correlated. The resulting correlated thermal model reduced uncertainties in the MLI and mat insulation performance and provided increased confidence in the flight predictions and design margins.