Space suits for advanced missions have baselined radiators as the primary means of heat rejection in order to minimize consumables and logistics requirements. While radiators have been used in the active thermal control system for spacecraft since Gemini, the use of radiators in spacesuits introduces many unique requirements. These include the ability to reduce the amount of heat rejection when overcooling or overheating of the crew member is a concern. Overcooling can occur with low metabolic rates, cold environments or a combination of the two, and overheating can occur with high metabolic rates in a warm environment. The main goal of the Radiator Advanced Demonstrator (RAD) program is to build and fly a radiator on the current Extravehicular Mobility Unit (EMU) in order to verify thermal performance capabilities in actual flight conditions. The RAD incorporates an aluminum plate separated from the primary water panel with a silicone gasket. The gap created is normally filled with a low pressure gas in order to facilitate heat transfer. In conditions where crew member overcooling or overheating is a concern, the gas gap is evacuated. This thermal switch design effectively creates a radiation barrier and significantly reduces the amount of heat rejected or absorbed by the radiator. A model of the RAD, including the gas gap was built using SINDA/FLUINT and TSS. A description of the RAD model, results and comparisons with experimental data are included.