The results of a thermal vacuum test on the Engineering Model Tower of the GLAST LAT Tracker and the performance expected for silicon detectors as a function of operating temperatures in the mission environment are presented. The tower thermal properties have been studied in the vacuum environment in order to determine the heat distribution in the tower and verify that the thermal gradient is within the requirements expected on the basis of the silicon detectors aging. Different powers have been supplied to the tower at different temperatures and complete maps of the temperature distribution have been reconstructed in order to evaluate the silicon detectors efficiency and performance with increasing operating temperatures.