The duration and frequency of extravehicular activity (EVA) is expected to increase with the anticipation of challenging missions ahead. This necessitates the development of an automatic controller for astronaut thermal comfort regulation. A reliable human thermal model is essential in order to predict the thermal response of subjects under various conditions to aid in automatic controller development. This paper examines thermal response sensitivity to several parameters and input modifications using a popular human thermal model. These parameter and input variations are based either on values reported in the literature or realistic estimates.