The overhead layer of buildings, often serving as semi-open transitional spaces, plays an important role in moderating outdoor microclimates and improving pedestrian comfort in urban planning. Thermal indices, particularly the Standard Effective Temperature (SET*), are commonly used to evaluate human thermal sensation; however, their applicability in dynamic outdoor and semi-open conditions remains limited. To address this, comparative experiments were conducted in an indoor badminton hall and on the overhead floor of a university building in Guangzhou. Thirty participants performed walking trials at different speeds followed by rest, during which environmental parameters and mean thermal sensation votes (MTSV) were recorded. Results demonstrated that the predictive performance of SET* was more reliable in transitional overhead spaces than indoors under dynamic walking conditions. Building on these findings, an enhanced SET* model (αSET*) and revised stress categories were proposed, extending the applicability of SET* to semi-open overhead environments. The refined model provides a thermal comfort range of 24–37.5 °C (MTSV within ± 0.5), offering practical guidance for integrating overhead structures into urban planning strategies to alleviate thermal discomfort.