In hot and dry climates, underground spaces are recognized as highly effective vernacular architectural solutions for improving thermal comfort and reducing energy consumption. This is primarily due to their lower elevation and increased contact with the surrounding soil. This research quantitatively evaluates the thermal performance of vernacular residential underground spaces in Semnan, Iran. Using DesignBuilder and EnergyPlus simulation software, coupled with on-site measurements of temperature and relative humidity, the study analyzes the impact of architectural parameters, including soil contact ratio, orientation, spatial configuration, and opening size, on thermal comfort conditions. A sensitivity analysis was performed on the optimal design using factorial and linear regression methods. Results indicate that underground spaces with three sides in contact with the soil achieve up to 3% more annual hours within the ASHRAE 55 thermal comfort zone compared to other typologies. To identify the most impactful factors, a sensitivity analysis using factorial and linear regression methods was performed. The analysis revealed that the percentage of opening area, wall thermal resistance, and the ratio of buried surface to total area are, respectively, the most significant parameters affecting thermal discomfort hours. Based on this analysis, a linear regression model was developed to predict thermal discomfort hours, serving as a rapid design tool. By emphasizing the optimization of soil contact ratio and the precise control of openings, these findings provide quantitative design guidelines for significantly enhancing the thermal comfort of underground buildings. These principles can substantially reduce reliance on active heating and cooling systems and promote environmental sustainability in similar climates.