Underground residential spaces are an integral part of vernacular architecture in Iran's hot-arid climate, playing a significant role in providing passive thermal comfort. However, the morphological principles governing their thermal effectiveness have not been systematically classified. This research addresses this gap by developing a new typology based on a case study of 80 underground spaces in Semnan (35 degrees 34 ' N, 53 degrees 23 ' E), Iran. The analysis involved a descriptive frequency study, a relational analysis of key variables, and a cumulative performance scoring system. The results identify an optimal typology characterized by multi-sided earth contact, north-south orientation, composite form, and no direct openings. A new performance scoring reveals a significant gap between this ideal and common practice: none of the 80 spaces met all four optimal criteria, and only 10% achieved a high-performance score of 3 out of 4. The analysis also uncovers key correlations, such as the tendency to pair greater earth contact with fewer openings, demonstrating a sophisticated, integrated design logic. These findings provide an evidence-based framework for architects, demonstrating how principles from vernacular architecture can be leveraged to create sustainable, low-energy buildings. The developed typology serves as an actionable guide for optimizing new residential designs in hot-arid climates worldwide.