Workmanship plays a vital role in establishing construction quality in general, and particularly in setting foundations for thermal application performance. In Bahrain, reducing cooling demand and achieving sustainable energy use are key reasons for studying and controlling this area. While polyurethane (PU) foam is favored for its low thermal conductivity and flexibility in application, its effectiveness in real-world conditions often diverges from expected theoretical measurements. This discrepancy is largely attributed to workmanship quality during on-site application, which affects insulation uniformity and continuity. This study investigates the relationship between workmanship quality and the resulting thermal transmittance (U-value) of PU foam- insulated roofs across 50 buildings in Bahrain. Workmanship was scored using a structured Workmanship Quality Index (WQI) from local engineering office, while thermal performance was evaluated both theoretically and empirically through site testing. Using regression models and statistical classification analysis, findings indicates that about 91% of the overall variance in U- Values may be attributed to variations in craftsmanship quality. And the results indicate that more than 90% of thermal performance is in inverse correlation to workmanship quality. Recommendations emphasize that superior workmanship significantly improves insulation performance. The findings advocate improved site practices and inspection protocols to bridge the gap between design performance and in-use thermal efficiency. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2026.