Reflective insulations are increasingly utilized in building envelope components such as walls, roofs, windows, curtain walls, and skylights. These insulations incorporate single- and multi-airspaces with at least one low-emittance surface, where heat transfer occurs through conduction, convection, and radiation. The thermal performance of airspaces depends on multiple factors, including their dimensions, orientation, surface temperatures, emittances, and the direction of heat flow. Current evaluations, primarily based on data from the U.S. National Bureau of Standards and represented in the ASHRAE Handbook-Fundamentals and ISO-6946, are limited in scope and do not account for all critical parameters affecting thermal performance. Advanced and extensively validated numerical model has been employed to assess the energy performance of building envelope components. This study explores the use of this model in several key areas: the development of the innovative and user-friendly Airspace Reflective Tool for evaluating enclosed airspaces across different building applications; the assessment of energy performance in reflective insulations subjected to air intrusion from infiltration and wind washing, considering different exterior air temperatures; the evaluation of multiple airspace configurations, including the effects of defects and cross airflow; and the analysis of residential attics with radiant barriers. The findings from these case studies provide critical insights into optimizing reflective insulation systems for enhanced energy efficiency in buildings.