The housing industry is considered one of the largest energy consumer sectors in the world. Residential building in regions with harsh climatic conditions use large amounts of energy for heating and cooling. Therefore, there is a critical need for energy-efficient buildings that minimize energy consumption and optimize the performance of individual systems and components of the building. To achieve energy efficiency in residential buildings, several methods are available; notable among them is the use of reflective insulation materials (Radiant Barrier). The application of the radiant barrier insulation system in the attic of buildings as a means for building energy conservation has significantly increased. However, quantifying the benefits of radiant barrier system is complex because the energy savings provided by this system depend on various factors including local climate conditions, building geometry, and other building operation parameters. Therefore, the objective of this study is to develop a simple estimating tool that may be used by homeowners, state agencies, contractors, and designers to assess the effectiveness and economic benefits of radiant barrier insulation systems in the southern region of the United States. The developed tool is transient Three-Dimensional (3D) Finite Element (FE) models that were validated based on the results of an experimental field study. The results of 3D finite element models were used to develop a set of regression equations to predict the thermal and energy performances of radiant barriers under a wide range of climate conditions in Southern region of U.S. While the theoretical basis behind this tool is robust and accurate, the developed tool is simple, flexible, and user-friendly to encourage its use among practitioners and homeowners with minimal background about this system and heat transfer mechanisms. It is anticipated that the developed tool will facilitate the integration of energy efficiency in residential design and construction.