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The thermal resistance (R-value) of airspaces depends on the emittance of surfaces around the airspace, dimensions, heat-flow direction, and the temperatures of bounding surfaces. Assessing the energy performance of building envelope components and fenestration systems requires accurate results for the R-values of any enclosed spaces. The evaluation of reflective insulation R-values has evolved to include use of computational fluid dynamics and surface-to-surface radiation to quantify convective and radiation contributions to the heat transfer across airspaces of all types. This paper compares an advanced and validated computational tool for calculating enclosed airspace R-values with the widely-used ISO 6946 and airspace R-values in the ASHRAE Handbook-Fundamentals. The tool evaluates construction defects, air-infiltration impact, and dimensional aspect ratios that 1-D methods do not address. The differences between the methods that are currently being used to evaluate the R-value and the advantages of the advanced method for evaluation of reflective insulation applications are discussed.