ASTM C 1199-91 requires that the measured air-to-air U-factor, US, is standardized to the U-factor that the test specimen would have if it was tested with standardized surface conductance coefficients on both sides. This produces the standardized thermal transmittance, UST, which is typically a smaller value than the thermal transmittance (the air-to-air U-factor), US. ASTM C 1199-91 currently allows for two methods of determining standardized thermal transmittance, and this paper proposes a variation of one of those methods. In addition this paper compares the results from all three methods of standardization using actual test results from five NFRC accredited testing laboratories. By standardizing the measured thermal transmittance, the results from different laboratories that may have different wind machines and thermal chamber configurations can be directly compared. In addition, the test results can be directly compared with computer simulation results that have also been determined using standardized surface heat transfer coefficients. This latter point is critical to the National Fenestration Rating Council’s certified product validation program. This paper recommends that the proposed method of determining the standardized thermal transmittance replace one of the existing methods, and that there be a criteria established that clearly identifies which of the two methods of calculating the standardized thermal transmittance is to be used when testing different fenestration products.