The accuracy of the thermal transmittance, or U-factor, of a fenestration system measured in a building assembly thermal test facility depends upon the test apparatus, test conditions, operating procedure, and the specimen properties. By conducting the uncertainty analysis of a specific thermal test facility, the measurement uncertainty can be identified and quantified. The estimation procedure for the uncertainty associated with the U-factor result for a test apparatus should be established and reported with the measurement of the U-factors of fenestration products. In a study on the University of Massachusetts Research Calibrated Hot Box, detailed analyses of the elemental uncertainties associated with the basic measurements and uncertainty propagation on U-factor results have been performed. In the analysis of uncertainty ropagation, the Root Sum Square (RSS) method was used extensively. The procedure for uncertainty analysis used for this test apparatus can be used in any ASTM C 1363 hot box thermal test facility including those also using ASTM C 1199 for fenestration U- factor measurements. Using the uncertainty analysis procedure described in detail in this paper, a plastic-faced Calibration Standard Transfer (CTS) is analyzed. The results show a reasonable uncertainty level for the U-factor measurement made using ASTM C 1199.The improvement of the U-factor uncertainty can be achieved by reducing the uncertainties associated with the test specimen flanking heat transfer and the metering chamber extraneous heat transfer in the hot box test apparatus.