A complete calibration on any Guarded or Calibrated Hot Box is critical for improving the accuracy of U-factor measurements of fenestration products. Any heat transfer that does not occur directly through the test specimen should be determined from the results of detailed calibration experiments. In addition, appropriate test conditions should be verified using national laboratory traceable calibration standards. In this paper, complete sets of calibration procedures using the University of Massachusetts Research Calibrated Hot Box under both the ASTM and ISO test methods are presented. Both the ASTM and ISO procedures include calibrations of the metering chamber extraneous heat transfer and the test specimen flanking heat transfer. The metering chamber extraneous heat transfer includes the metering chamber wall heat transfer, an insulated surround panel flanking heat transfer and other heat losses such as those through gaskets, mechanical fasteners, and less obvious heat transfer paths through the metering chamber wall or surround panel edges. The calibration procedure was conducted under ASTM and ISO test conditions. The calibrated heat transfer rates were characterized by the resulting surface temperature difference of the surround panel and the metering chamber the chamber wall thermopile voltage. In addition, the ASTM calibration procedures included the surface heat transfer coefficients calibrated using a plastic-faced Calibration Transfer Standard (CTS), and the ISO calibration procedures included the total surface thermal resistance calibrated using two Calibration Panels (CP).