The intent of this paper is to describe and compare the two different two-dimensional frame/spacer heat transfer calculation methodologies used in North America (FRAME [EEL. The FRAMEplus Toolkit for Heat Transfer Assessment of Building Components, Version 3.0, Enermodal Engineering, Kichener, Ontario, Canada, 1995], THERM [LBNL. THERM 2: PC Program for Analyzing Two-Dimensional Heat Transfer Through Building Products, LBL-37371, Windows and Delighting Group, Lawrence Berkeley National Laboratory, Berkeley, CA, 1998], ASHRAE SPC 142P [ASHRAE. Standard Method for Determining and Expressing the Heat Transfer and Total Optical Properties of Fenestration Products, Public Review Draft of Standard 142P, American Society of Heating, Refrigerating and Air Conditioning Engineers, Atlanta, 1998]) and in Europe [ISO 10077-2. Thermal Performance of Windows, Doors and Shutters-Calculation of Thermal Transmittance-Part 2: Numerical Method for Frames, International Standards Organization, Geneva, 2003]. The two approaches, called the ASHRAE and ISO methods, are different in the way they treat the effect of the glazing spacer on the heat transfer through the frame and the glazing unit near the frame. The ASHRAE method assumes that the spacer effects both the heat transfer through the frame and the heat transfer through the glazing in an "edge-of glass" region 63.5mm (2.5in.) from the glazing/frame sight line. The ISO method assumes that the additional heat transfer due to the existence of the spacer is proportional to the glazing/frame sightline distance that is also proportional to the total glazing spacer length. An example calculation of the overall heat transfer and thermal transmittance (U-value or U-factor) using the two methods for a thermally broken, aluminum framed slider window is presented. The fenestration thermal transmittance calculations analyses presented in this paper show that small differences exist between the calculated thermal transmittance values produced by the ISO and ASHRAE methods. The results also show that the overall thermal transmittance difference between the two methodologies decreases as the total window area (glazing plus frame) increases. Thus, the resulting difference in thermal transmittance values for the two methods is negligible for larger windows. This paper also shows algebraically that the differences between the ISO and ASHRAE methods turn out to be due to the way the corner regions of the window frame and glazing are treated in the assembly of the overall thermal transmittance for a three-dimensional window from the two-dimensional calculations. Three-dimensional heat transfer calculations can be made and correction factors can be applied to both the ASHRAE and ISO two-dimensional results to bring them into agreement with the three-dimensional results.