This paper presents a comparison between calculation procedures being formulated in the United States, Canada, and Europe for winter or nighttime window Uvalues. The three calculation procedures are those presented in the U.S. computer program WINDOW3 and the Canadian computer program VISION, which are currently undergoing revision, and the computer program ISOWIN04, which is based on the current draft version of a procedure being developed by the International Standards Organization (ISO) working group ISO/TC59-160- 162-163/WG2, Thermal Transmission Properties of Windows.

Comparisons are made between the window input properties, the indoor and outdoor surface and gas space heat transfer coefficients, and the spacer and frame heat transfer coefficients for the three different calculation procedures. Results for the three principal U-values (central part of glazing, central part of glazing with spacer [Insulated Glazing Unit], and overall window [Insulated Glazing Unit with Frame]) as a function of the gas space thickness, glass surfacemissivity, orientation, gas fill, and different frames are compared for the three source programs. In general, the results are in agreement; however, some differences exist and are discussed.

Specific recommendations are made for future work involving the determination of the specific differences between the three procedures.