A new draft standard (ISO/DIS 12567) for measuring the coefficient of thermal conductivity of windows (the U value) has been under discussion for several years, and is now approaching the stage of introduction and application. It will be voted on by ISO and CEN in parallel. Comparison tests between the results obtained by different hotbox arrangements can be arranged in order to obtain practical experience. The new method will include procedures for the following (and other) aspects: " the production of calibration panels " calibration of the hotbox " determination of surface thermal resistance in the hotbox " determination of the thermal resistance of windows. A round robin calibration within the Nordic countries has therefore been arranged, in order to obtain experience and knowledge of the various elements of ISO/DIS 12567. Using common instructions, the laboratories have each manufactured and determined the thermal resistance of two calibration panels, after which the various hotboxes have been calibrated. Two different types of windows were used as the test items: a basic double glazed wooden casement window with a U value of about 2.5 W/(m²K), and a linked casement triple glazed window having two low emission coated surfaces, with a U value of about 1.2 W/(m²K). All the participating laboratories were sent one window of each type. Before the windows were sent out, SP measured the U values of all of them, from which it could be ascertained that the differences between nominally identical windows were very small. The laboratories reported that they measured U values in the range 2.4 2.7 W/(m²K) for the Type A windows, and 1.1 1.2 W/(m²K) for the Type B windows. This indicates that the measured values are respectively 2.55 ± 5 % and 1.15 ± 5 %.