This International Standard specifies a measuring method used to determine the U value (thermal transmittance) of multiple glazing with flat and parallel surfaces. Structured surfaces, e.g. patterned glass, may be considered to be flat. This International Standard applies to multiple glazing with outer panes which are not transparent to far-infrared radiation which is case for normal glass. However, internal elements may be far-infrared transparent. This International Standard allows the U value in the central area of the multiple glazing to be determined, Edge effects, due to the thermal bridge through the spacer of a sealed glazing unit or through the frame are not included. Furthermore, energy transfer due to solar radiation is not taken into account. The determination of the U value is performed for conditions which will correspond to the average situation for glazing in practice. In this way a fair comparison between different products is possible.