Existing buildings in Germany have a total of over 300 million windows that are outdated in terms of energy conservation. However, the question as to whether window replacement is worthwhile needs to be carefully ascertained for each, individual case. This is not as easy as it sounds. The Ug-value can be determined with a certain degree of precision on the basis of the windowpane’s structure, but only in the case of older, air-filled window glazing without a low-E coating. In the case of glazing with a noble gas filling, or with low-E coatings, such an estimate is questionable, as the actual degree to which the glazing is filled with gas and the exact emissivity of the low-E layer are usually unknown quantities.