Measuring determination of U-values of external walls in comparison with typical catalogue values for existing buildings

The economic benefit of retrofit actions for old buildings depends on possibly increasing building value and rents and also on the achieved energy savings. To estimate the savings, usually the calculated energy demand before and after the renovation is compared. Several studies show, that calculated energy demand and consumed energy often show great differences, especially for old buildings before the 1970s. One important input parameter for the energy demand calculation is the u-value of the facade which is commonly chosen by the energy expert on site, out of a catalogue, where typical values for certain wall constructions are presented. In this publication, results of heat flow measurements and dynamic simulations of different typical wall constructions of old buildings are presented. The measured and simulated u-values for those facades were between 0.9 and 1.2 W/(m(2)K) and significantly lower, than the values in the Luxembourgish catalogue, which range from 1.4 to 2.1 W/(m(2)K), depending on the construction material. The deviations between calculated building energy demand and actual consumed energy can at least partly be explained by false assumptions of energy experts on site, who use the u-value catalogue for choosing the input parameters of their calculation.