The hygrothermal performance of various lightweight sloped roof designs was monitored in the KULeuven VLIET testbuilding for 2 years (Janssens et al. 1998). One of the aims of the investigation was to evaluate whether the design U-value of 0.18 W/m²K was effectively met in practice. For this reason 3 heatflux transducers were installed at the inside surface of each component together with a number of thermocouples at the surfaces of the composing material layers. The U-value was derived from the measurements by averageing the collected data on a daily basis. The observed deviations between the measured and design U-values are analysed and explained by quantifying the inconsistencies between the assumptions for the calculation of the design thermal performance and the conditions of the in-situ measurement. The following causes of deviations are analysed in detail, and related to the position of the heat flux transducer on the test components: (1) heat transport affected by wind-washing in the roofs, (2) 2-dimensional heat flow through framing elements in the roof
