The influence of a humidity dependent vapour retarder on the annual moisture distribution in thermally insulated flat roof constructions has been investigated by means of long term in situ measurements and two numerical simulation methods. It is shown that the simple Glaser method is not suitable to predict the moisture distribution in this type of construction under real climatic conditions and that only a transient hygrothermal analysis is able to reproduce the measured temperature and relative humidity. Based on both measurements and transient numerical simulations, the role of the vapour retarders for this type of constructions has been evaluated.