PASSYS test cells are outdoor test facilities for the evaluation of the thermophysical parameters of real scale samples of facades and building components. The most important application is the testing of passive solar facades. While we have the advantage of measurements under real weather conditions (real solar radiation) there is the disadvantage of non steady state boundary conditions. Various strategies are presented to enhance measurement accuracy. A so-called pseudo adiabatic shell(PAS) was introduced to reduce the heat losses through the test cell envelope and the time constant by using a controlled electric heating foil. A similar approach is described here using heat flux sensitive tiles (HFS). A special heat flux sensor was designed and tested at ITW to meet the requirements of an accurate measurement of the overall heat flux passing the test cell envelope. By the application of a movable cold box in front of the tested facade steady state outside conditions can be provided for the evaluation of the U-value according to the requirements proposed by CEN/TC 89/WG7.