Due to solar radiation and night time sky radiation flat roofs with dark roofing membranes experience higher thermal loads than other building components. Therefore this construction type was chosen to evaluate extreme hygrothermal conditions in mineral fibre insulations. To increase also the moisture content two litres of water per m² were inserted to a test roof on the field test site of the Fraunhofer IBP in Holzkirchen before closing the construction. The measured conditions in the roof serve to validate hygrothermal simulations and allow in a second step to “transfer” the test roof to other locations in Northern and Southern Europe. As result the maximum temperature and moisture conditions and their coincidence occuring in an insulation layer of a building envelope in Europe can be determined. These results are compared to the climate conditions of current test procedures to determine the durability of mineral fibre insulation materials. The rather extreme temperatures and humidities (above 60 or 70 °C and 95 to 100 % RH) applied during these tests lead to a significant reduction of the tensile and compressive strength of glass fibre materials which cannot be observed in real life. Neither in the insulation layer of an ETICS - which was subject of earlier investigations - nor in the flat roof insulation layer such combinations of high temperature and RH were detected. A test procedure shall accelerate the normal degradation process but not make fail a solution which performs well in reality. Therefore a new test procedure is proposed which is closer to the real maximum conditions and should therefore lead to a more realistic performance assessment of the insulation material.