During the last few years, many houses in Sweden have had problems with the so-called Sick Building Syndrome (SBS). People become sick because of the indoor climate, with health hazards like asthma, allergies among other problems. These problems can for instance be caused by durability failures in a wall structure. If the mass content in a wall structure is increasing from year to year, due to moisture accumulation in the wall structure or reach too high levels, the building component can cause health hazard for the inhabitants for instance due to mould growth in the building envelope (Hagentoft 1998, Samuelson, 1985). To prevent these problems from occurring it is of great importance to estimate the risks already in the design stage. One way of getting these estimations is by simulations using accurate material properties and accurate in- and outdoor conditions (Harderup 1998). For the engineers a design tool, which easily shows the advantages and disadvantages of a certain structure, can help predicting service life and durability. This paper shows a method to support the prediction of service life and/or to make durability checks on a multi-layered building component. The indoor and outdoor climate, the design of the building structure and the material properties, govern the hygrothermal behaviour of a building component. This study will provide a method of how to show hygrothermal stresses required to determine the durability of building component or/and material layers in the structure.