Directed design of a material with exactly specified properties requires a combination of computational and experimental techniques and a continuous interchange of information between material developers and users. This kind of design is usually necessary in multi-layered systems where the required properties of the components of the system may not be exactly known in advance and have to be specified quantitatively before the process of material development begins. In such systems the requirements to different parameters can be contradictory so that their balancing cannot be performed by a simple estimate. The directed design of material properties, which has to be done in such cases, requires a more or less extended set of computer experiments. Therefore, suitable computer simulation tools for modeling the hygrothermal and mechanical performance of the multi-layered systems where the particular materials should be applied are to be employed in order to identify the weak points of the systems in advance and to adjust the necessary material properties in an appropriate way. The possibility of future damage of the multi-layered system can be substantially reduced in this way. In this paper, the application of water vapor retarders in interior thermal insulation systems on hydrophilic mineral wool basis is studied as an alternative to the traditional water vapor barriers. The retarder permits a part of water vapor to diffuse further to the load bearing structure. Then, even if it is placed between the thermal insulation layer and the load bearing structure, the amount of condensed water in the thermal insulation layer is lower. The structure is not damaged in that case because it is exposed to such a water vapor flux only, that can be transported through it without condensation. As the water vapor flux must be adjusted according to the properties of the load bearing structure and the climatic zone, the water and water vapor transport and storage properties of the retarder layer have to be chosen in an appropriate way. This design can be done most effectively by a computational simulation. The TRANSMAT computer simulation tool is used for that purpose in this paper. As a result of computational simulations, the optimized values of moisture diffusivity, water vapor diffusion resistance factor and sorption isotherm are obtained, making possible to choose a proper material for the retarder which is available in current material basis.