Moisture dependent vapor retarders (smart vapor retarders) are widely used in building construction especially roofs in order to prevent moisture accumulation or to support drying out processes. The current modelling approaches mainly use only a relative humidity dependent vapor resistance. Recent studies have shown more complex behavior like direction depending flow or vapor pressure gradient depending flow. This paper shows a new procedure for including all these effects into physical model. Furthermore description of necessary measurements with evaluation of results and selection of a proper model with parameter adjustment will be shown based on three different example membranes. Finally the implementation of this model into a hygrothermal simulation code is described.