This paper shows that the theory describing gas diffusion with no consideration of the total gas pressure leads to an underestimation of the water vapor diffusive permeability. This experimental method, called cup method, can be considered as one of the most popular tests in the domain of heat and mass transfer for building applications. Thus, it is likely that the water vapor permeability defined for most current building materials is underestimated. Thanks to the approach proposed in this paper, provided that there is an advective gas permeability, it is possible to know whether a given water vapor permeability has been underestimated or not. However, it is not possible to estimate the magnitude of this measurement error. In the last part, a new experimental procedure is proposed, integrating a total gas pressure measurement on both sides of the sample. This modified test method makes it possible to simultaneously determine the advective gas permeability and the diffusive vapor transfer coefficient.