The extended cup method: Cross-effects of water vapor diffusivity and gas permeability in porous media

The cup method is a standard method for measuring the mass diffusivity of building materials, a crucial parameter for assessing the energy performance of buildings. However, during this test, the respective gradients of air and vapor concentration give rise to a cross flux of these two species by binary diffusion. A gradient of the total gaseous pressure therefore exists to ensure, via a Darcy’s regime, a total flux of air equal to zero. The extended cup method consists in measuring this pressure gap during the cup method. This paper proposes a large set of challenging experiments performed with an original experimental device. The results are analyzed using both a rigorous computational solution and an approached analytical solution. The theory was first confirmed by measurements performed with light concrete at various temperatures. Then, the results obtained for various porous media were plotted on a synthetic diagram, which allowed the following conclusions to be drawn: • By choosing suitable operating conditions using the synthetic diagram, the extended cup method allows a rigorous determination of both the mass diffusivity and the permeability in the case of an inert solid phase, • In the case of bound water migration, the extended cup method combined with conventional permeability measurement can quantify the respective contribution of vapor and bound water diffusion, • As practical outcome, the correction due to the pressure gradient can be discarded for routine measurement of mass diffusivity. • The combined effects of Fick’s and Darcy’s laws were checked for light concrete. • The extended cup method can determine both mass diffusivity and permeability. • A chart is proposed to use the extended cup method for routine measurements. • The pressure gap has negligible effect for routine mass diffusivity characterization. • The respective roles of water vapor and bound water diffusion can be quantified.