FAQ:General:ConvergenceFailureCausedbyVapor-PermeableLayer

(7): Convergence Failure Caused by Vapor-Permeable Layer

I tried to perform a WUFI simulation, but the water balance never adds up, regardless whether I make the grid as fine as possible or whether I choose stricter numerical parameters, as suggested in the on-line help. What can I do?

One situation where serious convergence failures tend to occur is a component with a vapor-permeable layer (e.g. air or mineral wool) which has accumulated a lot of moisture (RH ~ 100%) and which is now exposed to a high temperature gradient (e.g. caused by intense solar radiation). WUFI originally wasn't developed to treat these cases which sometimes prove too demanding for the numerics that are mainly tuned to massive porous materials.

If everything else fails, you may try an alternative moisture storage function. In the material database, the moisture storage functions for materials like air or mineral wool are left undefined, so that WUFI uses an internally defined default moisture storage function (see the preceding two questions).

This moisture storage function assumes that for RHs above ca. 50% capillary condensation occurs which leads to increasingly higher moisture contents until free saturation is reached at 100% RH. This is not really realistic for air layers or hydrophobic mineral wool (it may be more appropriate for non-hydrophobic mineral wool).
Since it seems that the problem is mainly caused by the high water content, reduction of the water content by choosing a different moisture storage function often remedies the problem.
Please note that the relative humidity in the material will remain largely unaffected by the specific choice of the moisture storage function, as explained above. So if you are interested in the relative humidity in the layer, your results will be affected only slightly (but please perform a few test calculations with different choices of the moisture storage function to be sure), and if you are interested in the moisture content, you should not rely on the default moisture storage function anyway, but use measured data instead which represent your particular material.

A possible choice for the moisture storage function in these cases is a table like this:

phi:	w:
0	0
1	wf

Use a low value for wf (the numerics may not be able to cope with very low values, you'll need to experiment a bit.) (*).
This linear function is even more realistic than the default function in that it avoids the capillary condensation for RH= 50..100%. The moisture content remains low up to RH=100% (as it should be in air or in hydrophobic insulation materials), and at or above 100% condensation may occur and increase the moisture content beyond wf and up to wmax.

In particular if you are interested in moisture accumulation by condensation in these materials, use such a linear moisture storage function with low wf. Then you know that any moisture content exceeding wf must have been caused by condensation. You can then analyse this excess over wf (test calculations show that this excess is only slightly dependent on the specific choice of wf).

(*) Note, however, that the porosity and thus wmax should remain high. If the water content exceeds wf, WUFI reduces the vapor permeability, in proportion to the excess, to reflect the fact that the pore volume gets increasingly filled with water and thus vapor transport decreases. At w=wmax the permeability reaches zero (all pores are filled). For vapor-permeable materials like air layers or mineral wool where moisture transport occurs mainly via vapor transport, wmax should therefore remain at a realistic value.