FAQ:General:WaterContentofanAirLayer

(6): Water Content of an Air Layer

I did a WUFI calculation with an assembly that includes an air layer. However, I get completely unrealistic water contents for the air layer. What went wrong?

WUFI was developed to simulate the hygrothermal processes in porous building materials. The detailed simulation of heat and moisture transport in air layers (including convection, turbulence etc.) is much more complicated and is outside WUFI's scope. Furthermore, it does not make much sense to try and implement these inherently two- or three-dimensional processes in a one-dimensional simulation program.

Air layers can therefore only approximately be simulated by treating them as a 'porous' material. It is possible to allow for the amplifying effect of convection on heat and moisture transport by employing appropriate effective heat conductivities and vapor diffusion resistance factors.

However, the moisture storage function of an air layer can only very crudely be approximated by the moisture storage function of a porous material. The latter is largely temperature-independent (and implemented as such in WUFI), so that the functional dependence of the moisture content in air on the relative humidity and temperature cannot be reproduced.
Furthermore, the default moisture storage function used by WUFI for materials for which the user has not defined one assumes that capillary condensation will occur in the material already at relative humidities less than 100%, which is not true for an air layer (it has been modeled after the moisture contents of dense mineral wool).

As a result you will get unrealistically large moisture contents for air layers. Note, however, that WUFI uses the relative humidity as the driving potential for moisture transport and computes the water content as a secondary quantity from the resulting relative humidity (using the moisture storage function of the respective material).
So the resulting distribution of relative humidity should in general be quite realistic, its temporal behavior will just be damped much more than in reality (the moisture content acts as a 'capacity term' for moisture transport in the same way the heat capacity acts as a capacity term for heat transport). If short-term fluctuations don't play a major role, the general trend in the behavior of the relative humidity should be tolerably realistic.
This also means that quantities that depend on the relative humidity in or near the air layer (e.g. mould growth rates) can be evaluated more realistically than quantities that primarily depend on the moisture content (e.g. heat conductivity, heat capacity).

Please note that the unrealistically large moisture capacity of an air layer may also affect other layers. If you are interested in the moisture distribution in an assembly that contains an air layer, the air may (or may not) take up more moisture than realistic, so that less moisture remains for distribution among the other layers.

You may mitigate these problems by explicitly defining a slightly more realistic moisture storage function for the air layer. To this end, use a linear function like

phi:	w:
0	0
1	wf

with 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 avoids the spurious capillary condensation.

Also see the next question for a related problem.

(*) 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.