How can I simulate a wall whose exterior surface has been treated with a water-repellent agent? Is it correct to set the rain water absorption factor to zero? Do I need to change the sd-value of the exterior surface, even though I use a diffusion-permeable treatment?
The rain water absorption factor must be set to zero if the water absorption is indeed completely stopped by the treatment. If water absorption is only reduced, you must determine the water absorption coefficient for the treated material and replace the part of the wall which corresponds to the penetration depth of the treatment with a layer of the treated material.
If the treatment does not change the diffusion permeability of the material, no sd-value needs to be specified for the exterior surface.
Many treatments do, however, increase the diffusion resistance factor (µ-value) of the material. In these cases, this additional resistance should be allowed for by an appropriate sd-value. Alternatively, and even better, you can replace the part of the wall which corresponds to the penetration depth of the treatment with a new layer that has the same material properties but an appropriately increased µ-value.
Even if the water absorption is negligible (so that adjusting the rain absorption factor instead of the liquid transport coefficients would be sufficient) and vapor diffusion is not hindered by the treatment (so that no µ-value needs to be adjusted), it might nevertheless be preferable to model the treated part of the wall by defining a separate layer whose liquid transport coefficients have been reduced or even set to zero.
This is because the capillary conduction in this layer does not only determine the amount of absorbed rain water; it also influences the wall's drying behavior.
Drying-out proceeds faster if water from the interior of the wall can be conducted to the surface by capillary transport and can evaporate from there. Drying-out is impeded, however, if capillary transport stops a few centimeters behind the surface and moisture can only dry out after crossing this treated layer by vapor diffusion. So this is another mechanism by which water-repellent treatment may reduce the drying potential of a wall, in addition to a possibly increased µ-value.