What is the right choice for the rain absorption factor for an unrendered natural sandstone wall? When I use the value of 0.7 suggested by WUFI, then the entire wall gets wet like a sponge. When I reduce the absorption factor to 0.5, the same happens, it just takes longer. What's wrong?
This should not happen, but the rain absorption factor is very likely not to blame. It does not depend on the material of the wall (it depends a bit on its surface structure and, of course, on its tilt). After all, it simply expresses the fact that some of the rain water splashes off when it hits the wall surface and is no longer available for absorption.
Are you sure that the amount of rain is okay? Maybe you created your own *.KLI file and used normal rain instead of the correct driving rain?
Several kinds of sandstone have a very high water absorption (e.g. Rüthener) and may accumulate an inacceptable amount of moisture when exposed to a wet climate such as the Holzkirchen weather. Maybe you used one of those?