FAQ:General:100RHattheFacade

(17): 100 % RH at the Facade

OK, this explains why I didn't see dew conditions in the wall. But shouldn't condensation at least happen at the facade on days with high humidity and little sunshine?

The surface of a normal wall in temperate or cool climate regions will always be somewhat warmer than the surrounding air. By day because of solar radiation (even on foggy or overcast days), by night because of heat flow from indoors (exceptions: air-conditioned dwellings or nightly emission, see below).
Since the RH in the air can't be greater than 100% and the RH at the warmer-than-air wall surface will always be less than the RH of the air, you usually can't reach or surpass 100% there.

You'll have free saturation (i.e. 100% RH) at the surface when enough rain is absorbed, but this is not due to dew conditions.

The surface temperature will fall below air temperature when the wall emits more long-wave radiation than it gets back from surrounding surfaces. If it even falls below the dew-point temperature, you will indeed get dew conditions at the surface.
This happens routinely during the night, especially during clear nights, when the long-wave emission of the water vapor in the atmosphere is at a minimum.

In these cases you may get repeated and regular wetting of the surface which may lead to dust accumulation or algae growth, especially with exterior insulations whose surfaces cool down particularly strongly.
Currently, WUFI does not routinely allow for this effect, since the necessary data on atmospheric and terrestrial counterradiation are rarely available. If these data are provided, WUFI can compute nightly emission cooling in principle, but only approximately. Future WUFI versions will have a more sophisticated emission model incorporated.