Quelltext der Seite FAQ:General:100RHattheFacade

= (17): 100 % RH at the Facade =

<B>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?</B>

<P>
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).<BR>
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.
</P>
<P>
You'll have
[[Details:MoistureStorageFunction | free saturation]] (i.e. 100% RH) at the
surface when enough rain is absorbed, but this is not due to dew conditions.
</P>
<P>
The surface temperature will fall below air temperature when the wall
[[Details:LongWave | 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.<BR>
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.
</P>
<P>
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.<BR>
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.
</P>