Environmental Engineering Reference
In-Depth Information
- water intake from driving rain is limited by the cumulative amount of each rain
event and by the local
interception
and
absorption
conditions on the surface
considered;
- depending on the orientation of the stone that receives water and its water-
related properties, gravitation can play a positive, negative or negligible role in the
resulting infiltration phenomenon.
Rain can also have indirect effects: if the amounts of driving rain exceed the
infiltration capacity of a surface,
run-off
appears that can supply water to other,
unexposed parts of a building. Run-off can also originate from defects in the
structure (roof or gutter leaks, for instance) or from human influences (splashes
from traffic, uncontrolled disposal, etc).
9.1.1.3
. Condensation
Condensation
is the phase change of water from vapor dispersed into the air to a
liquid state. In building physics, and concerning cultural heritage in particular,
generally speaking condensation is often taken to be an exclusively thermal effect: if
a wall surface becomes colder than the surrounding humid air −
dew-point
temperature
that depends on its
relative humidity
and temperature
2
− condensed
water forms as small droplets on its outer surface. Of course, this is true and must be
taken into account when choosing heating or air conditioning strategies for
monuments and to determine what quantity of visitors is acceptable from that point
of view. But it should also be pointed out that condensation is not just a surface
phenomenon: it can also appear inside the masonry. Moreover, the porous
characteristics of the materials are also decisive. This last issue will be examined in
section 9.2.3.
9.1.2.
Drying
Fortunately, the mechanisms of water intake described above are often
reversible:
- Under the combined effects of wind and sun outside - or ventilation (natural
or artificial) and heating inside - water contained in the masonry can evaporate.
This
evaporation
phenomenon creates a
drying front
inside the materials that goes
from the surface to deeper layers.
2 For more details on humid air thermodynamics, the reader can refer to Chapter 2 in
[CAM 98]
