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Condensation
In buildings, condensation can occur when water vapour, usually
produced by the occupants and their activities, condenses on exposed
building surfaces (surface condensation), or within building elements
(interstitial condensation).
Condensation can be categorised into three types:
• Inconsequential condensation
– condensation that is not harmful;
• Nuisance condensation
– surface condensation that is not harmful, and;
• Harmful condensation
– interstitial or surface condensation that is likely to cause damage
to building fabric, degrade its thermal or structural performance or
support mould growth.
Condensation
will occur when the temperature of any part of a wall is lower than the
dew point temperature at that point. The amount of moisture vapour
that the air can hold is a function of the temperature, with warmer air
able to hold more. The point at which the air becomes saturated (can
not hold anymore moisture) is called the dew point, Td .
By ensuring that the temperature of the wall is high enough and/or the
moisture content of the air is low enough the risk of both surface and
interstitial condensation can be reduced significantly.
Assessing condensation risk
Standards BS 5250 and BS EN ISO 13788 specified the method of
assessing the risk of condensation in buildings. These Standards
only deal with basic one-dimensional problems. For more complex
structures, different methods are needed. These take the form of
computer simulations that will more accurately model the flow of moisture
and characteristics of the materials involved.
Design rules for avoiding condensation
There are several design rules to avoid condensation risk.
Firstly the main insulation layer should be placed as near to the cold
side as possible. This ensures that the temperature throughout the
structure is as high as possible which reduces the risk of the temperature
at any part being below the dew point temperature.
Secondly there should be one vapour barrier which shall be placed
as close to the warm side as possible. For UK conditions the air
in the internal environment contains more moisture than the external
air. This means that there will be a moisture gradient with moisture
trying to flow from the inside to the outside. By having a vapour
control layer near the internal surface, the amount of moisture in the
wall is reduced and therefore reduces the risk of any part of the wall
being below the dew point temperature. Typical vapour control layers
include a dense plaster finish to a blockwork wall or foil backed plasterboard
as a dry lining behind a blockwork wall. See CWCT Technical Note 33
for further details.
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