Condensation and mould that results
from high humidity has become one of the most
common causes of dampness within buildings. A
Report by the Building Research Establishment
estimated that one and a half million homes in
the UK are badly affected by condensation.
Condensation occurs when warm
moist air is cooled that it no longer has the
capacity to hold so much water in vapour form.
Typically this can be seen to occur at cold surfaces
such as windowpanes as mist or cold water pipes
as droplets of water.
A brief summary of the terminology
The amount of water (or moisture),
which can be held by air, (the humidity )
depends on the air temperature. The warmer the
air the more water (or moisture) it can hold.
Cold air at 0 C is unable to hold any moisture.
The temperature at which the air cannot hold any
more moisture is described as the dew point. The
dew point is the point at which condensation occurs
(water or moisture that was in the air has condensed
to liquid on a surface that was
at or below dew point) As the air temperature
increases the dew point temperature will be determined
by the amount of moisture in the air. The relative
humidity is the amount of moisture in the
air relative to how much moisture the air can
hold at that temperature. So, at any given temperature,
increasing the moisture in the air will increase
the dew point temperature. As the humidity increases
the relative humidity increases and the risk increases
that relative humidity will reach 100% Dew point
temperature occurs when relative humidity reaches
100% (and vice-versa). The inter-related nature
of temperature humidity relative humidity and
dew point can be seen from examination of a “psychrometric
Some points to
The amount of moisture within
the air in a building varies continuously. Certain
lifestyle activities generate moisture and these
will increase the amount of moisture in the air.
Examples of these are cooking, drying clothes,
bathing and even breathing. Such activities need
not always produce a condensation problem. The
moisture can be dispersed out through
an open window or extractor fan, or it may condense
on a nearby windowpane and be easily wiped away.
It is to where the moist air
goes and the temperature of the places it goes
that will determine whether a condensation
problem is produced.
When the air is heated throughout
a property the air is capable of holding the moisture
as vapour to the extent that can be seen from
the psychrometric chart. Most properties are not
heated to the same temperature all the way through
all the time. For example it is not unusual to
heat a living room to a higher temperature than
a bedroom. This means that the
air in the living room can hold more water vapour
than the air in the bedroom.
Typically moisture generated
in the kitchen and bathroom might be allowed to
escape through the whole house. This
might typically result in some local visible condensation
in the kitchen and bathroom, no signs of a problem
in the living room, but some damp patches and
mould growth on the external wall of a back bedroom.
Unless a study is made of the
varying temperatures and humidities within a property
it can be difficult to predict whether the property
is likely to be affected by a condensation problem.
The factors that cause the problem, vary all the
is certainly true that the tendency in recent
years to draughtproof houses and insulate to conserve
energy has reduced
the amount of air (and therefore airborne moisture)
escaping to the outside. This has had the effect
of increasing the overall humidity levels.
The reduction in ventilation
and increase in humidity can also encourage the
growth of mould fungi. These are often the first
visible sign that a problem is present. Relative
humidity does not require to be as high as 100%
for mould growth to occur. Mould growth can occur
when the relative humidity is around 70%. This
often happens in a part of a property that
is less well heated and where the ventilation
is reduced. For example: a cupboard or wardrobe
on the external wall of a back bedroom. Although
there may not be visible wetness due to condensation
The temperature in this location may not be as
consistently high as that throughout the house,
and as a result the relative humidity can be locally
increased to above 70% at certain times of the
day. That can be sufficient to support mould growth.
It is fairly easy to spot a
condensation problem when there is visible evidence
of moisture on a window or cold water pipe. Fungal
growth (usually moulds) can occur before the problem
is visible in that way.
The conditions, which produce
condensation and related problems of mould growth
depend also to a great extent on the temperature
and humidity of the air outside the building.
It is not uncommon therefore for a house to be
affected by condensation or mould growth only
at certain times of the year. In such cases it
would be unlikely that altering the ventilation
alone would be sufficient to reduce or eliminate
A successful strategy for combating
condensation will usually depend on a number of
factors. Since these will be most likely to produce
a successful result.
New digital technology has facilitated
studying how and when condensation problems are
Where condensation is suspected as a source of
dampness it is useful to monitor internal climate
conditions. This is best done over a continuous
period, and should be compared to external conditions,
in order that a comparison of vapour pressure
can be made. The methods are described in the
British Standard for Condensation (BS5250, 2011)
Dataloggers provide a means of monitoring atmospheric
temperature, dewpoint and moisture conditions
continuously. This provides useful information
about how temperature and relative humidity change
on an hourly basis. By analysing the data and
calculating vapour pressure, a comparison can
be made between different locations at different
temperatures. That comparison can be used to determine
the source of condensation within a building.
Vapour pressure provides a measure of atmospheric
moisture that is independent of temperature. Vapour
pressure can be calculated from relative humidity
and temperature measurements. Vapour pressure
comparison is the method described in the British
Standard (BS5250) for condensation.
Vapour pressure is that proportion of normal
air pressure that is made up of water in vapour
form. This is typically around 1% of atmospheric
pressure, but varies continuously. The moisture
within the air within a building is made up of
the moisture in the air outside plus that which
is generated within the building by normal living
activity, such as cooking washing and bathing.
Vapour pressure provides a useful means of comparing
airborne humidity between different locations
within a building, and of determining the direction
of moisture movement. The direction of moisture
migration is from areas of higher vapour pressure
to lower until equilibrium is reached.
A way of determining the source of condensation
within a property is to track vapour pressure
migration because vapour pressure will generally
migrate from the source location to other parts
of a building.
ELEMENTS NEED TO BE CONSIDERED:
1) Reduce humidity (Extractions
fans in kitchens and bathrooms help to do this)
2) Improve air circulation
3) Improve insulation
4) Improve overall background
There are a number of ways of
dealing with each factor and the most appropriate
will depend on the individual property.
It is generally true that more than one factor
will require modification.
It makes good sense to start
with the simplest and least expensive option which
typically might be the installation of extractor
fans in kitchen and bathroom, the places where
most of the airborne moisture is generated.
A monitoring period should be
allowed after each course of action is taken so
that the contribution of each factor can be evaluated.
Further details and
advice can be found in BS 5250 (2011)
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