CONDENSATION - THE BASICS
Condensation is the most common form of dampness in buildings. Indeed, it appears to be
more of a problem in modern properties than our historic buildings due to the introduction of
double glazing, draught exclusion which basically cut down the natural ventilation of the
property. Older properties, say, with sash windows, open fire-places and gaps around the
original doors and windows are far less likely to be severely affected by surface
condensation.
The water in the air which causes surface condensation is basically derived from 'life-style',
mainly cooking, bathing and just general activities and breathing; these, coupled with a lack
of ventilation cause the greatest problems. It must be fully appreciated that the amount of
water contributed to the internal environment of a property from dampness in walls, floors,
etc, is considered to be negligible
Moisture and relative humidity
Before looking at condensation it is necessary to understand a little about water vapour in
the air.
At any given temperature the air can hold a given le vel of water as vapour - the warmer the
air the greater the potential amount of water vapour that can be held. For example:
Air at 10ºC is saturated when it contains 7.6g water per kg dry air
and,
air at 20ºC is saturated when it contains 15.3g water per kg dry air - just over double.
So if we know the maximum amount of water that can be held it is very useful to know how
'saturated' the air actually is, i.e., what is the proportion of actual water vapour compared to
the maximum amount that can be held at a given temperature. This proportion is known as
the RELATIVE HUMIDITY (rh) and is expressed as a percentage.
Relative humidity can therefore be defined one way as the actual amount of water vapour in
the air expressed as a percentage of the maximum amount of water vapour that could be
held at the same temperature.
So air, say, at 10ºC could hold 8 grams of water vapour at its maximum, and if in reality only
4 grams was actually found, then the relative humidity would be 4/8 x 100 = 50% i.e., the air
is 50% saturated. Similarly air at say 20ºC could hold around 14 grams of water vapour at
maximum, but if we found only 7 grams in the air then the relative humidity would also be
7/14 x 100 = 50% at that temperature.
Condensation and dew point:
What happens if we cool moisture at laden air?
We know that cold air cannot hold as much water vapour as warm air, so as the
temperature drops the relative
humidity increases.
The figure to the right illustrates
why this is so Imagine the air as
a 'bucket' holding a proportion of
water. As the air is cooled the
'bucket' get smaller and therefore
the proportion of water increases
with decline in bucket size. If air
is continued to be cooled, the
'bucket' will diminish to a size
where it is now full with water,
i.e., it is 100% full; if the air is
cooled any further the 'bucket'
will become even smaller and the
water overflow. In reality this
occurs where the air temperature
has cooled so much that it can no longer hold the water as vapour. When this happens
liquid water drops out of the air as CONDENSATION. The temperature at which
condensation begins, i.e., when the relative humidity reaches 100% (air is fully saturated) is
the DEW POINT temperature.
Surface condensation
The cause of surface condensation is where
moisture laden air comes into contact with a
suitably cold surface - any surface including walls,
floors, sub-floor areas, roof spaces, etc
As moisture-laden air gets close to the cold
surface it starts to get cooled and so the
relative humidity increases; the greater it is
cooled the higher the relative humidity
(remember water from a large bucket
passing to a small bucket as explained
above). Against the cold surface the
temperature of the air now drops below the
dew point temperature and liquid water
drops out as condensation.
Where does the water come from?
Water comes from the 'life-style' - just
normal everyday living (see table below).
The amount of water produced from normal household activities can be quite considerable.
Certain other activities such as using bottled gas and paraffin heaters add significant
amounts of water to the air, the by-product of burning these fuels. Drying clothes over
radiators will also significantly add water vapour. Also consider that the surface area of your
lungs is in excess of 75 square metres and warm air is passing over this wet surface as we
breathe 15-20 times per minute; this is being breathed back into the environment! Indeed, it
is reported that a large dog can give off even more water vapour than the average adult!
Water vapour source
(average house/day)
4/5 people asleep:
Approx water
generated
(in litres)
1.5
2 people active:
1.6
Cooking:
2.6
Washing up:
1.0
Washing clothes:
4.0
Drying clothes:
Bathing/washing:
4.5
0.5
Approx. total
15.7
Contrary to popular belief, damp walls form rising/penetrating damp, and damp floors do
not add significantly to the water burden in the air because water evaporation from such
'static' surfaces is very low compared to breathing and other active water producing
activities. Indeed, recent figures obtained from Building Research Establishment using a
validated model showed that a "saturated" floor slab of 8sq.m in a room at 60% rh and
20ºC lost around 36mls water per day, ie, 5 tea spoons full! This compares to around the
15 litres of so (nearly 4 gallons) produced from normal household activities. Indeed, and
individual often produces 10 litres of water per day just form simple occupati onal activities.
Furthermore, it becomes quite evident that given the rate of drying of a wall (1 month for
every 25mm in thickness) then water is lost very slowly to the environment and even then
most of the water passes outwards. Why? Water vapour exerts a pressure (it is part of the
atmospheric pressure) and over most of the year there is more water vapour in a building
that externally. In an unoccupied property external water vapour will balance with internal
water vapour, but as soon as the building becomes occupied water vapour is generated
internally and adds to the environmental water burden - the more water vapour, the greater
the vapour pressure. This now means that there is a greater vapour pressure internally
than external and so water vapour now passes down its vapour pressure gradient, ie, from
inside to outside.
Thus, the most likely direct cause of surface condensation is 'life-style', ie, water produced
by the occupants activities, coupled with insufficient ventilation. Occasionally one can find a
'normal' life-style but certain areas of walls or cold spots (e.g., dense concrete lintels) are
sufficient cold to allow condensate and mould growth to form.
Mould growth
Water vapour in the atmosphere alone causes no problems - certainly not health problems.
Indeed, constant inhalation of very dry air can. However, condensation and maintenance of
high humidities does lead to mould growth. This can usually be detected frequently by the
musty odour associated with damp. Where such conditions occur it is mould spores in large
numbers that may cause some to experience health problems.
The most common mould associated with condensation is the 'black spot' mould,
Aspergillus niger. However, other moulds may also develop - it depends on the substrate
and conditions. For example, some moulds will
readily colonise leather at relative humidities
maintained around 76% whilst on brick and paint
relative humidities in excess of 88% are reported
required. Green and yellow moulds may be
present; some white moulds are occasionally
mistaken for efflorescent salts.
It should be appreciated that some black moulds
may be one of the 'toxic moulds', the most well
know being Stachybotrys chartarum. This
particular mould is black and slimy; it also requires
a cellulose based substrate, i.e., paper and cardboard. So care may need to be taken when
investigating the nature of mould growth.
It is the mould growth that tends to cause the most concern because not only do they
produce the musty odour but also cause decorative spoiling, and also spoiling of fabric in
some cases. Moulds, once germinated, require the maintenance of persistently high
relative humidities, usually over 75%, but frequently much higher. Moulds therefore have a
tendency to develop in those areas where air
flow is limited and the air remains damp and
stagnant, e.g., corners, floor/wall junctions,
etc, where we can frequently see 'triangular'
patterns of moulds very typical of a
condensation problem (photo above).
Don't expect to maintain relative humidities
less than 75% during periods the summer;
moisture contents of the external air are such
that relative humidities internally in excess of
this will naturally occur.
Beware of relative humidity figures alone
without knowing the temperature! It can lead to
misdiagnosis! For example in a recent case
the air was reported to be at 65% relative
humidity. The surface of the solid floor was
pronounced to be 85% relative humidity from
which it was stated that the floor was damp,
possibly a damp-proof membrane defect.
However, investigation showed the floor to be
dry (no capillary moisture) and, as one would expect, several degrees cooler than the
ambient air temperature. This would mean that the relative humidity at the floor surface was
higher! Someone hadn't considered that the relative humidity increases as the temperature
falls!
And on the same principal, don't stick a relative humidity probe into a wall as a
measurement of possible dampness - the wall is likely to be colder than the internal air
temperature, and the coldness will increase the relative humidity with the same amount of
water vapour in the air (NB the 'buckets' described above)-- the higher relative humidity
obtained may not reflect 'dampness' in a wall, just the difference in temperature! You have
been warned!
Finally, on the use of electronic hygrometers. Some recent tests showed that for some
electronic hygrometers to come into equilibrium with the surrounding environment took
some considerable time. Thus, taking the instrument out of a cold car and using it
immediately in a property would certainly give VERY misleading results. The instrument
MUST be allowed to come up to room temperature (or down). Some initial tests suggests
that as a rule of thumb you give a minimum of 10 minutes plus 3 minutes for each degree
change in temperature. For example coming from a cold car, say 10ºC into a room at
around 20ºC will take 10 + (10 x 3) = 40 minutes before one should contemplate recording
data.