condensation - the basics.pdf

8/16/2019 Condensation - The Basics.pdf

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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 ofdouble glazing, draught exclusion which basically cut down the natural ventilation of theproperty. 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 surfacecondensation.

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 ofwater contributed to the internal environment of a property from dampness in walls, floors,

etc, is considered to be negligible

Moisture and relative humidityBefore looking at condensation it is necessary to understand a lit tle about water vapour inthe air.

At any given temperature the air can hold a given level 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 asthe RELATIVE HUMIDITY (rh) and is expressed as a percentage.

Relative humidity can therefore be defined one way as the actual amount of water vapour inthe 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 airis 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 be7/14 x 100 = 50% at that temperature.


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Condensat ion 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 relativehumidity i ncreases.

The figure to the right illustrateswhy this is so Imagine the air as

a 'bucket' holding a proportion ofwater. As the air is cooled the

'bucket' get smaller and thereforethe proportion of water increaseswith 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 iscooled any further the 'bucket'

will become even smaller and thewater overflow. In reality this

occurs where the air temperaturehas cooled so much that it can no longer hold the water as vapour. When this happensliquid water drops out of the air as CONDENSATION. The temperature at whichcondensation begins, i.e., when the relative humidity reaches 100% (air is fully saturated) isthe DEW POINT temperature.

Surface cond ensation

The cause of surface condensation is wheremoisture laden air comes into contact with asuitably cold surface - any surface including walls,

floors, sub-floor areas, roof spaces, etc

As moisture-laden air gets close to the coldsurface it starts to get cooled and so therelative humidity increases; the greater it is

cooled the higher the relative humidity

(remember water from a large bucketpassing to a small bucket as explainedabove). Against the cold surface thetemperature of the air now drops below the

dew point temperature and liquid waterdrops 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.


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Certain other activities such as using bottled gas and paraffin heaters add significantamounts 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 yourlungs 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, itis reported that a large dog can give off even more water vapour than the average adult!

Water vapour source

(average house/day)

Approx watergenerated

(in litres)

4/5 people asleep: 1.5

2 people active: 1.6

Cooking: 2.6

Washing up: 1.0Washing clothes: 4.0

Drying clothes: 4.5

Bathing/washing: 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 producingactivities. 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 and20ºC 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, andindividual often produces 10 litres of water per day just form simple occupational activities.

Furthermore, it becomes quite evident that given the rate of drying of a wall (1 month forevery 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 buildingthat externally. In an unoccupied property external water vapour will balance with internalwater vapour, but as soon as the building becomes occupied water vapour is generatedinternally 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 internallythan 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 producedby the occupants activities, coupled with insufficient ventilation. Occasionally one can fi nd 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.


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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 themusty 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 substrateand conditions. For example, some moulds will

readily colonise leather at relative humiditiesmaintained around 76% whilst on brick and paint

relative humidities in excess of 88% are reportedrequired. Green and yellow moulds may bepresent; some white moulds are occasionally

mistaken for efflorescent salts.

It should be appreciated that some black mouldsmay be one of the 'toxic moulds', the most well

know being Stachybotrys chartarum. Thisparticular mould is black and slimy; it also requires

a cellulose based substrate, i.e., paper and cardboard. So care may need to be taken wheninvestigating the nature of mould growth.

It is the mould growth that tends to cause the most concern because not only do theyproduce 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 atendency to develop in those areas where airflow is limited and the air remains damp andstagnant, e.g., corners, floor/wall junctions,

etc, where we can frequently see 'triangular'patterns of moulds very typical of acondensation problem (photo above).

Don't expect to maintain relative humiditiesless than 75% during periods the summer;moisture contents of the external air are such

that relative humidities internally in excess ofthis will naturally occur.

Beware of relative humidity figures alone

without knowing the temperature! It can lead tomisdiagnosis! For example in a recent casethe air was reported to be at 65% relative

humidity. The surface of the solid floor waspronounced 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


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dry (no capillary moisture) and, as one would expect, several degrees cooler than theambient 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 temperaturefalls!

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 airtemperature, and the coldness will increase the relative humidity with the same amount ofwater 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 havebeen 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 tooksome considerable time. Thus, taking the instrument out of a cold car and using itimmediately 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 degreechange in temperature. For example coming from a cold car, say 10ºC into a room ataround 20ºC will take 10 + (10 x 3) = 40 minutes before one should contemplate recordingdata.

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