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Condensation |
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One of the most common causes of dampness in buildings is condensation which can often lead to the appearance
of mould growth. It is the presence of water condensed on walls, ceilings and other cold surfaces which support
mould growth. The problem of condensation, particularly in homes, is very much a
problem of today and results from a series of relatively simple, totally invariable conditions, and is directly related to
standards and methods of heating, ventilating and insulating buildings.
Air normally contains water vapour in varying quantities and its capacity to do so is related to temperature - warm air holding
more water than cold air. Air is saturated when it cannot contain any more water vapour at the existing temperature; under these
conditions it is said to have a relative humidity (RH) of 100%. If the temperature of the air falls until saturation point occurs
the air is at a critical temperature at which it cannot hold any more water - this temperature is known as the dew point.
Any further fall in temperature will result in water vapour being forced to condense out as liquid water. The amount of water
vapour condensing out will be the equivalent to the amount of vapour excess of 100% RH of the air at its new temperature.
Therefore, when warm air comes into contact with either colder air or a cold surface the warm air is cooled, i.e. depressing
the temperature of the air to a level at which it can no longer contain all the water vapour and some of it is discarded as
condensation or liquid water.
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Condensation in a building usually occurs when warm air comes into contact with a cold surface. The air is cooled
below its saturation point causing its excess water vapour to change into liquid water. The condensed water usually appears as water
droplets or water film on non-absorbent surfaces such as windows or tiles. This form of condensation is SURFACE CONDENSATION.
It is obvious and always occurs on the surfaces which are at or below the dew point of the air immediately adjacent.
Condensation occurs when relatively warm moisture laden air comes into contact with surfaces, at or below its
dew point, which are relatively cold as a result of locally reduced insulation values between the warm air and a
relatively cold area. At the interface between warm moist air and cooler walls and windows, the water vapour will be cooled becoming
liquid again, creating many fine droplets.
Typical examples of this process are condensation at the base of external walls, where it may be confused with
rising damp, condensation on window panes where it often results in accelerated
decay to the lower parts of window frames. Surface condensation can also occur on relatively cold internal walls, furnishings and
fittings.
Uninsulated ceilings are prime locations as warm moist air rises and contacts cold ceiling and wall surfaces – often illustrating
ceiling joist or timber wall stud or batten locations which act like strips of insulation maintaining warm spots.
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As the temperature of moisture-laden air approaches dew point and the relative humidity rises, a number of moisture
associated problems can become apparent, even before condensation occurs. In particular, superficial and interstitial
mould growth can occur, especially on surfaces or in materials contaminated by dust or
other organic materials. This typically occurs in poorly ventilated areas such as behind furniture and pictures, behind the glazing of
pictures, in soft furnishings or carpets, and in poorly ventilated cupboards or corners of rooms, both at ceiling and floor level. This
can cause serious damage to decorative and historically important finishes, as well as representing a significant health hazard, especially
to sensitive individuals. Interstitial mould growth in contaminated carpets, soft furnishings or insulation materials
due to this raised moisture content and inadequate heating and ventilation can be an extreme health hazard, and appropriate respiratory
protection should be used in affected areas.
Interstitial Condensation
Condensation can also occur within the fabric of the building due to the internal air permeating through the structure because of its
greater pressure. Water vapour in the air exerts a pressure which contributes to the total pressure of the air.
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The more moisture present in the air the greater the contribution of water vapour to the total pressure of the air referred to as vapour
pressure. Air inside a heated building usually contains more moisture than does the external air. This means it is at a higher pressure
which tends to force the warm air through the structure taking the moisture with it. Most building materials, except metals, plastics
and certain lined elements, are to some extent permeable and do not obstruct the movement of moist air through the structure. The warm
moist air will eventually cool below its dew point within the fabric of the building resulting in condensation. This form of condensation
is interstitial condensation.
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Interstitial Condensation is rather more complex than the surface condensation and presents a greater hazard because
the resulting high moisture content can often go undetected for long periods until serious structural damage has developed such as timber
decay. It will also render ineffective any insulation within the component where it occurs.
Poorly ventilated external masonry wall cavities can also suffer from condensation which occurs within the cavity, (interstitial condensation),
and without adequate air flow the condensed water vapour can saturate masonry quite quickly. This can often be determined by using an electronic
moisture meter on the external wall surface. Higher readings may be revealed below brick
window sills which obstruct upward convection currents vital to removing condensed water vapour, and mid-way between external brick vents.
In many instances the masonry will be visibly wetter, (darker in colour), above the damp-course level. When investigating with hand held
electronic moisture meter readings should be taken on both sides of masonry walls and compared.
Electronic instruments are available that will measure and compare air temperature and wall temperature and calculate a "Dew Point"
to confirm the possibility or presence of condensation. These tests are only relevant when certain comparative temperature differentials are
present.
In cold weather, the best way to keep rooms warm enough to avoid condensation is to keep low background heating on all day, even when there is
no one at home. This is very important in flats and bungalows and other dwellings where the bedrooms are not above a warm living room. If
you have central heating set it to provide background warmth in all rooms, including unused rooms, ensuring effective circulation minimising
cold spots. Ducted heating systems should always have the “cold air” return installed at the lowest possible location preferably at floor level,
never in the ceiling.
Many houses remain unoccupied and unheated throughout the greater part of the day, allowing the fabric of the building to cool down. The moisture
producing activities are then concentrated into a relatively short period. This sudden increase in warm air can produce condensation as the
air comes into contact with the relatively cold structure which is still warming up.
Further problems are encouraged by poor air circulation where stagnant air pockets are created. There is a real danger of condensation occurring
where air is left undisturbed behind furniture and cupboards, often recognised by the appearance of
mould growth.
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