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 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 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.
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