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There is always moisture in air in the form of water vapour. The warmer the air is, the more water vapour it can hold. To measure the amount of moisture in air one needs to refere to relative humidity. Relative humidity is a measurement of how much moisture is held by the air in comparison to the maximum amount of moisture the air can hold at a given temperature. For example air at 75% relative humidity holds three quarters of the moisture it is capable of holding at that particular temperature.

When air is warmed the relative humidity of it will decrease. Any air that leaks in to a building from outdoors in wintertime will warm up and the relative humidity of it will decrease. So in winter a building that leaks in a lot of air will have a very dry atmosphere.

When air is cooled the relative humidity of it will increase. Any warm air that leaks out of a house in wintertime will cool and the relative humidity of it will increase. So if warm air leaks out of a house in to the cold attic for example, condensation will be formed and moisture problems can occur in the attic and the walls of the building.

Air Leaks In To The Home


Air does not have to leak in to or out of a building to have its relative humidity changed or to form condensation. Cold surfaces are a great contributer to condensation.

In winter a buildings coldest surface is usually the windows. This is why condensation is very common on cold days.

Cold surfaces also cause condensation in summertime which may surprise you. Late Spring and early Summer are the most frequent times this occurs because the ground has not warmed up properly from the previous winter.

When the day is warm and humid you may find condensation on walls and floors that come in to contact with the ground. Opening windows and allowing air flow in may not reduce this condensation problem, but increase it in certain situations.

That is because you are allowing warm air in but the walls and floors are cold from the contact with the cold ground.

If relative humidity is high in a home during winter, condensation will form on cold surfaces. The air in homes can have an increased humidity level during winter from several moisture sources.

These sources can be ;


Just normal living and breathing can produce 30-40 litres of water vapour by a family of 4


Drying firewood indoors can produce 15-30 litres of water vapour


Showering, cooking and washing can produce 15-20 litres of water vapour a week


A damp basement can produce 15-20 litres of water vapour a week


Drying clothes indoors can produce 10-15 litres of water vapour a week


Summer moisture absorbed by the home that is released in autumn can give 20-40 litres of water vapour a week


As well as the contributing issues above any indoor humidity levels can be affected by air exchange. This replaces the moist indoor air with dry outdoor air.

How much air exchange in the home that takes place is dependant upon ventilation and air leakage.

It is the combination of the indoor moisture sources, any air exchanges and the number of cold surfaces in the home that will determine how much condensation is formed in your home.





Moisture - Basic Facts


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