The condensation of water vapour leads to dampness, and as we all know, dampness is nothing but bad news. In this article I will be outlining the condensation process in distinctly non-scientific terms, and will provide a few tips on how to prevent condensation from happening in the first place.

The amount of water vapour that air can contain will depend on the specific temperature of the air. For instance, the reason dew appears on grass in the mornings, is that the drop in air temperature overnight causes some of the moisture in the air to condense on the grass as water droplets. During the day, as the air temperature rises, the dew evaporates back into the air. So as we can see, the warmer the air temperature, the more water vapour it can contain.

The water vapour inside a building starts to move outwards as its concentration increases.

Everyday activities inside a building such as breathing, cooking, showering and so on will create water vapour inside a building. The water vapour inside a building starts to move outwards as its concentration increases. If the temperature of the air outside the building is lower than the inside, the difference in moisture content is deposited as condensation on the cooler surfaces. This is why your bathroom mirror fogs up when you have a shower.

The following is a few examples of when and where condensation problems can occur:

Winter + air pressure + moisture

In winter, the water vapour in the air within a building is under slight pressure. This pressure is enough to cause the air to move outwards in all directions. If the construction material is permeable (and most building materials and many paints are) water vapour will penetrate upwards and sideways into the construction fabric, just as easily as downwards. Pockets of air in the material will be humidified as the water vapour reaches them. They then create little pockets of moisture in the construction fabric.

Wall cavities + decrease in temperature

The temperature of air in the cavities in external walls decreases towards the outside. This reduction in temperature may lower the ability of the air to contain water vapour to the point that some of it may be deposited as water on the cold surfaces of the structure.

Domestic flat roofs + winter water vapour

Some types of domestic flat roofs speed up condensation in winter. Water vapour is able to enter the roof space, but as the waterproof roof membrane is also a vapour barrier, the vapour cannot escape to the outside air. So it either lodges within the roof deck or condenses under the roof sheeting and drips from it.

Ceilings near wall cavities + cold air

Watermarks may occur on ceilings close to and usually parallel to brick cavity walls where eave soffits are ventilated. Concentrations of condensation like this are most likely when moist air in the structure and cold air from the outside meet in a confined space. As the moist air is chilled, its ability to contain water vapour is reduced, and the surplus moisture condenses on the coldest nearby surfaces. If this continues for long enough, water may drip from these surfaces.

No simple statement can be made to cover all the circumstances in which damaging condensation is likely to occur. The variety in climatic conditions and the type of construction involved will ultimately determine the problem, and how best to combat it. Generally, your first port of call should be to ensure that there is adequate ventilation in your building as well as your cavity walls. Extractor fans are very effective in areas such as kitchens and bathrooms. In other areas, if condensation is a problem, your best and cheapest bet is simply to leave your windows open as much as possible. This will allow water vapour to escape before condensation takes place.

“…ensure there is adequate ventilation in your building as well as your cavity walls…”

One should also look at vapour barriers, which come in the form of special paints, polythene sheeting, metallic foils and bitmastic felts. These should be placed on the “warm” side of the construction. In the case where the ground provides a source of unwanted moisture, a vapour barrier should be laid over the ground. Alternatively, it may be interposed between a concrete slab floor and its concrete topping.

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