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Condensation is by far the most common cause of dampness in buildings, probably accounting for the majority of damp problems reported. It affects both old and new buildings, but it appears to be a significant problem where the building has been modernised.

Condensation is directly associated with mould growth. It is this that the occupier sees first, and it gives an indication of the potential scale of the problem. The mould is usually found on decorative surfaces, especially wallpapers, where it can cause severe and permanent spoiling. In many cases, the mould and its spores ('seeds') give rise to complaints about health, and cause the "musty" odour frequently associated with a damp house.

The obvious places for condensation to occur are on cold walls, wiondows, and floors, but it can also occur in roof spaces and in sub-floor areas where there is a timber suspended floor; in the latter case, it can lead to dry rot or wet rot developing in floor timbers.

The Cause

It is a fact that warm air can hold more water as vapour than cool air. Condensation is caused when moisture-laden air comes into contact with a cold surface - the air is cooled to the point where it can no longer hold its burden of water vapour. At this point, known as the "dewpoint," water begins to drop out of the air, and is seen as condensation on surfaces. On impervious surfaces such as glass and gloss paint, beads or a film of water collect. On permeable surfaces such as wallpaper and porous plaster, the condensing water is absorbed into the material. Therefore, the problem is not always initially obvious.


Condensation is very much a seasonal problem, occurring during the colder months - October to April. During the summer, the problem is seen to go away.

During the winter, ventilation of the house is usually low (due to windows and doors being closed, draught-proofing takes place). This allows build up of water vapour in the house, which, in some cases is sufficient to cause condensation. This condensation becomes apparent from the following symptoms:

  • Water droplets form on cold, impervious surfaces such as glass and paint.
  • Slightly damp wallpaper (often not noticed).
  • Development of moulds, usually black mould.

In some cases, condensation may be long term, but intermittent, forming only at certain times of the day or night. In these cases, the only sign of condensation may be mould growth, as the moisture may have evaporated by the time moisture measurements are taken.

One should also be aware that the problem can occur well away from the site of most water vapour production. E.g. water vapour produced in the kitchen may diffuse through the house into a cold bedroom where it will condense on cold walls.


If one wishes to confirm that there is a condensation risk, then a Humiditect card can be affixed to the surface where condensation is suspected for 7 days (due to the intermittent nature of the problem). Spots of colour printed on the card will gradually bleed depending on the severity of the problem.

Control of Condensation

The control of condensation is based on two very simple primary measures, supported by a number of secondary measures.

Primary Measure 1 - Improve Ventilation
This will sweep away the internal moisture-laden air and replace it with drier air from the outside (yes, external air is drier than internal air most of the year!)

Mechanical ventilation such as Humidistat Fans and the PPF9 Positive Pressure System should be considered.

Primary Measure 2 - Improve Heating
Coupled with ventilation, heating should be set or applied to give a low-level background heat. This will ensure no rapid changes to the environment, and will facilitate slight warming of wall surfaces over a period of time, thus reducing the risk of condensation.

Secondary Measures

  • Remove excess moisture sources - e.g. paraffin heaters, indoor drying of clothes.
  • Insulate cold surfaces.
  • Prevent possible water penetration.
  • Install a dehumidifier.
  • Use an anti-mould paint.

Professional Assistance

In situations where a condensation problem is persistent, it may be necessary to employ the services of a professional company to correctly diagnose the factors causing the condensation problem and implement measures to control it. If you would like a quote from a professional company in your area that offers a condensation control service, please fill in the form below or call us on 01403 210204.

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Brief description of the property affected.

Condensation on Window
Condensation in window
Mould Caused by Condensation
Mould caused by condensation.
How Condensation Occurs
How condensation occurs in the home.
Download Datasheets
Condensation Guide Humidistat Fan Raincheck Plus
Guide to Condensation (472k) 6-inch Humidistat fan (104k) Raincheck Plus (40k)
Price List    
Price List & Order Form (85k)    
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