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Condensation a dampener for tenants and landlords


Chartered Surveyor Charlie Kemble-Davies looks at the seasonal problem of condensation and offers tips for breaking the ‘mould’.

We have reached  that time of year when condensation can become an issue for both residents and landlords. From October onwards the enquiries start coming in about why mould and mildew are appearing and what can be done about it.

The condensation warning signs usually appear between September and the end of  May. But it’s generally not until mould and mildew starts to appear on surfaces of walls, furniture and stored items that we as Surveyors are instructed to undertake an inspection of a property and report back on the issues causing the problems.

Below we have identified what condensation is and the main causes and practical ways to control condensation issues within a building.

What is Condensation?

Condensation frequently happens in warm, wet rooms like kitchens and bathrooms. It can be seen on windows in the form of water droplet following ‘steaming up’ of the surface or as patches of dampness on walls and ceilings.

Condensation frequently gives rise to the growth of mould, especially when there is a sustained level of high humidity.  The mould requires pure moisture to survive and this is what is produced by condensation. Without treatment the mould will continue to grow.

What causes condensation?


Condensation occurs naturally, as a result of changes in temperature, or artificially by the actions of people themselves.  At any temperature air is capable of containing a limited amount of water vapour. The warmer the air the more water vapour the air can contain before it becomes saturated, often creating a humid environment.

When warm air is cooled, such as when warm air comes into contact with a cold surface, the temperature of the warm air will reduce and water vapour will condense on the cold surface. Condensation will be created at the temperature at which the air becomes saturated (the dew point).  It will deposit the water that it can no longer retain as condensation on a cold surface, such as that of a cold external wall or pane of glass.

Condensation is often only an issue during the winter months (September through to May) particularly where water vapour laden air moves from a centre of high concentration (typically kitchen and bathroom) to a centre of low concentration (typically cooler bedrooms) and deposits water on the colder surfaces. Water vapour in the air exerts a pressure, ‘vapour pressure’, so air containing a large mass of water vapour has a higher pressure than drier air, which causes water vapour to diffuse to lower pressure areas, such as cooler bedrooms. The term to describe whether air is dry or water laden is ‘relative humidity’.

When the air stays above 70% relative humidity for lengthy periods, there is a high risk of mould growth on the external fabric. The risks can be reduced by increasing the temperature, decreasing the water vapour or a combination of both.

Moisture can come from a number of different sources within a house such as:

  • Breathing (asleep) 0.3 kg of moisture produced
  • Breathing (awake) 0.85 kg of moisture produced
  • Cooking 3 kg of moisture produced
  • Personal washing 1.0 kg of moisture produced
  • Washing and drying clothes 5.5 kg of moisture produced

*(1 kg of water equates to 1 litre)

Water vapour is produced in relatively large quantities from just normal day-to-day activities. For example a five-person household puts about 10 kg of water into the air every day.

Mould Growth

Mould spores exist in large numbers in the atmosphere. To germinate, mould needs nutrients, oxygen and a suitable temperature. Nutrients and oxygen are generally widespread in occupied buildings. The environment in an occupied building also provides suitable temperature for growth. This generally means that mould growth is dependent on moisture conditions. It has been noted in studies that mould can grow in the absence of pure moisture if the relative humidity remains above 80%. The internal surfaces of the outer walls will be lower than the air temperature within the building in winter and would create a relative humidity of 10% on average higher than in the centre of the room. The better insulated the walls, the lower the relative humidity close to the external walls.

Mould grows on all types of surfaces and furniture. The closer the furniture or object is to the external wall, the higher the relative humidity will be. Unheated bedrooms, wardrobes and items placed close to external walls are particularly vulnerable.

Ways to control condensation

There are three primary measures that can be taken to prevent condensation. These are:

  1. Increase ventilation – to help remove moist air from the building and not allowing it to come in contact with cold surfaces.
  2. Increase insulation – to prevent a cold surface reaching below the “Dew Point” temperature.
  3. Maintain consistent heating – to prevent the structure from becoming cold or below “Dew Point”.

Practical measures which you can adopt

  1. Leave heating on a low setting throughout the day in cold weather, rather than switching it on and off at certain times. Most buildings take a long time to warm up, particularly older buildings with solid external walls.
  1. After a bath or shower, ventilate the room to the outside by opening the window. Try not to leave the bathroom door open so that moist air escapes into the house. Make sure the extractor fans are clean, maintained and working well.
  1. If condensation forms on the windows, wipe it off to prevent mould and mildew growth.
  1. Keep the bathroom doors shut when bathing/showering, keep the kitchen door closed when cooking and keep lids on pans. Try not to allow steam to build up in the house. Don’t let kettles and pans boil longer than is necessary. If you have an extractor fan fitted make sure you use it when cooking or bathing.
  1. Add or maintain mechanical ventilation/extraction fans to areas which produce a lot of moisture, such as kitchens and bathrooms. Extractor fans are available with an air-moisture switch so that they operate automatically while there is moisture in the air.
  1. Ideally dry clothes outside. When this is not possible, try drying clothes in a cool area of the house. While this will take longer, less moisture can be held in cool air and with good ventilation, the risk of condensation is lower.
  1. When people come into your house with wet coats/shoes etc., hang them outside the living area to dry. Try to increase ventilation, allow the air within the house to change regularly. It may be possible to install trickle vents to existing windows   and external walls.
  1. Consideration could be given to installing a heat exchanger. A heat exchanger will remove the moist air from within rooms and reuse the thermal energy. This helps maintain a low level of air moisture/humidity which would otherwise be wasted.
  1. Allow air to circulate behind furniture. Don’t overfill cupboards and wardrobes. Leave a space between the wall and cupboard/wardrobe/sofa to allow air circulation. Make sure   that air can circulate freely by fitting ventilators in doors.   Leave a space at the back of the shelves.
  1. Don’t block air vents. If condensation still persists, there are still some other things you can try:
  • Install secondary glazing – to help insulate, but remember to also ventilate.
  • Consider installing new double glazed windows – although this can be expensive and require planning consent the thermal efficiency will be much improved.
  • Insulation and vapour control barrier to the internal walls – this will add an extra layer of insulation and prevent relative humidity increasing close to the wall.

In many cases the cost of alterations to older properties may be so prohibitive that some homes will be unlikely to be completely free of condensation due to the original design of the building.  This means that on-going maintenance is essential along with being mindful about your living habits, lifestyle and surface finishes, you should be able to manage the amount condensation in your home.

For more information refer to the following on how to control and condensation in your home see the following;

British Standard BS 5250:2002 for Code of Practice for Control Condensation in Buildings.

The National House Building Council provides advice on how to deal with condensation in new homes in its publication: A Guide to Your New Home.    Visit:,15900,en.pdf

Posted on 25 October, 2016

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