What is Condensation?

The air contains moisture. The temperature of the air determines how much moisture it can hold, and warm air contains more moisture than cold air.

When warm, moist air comes into contact with either a surface or air that is colder than it is, the warm air is unable to retain the same amount of moisture as it did and the water is released either into the cold air or onto the colder surface, causing condensation to form, quickly followed by mould.

What are the Causes?

Day-to-day activities such as cooking, washing and drying clothes, heating and even breathing produce water vapour. Air can only hold so much moisture in the form of an invisible vapour, no matter what temperature it is.

When the air contains more moisture than it can hold, it reaches ‘saturation point’ and when this is reached, the moisture turns back into water and condensation occurs. The temperature reached at saturation point is called the ‘dew point’.

When this happens, the air has a relative humidity of 100%. The air in the majority of homes tends to have 50-70% relative humidity. Problems occur when structural defects in a building mean the moisture content has become too high; when old houses have no damp-proof course (DPC); and when there is inadequate ventilation in the home.

Period homes often have no DPC, which means moisture from the soil beneath the house rises up into ground floor rooms, whilst other homes suffer from bridged DPCs or damaged guttering.

There are several types of condensation. Cold-bridge condensation occurs when warm, moisture-heavy air comes into contact with surfaces at or below its dew point. This occurs at the base of external walls – where it is often mistaken for rising damp – on windows, where it may cause cills to rot, and on the underside of the roof.

Warm-front condensation occurs when warm, damp air gets into a cold house. This happens in the winter, when a ‘warm front’ from the Atlantic arrives, and is common in unoccupied houses.

Interstitial condensation happens when warm, moist air diffuses into a vapour-permeable material, such as fibrous insulation. If this material is warm on one side and cold on the other, the moisture will be deposited in liquid form within the material. This is a particular problem in heavily insulated or air-conditioned homes.

How to Treat it

There are three basic ways to control the problem of condensation, by looking at relative humidity, ventilation and insulation:

  • Control the relative humidity in your home through the use of extractor fans in kitchens and bathrooms. Shutting the doors to these rooms whilst the extractor fans work also helps
  • Ensure there is adequate ventilation. Trickle vents in windows work well, but a more sophisticated option is a heat-recovery ventilation unit. These replace the air in your home by taking the stale, damp air outside, then bring fresh air back in via a separate grille, passing it back over the heat exchanger to be warmed. It is also possible to buy central extract systems which connect all of the wet areas in your home to a central fan before discharging the stale, moist air outside
  • Add insulation so that internal walls are kept at a temperature above the dew point of the air inside

Articles like this Comments
  • Damp Proofing Specialist 22 Mar 2011 at 11:20 am

    Once again, a well written & informative article. For further advice on identifying and combating condensation take a look at the site below:


  • Terence Turner 24 Dec 2011 at 10:00 pm

    1 rodiator is very hot at the top and not as warm at the bottom what shall i do

  • Samuel Joy 3 Jan 2012 at 10:27 am

    It sounds like you may just need to bleed the radiator.

  • Don Brown 16 Jan 2013 at 12:28 pm

    We have found that several of our older properties have suffered from severe condensation even though they were reasonable well ventilated. We did find a company called the Condensation Shop (www.condensationshop.com ) who supply a type of air vent (passyfier) that wicks away about two litres of water per 24 hours. Absolutely stunning, mould disappears in a matter of days. Not that cheap to buy but they work and are worth it when you consider how much it costs to repaint and decorate.

  • Andy Ferguson 17 Jul 2013 at 10:03 am

    Who new condensation was such a problem. Moving into my new property, its the first time I have ever had to deal with the issue. It almost sounds the case that they have over insulated my house resulting in the excess moisture being trapped. Some real good tips within the article which I will be applying. For anyone else that reads this, I also came across a handy video that really helped to explain condensation. You can find it at http://www.wisepropertycare.com/condensation/.

  • karen burley 3 Oct 2013 at 5:59 pm

    We went to http://www.ableair.co.uk fantastic results, the mould was indeed gone in a few days, and its been on the verge of life changing in terms of the living air quality, would definatley recommened having piv installed to sort your condensation/mould out.

  • benson Pearce 28 Nov 2013 at 2:45 pm

    Its very clear from this article condensation is the most common form of damp in buildings, if you have any cause for concern then it may be possible that a condensation survey could highlight the cause of the problem,You can check more about condensation survey at http://www.penninepreservations.com/condensation-control

  • peter ward 22 Jan 2015 at 5:47 pm

    Condensation is a major issue in modern lifestyles – it is responsible for something like 95% of all ‘damp’ problems in houses and ultimately the cause of ‘rising damp’. Unfortunately there are a lot of people out there pushing cheap solutions to this problem, My advice is to ‘Do Your Research’ and don’t just take recommendations from websites – learn about it first. As consultants, we try to help people to solve their ‘damp house’ problems without resorting to ‘damp companies’. Publications like Home Building and Renovating are doing a great job of alerting people – we try and provide more depth of knowledge to fill in the gaps.

    Have a look at this site – read, learn, and THEN look for a solution armed with knowledge.

    Take humidity readings in your home – log the moisture content of your house in each room. Knowledge is power – buy a little thermo hygrometer from Amazon – and take readings. That way you won’t get conned by damp companies – look at any of the ads for condensation control – if those sites also link to rising damp control, or ‘damp proofing’ – you can be sure they are just out to con you. Be Warned! Here goes:


  • Sarah Collins 27 Mar 2015 at 4:19 pm

    I rented my house out to a young couple and they complained of a lot of condensation – this never happened whilst I lived there so searched on line how to get rid if it. Did a bit of research and found a product called the Drimaster by Nuaire. I have to say my tenants were amazed at the results and have never complained since. Online searches say condensation can be caused by flash heating and drying clothes on radiators. My tenants still dry their clothes indoors so it must be the Drimaster that is stopping the wet windows.

  • Simon Hooker 9 Nov 2015 at 10:19 pm

    After a good double glazing installation, on damp cold nights you WILL wake up to condensation on the outside. Given the physics, this is to be expected. Enough house heat is retained inside and the outside pane is cold enough to cause the outside humid air to condense on that cold outside pane.

    As for inside damp, a good dehumidifier is useful. Be careful, some of those recommended by a certain consumer magazine can be very noisy. An art gallery near us uses a much quieter one – which has a ‘laundry’ setting among others. I am thinking of putting one in the bathroom instead of an extractor, and use the drain hose into a suitable outlet (to avoid the refilling)

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