Managing and understanding condensation in buildings

Firstly:  Condensation does not just occur on wall surfaces.  

Condensation happens INSIDE walls.

Condensation on or within walls is a common problem in old houses, and to a lesser extent in new houses, which should comply with building regulations – these are meant to avoid problems such as condensation. Condensation in old houses causes the phenomenon most commonly called 'Rising Damp'.

Temperature + Humidity = Condensation...

How does water get into a wall you ask? The air around us has lots of water in it.  It is a gas - harmless, not wet - and moves around in everything.  It is passing through the walls around you now - together with oxygen and nitrogen and a bit of carbon dioxide.  Your old house walls always have a bit of water in them as a vapour and it does no harm.  If you cool the wall, water can turn to a liquid - condensation.  The coldest bit of the wall is the bit nearest the gound, or a very exposed outer corner of an upper bedroom, or underneath a bay window.  If the water which forms as condensation can evaporate - through lime plaster, or lime mortar or stone, it goes as soon as it forms - just evaporates away like rainwater on your paving stones dries when the rain stops.  If you trap it - with gypsum plaster, cement render or cement pointing on the outside of the wall - it can't get out and the wall stays wet.  THAT is what the damp wallies call rising damp.  Its nothing to do with rising damp - it's just condensation.

There are two ways of measuring humidity - Relative, and Absolute.  We like absolute - it is measured in grams per cubic meter.  A dry house will have around 6 or 7 grams/m3  and a wet house will have 10 grams or more.  6 grams is about half a small teaspoon.  Relative humidity can confuse the issue, because air at 50% Relative at 10 degrees, for example has half the amount of water dissolved in it than air at 20 degrees.  Warm air can hold a lot more water, which is why when we insulate houses, damp problems often start.

We have sophisticated, yet simple measuring equipment which allows us to monitor the levels of humidity and the temperature in your house. From these, we calculate the 'Dew Point' - the temperature at which condensation forms. It is then a simple process to check the temperature of walls and pinpoint those which are at risk, and those which actually have condensation forming on them.

A recent survey provided a fantastic example. A client asked us to survey his house and ascertain why mould was suddenly appearing on the walls around the house. He complained there were damp patches all over the house, and mould growing on clothes and belongings in cupboards. Plaster was peeling in some areas, together with paint finishes. As soon as we entered the house, we took ambient humidity and temperature readings - these showed humidity of over 70% and a room temperature of 22 degrees C. The dew point, at which temperature condensation will start to form, was in this case calculated at 13.5 degrees. A spot check of the walls in the worst affected areas (using a very sensitive thermocouple) showed that they were all at or below dew point, and thus condensation was actively forming - which is why the walls were wet.. We opened a window at the front of the house by an inch, and the back door by a similar amount, which was just enough to allow a flow of air into the house. Within two hours, condensation on the walls had disappeared. Checks of the humidity and temp levels showed that the humidity had dropped to below 60%, and the temperature only by 3 degrees - the house felt warmer, and there was now a big gap between the temperature of the walls, and the dew point, which had dropped considerably. Our client was delighted, his problems were essentially solved - we recommended a series of small changes in his lifestyle, some ventilation fans, and air vents - all at minimal cost.  The house is now starting to become dry and mould free. Luckily, we got to this one before the plaster dropped off and damage occurred to finishes.

In another survey, the client was using gas fires to heat the place. A huge woodburner in the middle of the house was never used. We made him cut off the gas fires and use the woodburner. A month later, he rang me to say that they did not need to do any of the re-plastering or damp works that other 'Timber and Damp Surveyors' had told him to do - and that just by turning the gas fires off and using the wood burner - "You've changed our lives - our asthma problems have stopped, the house is warmer, and dry as a bone - the mould has disappeared from our clothes''

These two photos below are probably the most significant examples of what can happen to a wall that I've ever seen. Look carefully at them, and the pattern of 'damp' that is repeating in the first picture - then have a look at the detailed explanation below the photos...

More examples

Thermal images of condensation in the corner of a room

These images are to show you how temperature varies in walls.  This is the corner of a room - to the right of the picture is the outer wall.  The wall in front of you - to the left of the pic, is also an external reveal, with another wall running off it away from you... This means that the corner is a very cold corner, exposed to wind and temperature.  Rising damp ?  No, of course not - but the coldest part of a wall is the bit at the bottom - the warmest - at the top.  Cold corner - cold in the corner.  So you get this textbook triangleof cold at the bottom of the triangle and warmer at the top, with the associated mould pattern from condensation in the cold bit.  

It's all very simple really - this is what condensation is all about - and yet people insist on calling every occurrence of mould as rising damp - now you start to visualise - its not!

What is Condensation?

Condensation takes place ON surfaces, but it can also happen INSIDE building materials - like rock, brick and timber.  So don't think it only happens on surfaces - if your walls are cold, it happens INSIDE the wall, dwhich is the real cause of the symptoms called 'rising damp'.

Condensation is the result of warm, moist air which finds a cold surface on which to condense. The two coldest places in a house are generally the windows, and the bottom of the outside walls, near the skirtings. Moisture comes from a variety of sources, but mainly:

  • We humans – the average human sweats and loses around 8 pints of water in a 12 hour period
  • Bedrooms - we breathe all that water out at night - where does it go?? Into our beds, clothes in the wardrobe where they get mouldy, carpets and so on..
  • Kitchens – cooking – steam from hobs and ovens, tumble dryers, fridges that auto-defrost
  • Bathrooms – showers, baths, wet towels
  • Plants – in pots on window ledges
  • Drying clothes on clothes hangers in the house
  • Wet cellars with no ventilation or air circulation

One of the things we often see in old houses, is water streaming down windows in winter and collecting in pools on the window ledge. This is because the rooms are not able to breathe – the air is not circulating and able to get away, and the warm, moisture laden air thus releases moisture on the windows. The commonest problems with old houses that stop air circulation are listed below:

  • Installation of plastic (UPVC) windows with no ventilation
  • Ventilation bricks blocked up
  • Fireplaces blocked up with no ventilation
  • Draught excluders around doors
  • Lack of ventilation to cellars
  • No extractor fans to kitchen and bathrooms
  • Bedroom windows shut tightly

By tackling each of these problems, air circulation will dramatically improve, and far from making the house colder, will actually make it warmer, because there will be a lot less moisure in the air. If you have a cellar, the best way to dry it out and stop the dank smell is to fit a ventilation grille to the door, with a little electric extractor fan (bathroom type) on a timer switch, which is fitted to the outside wall near the ceiling of the cellar. This operates for an hour or two a day, and draws warm, dry air from the house into the cellar, and pushes cold, damp air out from the vent fan to the outside. You must always ensure that air can ENTER the room as well - no point in a fan drawing air out, if nothing can replace it.

Problems arising from lack of ventilation:

Water condenses on windows and damages sills, and rots the frames at the bottom

Water condenses on external walls, near the skirtings, and makes the plaster fall off.

The consequences of these problems can cost a lot of money to fix, so rather than watching wallpaper and plaster fall off walls, and windows rotting, all you have to do is get that ventilation working again!


  • Ensure that UPVC windows have vents that work – if not, get vents fitted – they are available and easy to fit
  • Uncover ventilation bricks – they are there for a purpose
  • Make sure fireplaces all have a ventilation grille if they have been blocked up
  • Take some of the draught excluders off – I know it sounds daft, but it works!
  • Make sure your kitchen has an extractor fan and you use it when cooking
  • Make sure your bathroom has an extractor fan, and make sure you use it whenever the bath or shower is being used
  • Make sure tumble dryers are vented to outside walls – if not, use a condensing dryer
  • Leave a window partly open – open a top hung light a little – let some air into the house
  • Dont dry clothes inside the house
  • Turn the heat down – a 5 degree drop in temperature means a great deal less water can go into suspension in the air
  • Dont run your heating on an on/off cycle - turn the feed temperature right down, and leave it on very low, all the time
  • Make sure your cellar is properly vented
  • Ventilate wardrobes - allow a good flow of air into and around clothes - any stagnant pockets of air will encourage mildew

 If you follow all these rules, your house will be a lot drier, and condensation won’t be a problem.

Bear in mind that UPVC window vents are tiny. They don't allow much air exchange, and we don't rate them as a means of ventilating a room. You need several air changes a day, and tiny vents in a window will barely allow one air change a year! If you have them, great, but if you don't, then you are better off installing proper humidity controlled ventilation - such as the RHL / Vapourflow vents referred to elsewhere in this site. 

A Note about condensation problems in bathrooms...

I recently did some experiments with our humidity measuring equipment. I went into the bathroom as normal and had a shower. The ambient humidity in the house is around 55% - and within 4 or 5 minutes, the humidity in the bathroom had risen to nearly 100%. Measured at the floor, it was over 70%, and ceiling 100%. I had the extractor fan turned on, and the door open. When the shower was turned off, the humidity started to drop at ceiling height quite quickly - but only to about 85%. At floor level it dropped to 70%. The disturbing fact is that it took over 4 hours for the humidity in the room to drop below 70% - in other words, with average house temperatures there would have been condensation forming for 4 hours or more. It's worrying to see figures like this - I get similar figures all the time when cooking in the kitchen - what it means in reality, is that we are producing huge amounts of moisture as part of our everyday life, and our houses simply cannot cope. We NEED to be aware of this, and make sure that the house can breathe and ventilate. Ventilation means exchanging the volume of air in the house - you cant do it through a tiny crack or vent brick - the ventilation system needs to be able to exchange the entire volume of air in the house at least once a day - and that's hard to do without opening windows and doors...

Scary stuff, I know - but you need to know the facts!

Gas Fires - A major cause of damp problems in old houses...

One of the bigger problems we come across is the use of gas fires in old houses. When gas burns, it produces surprisingly large amounts of steam. This enters the atmosphere of your rooms, and floods the house with humidity. This then condenses at the coldest part of the room - the base of external walls.. Houses which are regularly heated with gas fires are almost invariably very humid and suffer terrible damp problems. Caravans and mobile homes are a classic example - they are always streaming with condensation when the fire is on - and often occupants use dehumidifiers constantly. Old houses are frequently equipped with gas fires - especially Victorian townhouses and terraced properties - where so called 'rising damp' damp problems are common. The cause can often be traced back to the use of a gas fire. The solution is simple - get rid, and install an electric fire, or open stove, or perhaps a small central heating system.

Damp and Condensation

Damp facts ...

'Rising damp' usually shows itself as damp patches towards the bottom of a wall, or plaster falling off, or sounding hollow. Building surveyors say its damp, because their silly little meter gives a high reading. This reading is nearly always false. The reading can be high because of salts which have been drawn from the wall by the moisture. The meter measures conductivity, NOT water. You DON'T have rising damp!!!

Rising Damp doesn't exist, except in the minds of the surveyors trying to sell you injection damp proofing... Research the qualifications of the surveyors - find out what their 'CSRT' actually means, and what they had to do in order to get it.. at the last count it was a half day course on injecting chemicals, run by an organisation funded and at least part owned by chemical companies.

Rising Damp does not exist! It has never been reproduced in laboratory experiments despite numerous attempts. All the 'Timber and Damp specialists' who recommend damp proofing, tanking, injecting, rendering - all of them are defrauding you. They are treating something that doesnt exist. Your 'damp' is easily diagnosed, and it won't be rising.

Almost all cases of 'rising damp' are caused by condensation. Some are caused by penetrating damp - simple things like high ground levels or leaky gutters and downpipes.

Don't trust surveyors damp meter readings!

We often check surveyors meter readings by drilling a hole into the wall behind the plaster. We take a sample of the actual brickwork or stone - and not surprisingly when we do a proper chemical test on the powder from the drillhole, its bone dry. Surveyors use a 'pingy prongy' meter - a silly thing with two prongs. It doesnt measure moisture - it measures conductivity - which ALWAYS gives totally incorrect readings. Supposedly wet walls usually arent.

The 'Damp' is only surface deep. Humans breathe - when we do, moisture loads the air in the room, and condenses at the coldest spot - the bottom of the wall. That moisture is concentrated by things that retain it - like plastic (acrylic paints) or gypsum plaster, waterproof plasters, anything with cement in it... and the only way it can then escape is to blow the plaster or bubble the paint.

Cement renders

The same problem occurs with cement renders or 'pebbledash'. There was a mistaken belief after the war that damp houses were caused by water getting in through the outside walls - this rarely happens. That belief resulted in houses being covered with thick coats of cement render - which traps condensed moisture in the fabric of the walls, and causes massive build up - which then blows out into the internal walls. The solution is to remove the render and let the walls dry out. If brickwork is damaged and needs repair or replacement, it should be done with lime mortar. If it is proposed to re-render, you must always use lime render, which can breathe freely, and allows trapped moisture to escape. Don't ever use so called breathable modern paints - they arent. They are plastic - they coat your house and trap moisture.

Common causes of damp...

  • Modern paints
  • Cement render
  • Brickwork pointed with cement
  • Gypsum plaster
  • Ground levels outside higher than inside
  • Broken guttering or missing downpipes
  • Vegetation growing near the wall
  • Trees creating shade and moist air near a wall
  • Lack of ventilation - double glazing, no vents
  • Blocked chimneys - fireplace blocked up, no vents
  • Furniture against walls creating cold, damp areas
  • Gas fires - produce huge amounts of steam when the gas burns - responsible for terrible condensation damage. Use a woodburner!


Building Surveys...

We will inspect and prepare a damp survey and report on your home, with recommendations and solutions to any problems found. We will NEVER recommend injection damp proofing. If you are buying a property, contact us to arrange a survey before you commit to buy - our report will give you a good idea of how much money you may need to spend on a property to bring it up to scratch - and will give you a lot more confidence in your purchase. Note that your survey does not just include damp and condensation problems - we check the entire fabric of the house for any other problems - structural, timber, plumbing, roof and so on - in other words, a full health-check on the entire house.

We've had clients go back to their vendors and negotiate substantial (£15,000 and £25,000 in two cases) discounts to the purchase price as a result of what we have found. We have also had a LOT of people refuse to pay for Homebuyer surveys when they find that the surveyor recommends a standard clause; 'Engage Specialist Timber and Damp surveyor to inspect and report on timber and damp issues'

Um - what the hell are you paying for a homebuyer survey for, if the idiot of a surveyor can't even diagnose the real reasons for timber and damp problems - he has to recommend a chemical salesman who has NO interest in solving your problems, and a HUGE interest in making money.

Costs are typically around £700 plus costs for the survey, and if required, scheduling of the build from £250 to £1000 depending on the detail of reporting and costing required - we can even draw up full plans and Listed Building Consent applications.

Timber and Damp Surveys - The truth about Those Guarantees they all talk about!!

Every time we hear from an angry client when one of the 'Timber and Damp' companies has refused to honour a guarantee, we laugh. You need to look for the 'get out' clause in all of their contracts. Its always going to look something like this:

"We guarantee that our rising damp treatment will stop rising damp, and if you ever get rising damp we'll come back and re-treat it. Please note, this guarantee does not cover problems subsequently caused by other sources of damp out of our control such as condensation....."

Have a look at this example if you dont believe me: click here

Amazing isnt it. These fraudsters know from the outset that rising damp is non existent. They are taking your money to treat what was probably condensation related. They know they aren't curing the problem, and they know you'll be back - but they know how to get out of it, and they are going to hang onto your money, come what may.  Peter Cox damp proofing are really good at getting out of guarantees - we have loads of examples.

Building Societies, Banks and Solicitors often ask for 'Timber and Damp Surveys', and frequently ask for some sort of damp treatment guarantee. If you are victim of such a request, contact us (We can explain the real options, and can often show the banks that there is no need for any 'damp' work to be done - DO NOT just google the closest 'Timber and Damp Surveyor' - you'll end up with a chemical salesman who will convince you there is rising damp everywhere, and draw you some really nice plans of your house with little red lines where they are going to inject it. He wont charge you for the survey either - he's really hoping you are going to fall for his fraudulent sales patter and buy more of his useless chemicals.

If you have paid for one of these guarantees - take it to a lawyer - they are the next PPI - we know the banks are worried already that GPI - guarantee protection insurance - will hopefully overtake PPI as the most fraudulent miss sold product in the building industry.

Measure humidity in your home
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