Managing and understanding condensation in buildings
Firstly: Condensation does not just happen on the wall.
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? Simple really - 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. So 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 really - just evaporates away like the rainwater on your paving stones dries when the rain stops. If you trap it - with gypsum plaster or cement render, or cement pointing on the outsdie of the wall - it can't get out and the wall gets wet. THAT is what the damp wallies call rising damp. Its nothing to do with rising damp - it's just condensation.
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 the best example we could ever quote. Our client asked us to survey his house and try to ascertain why mould was suddenly appearing on the walls around the house. He complained that 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...
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 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.
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.