Property Repair Systems

Stop Condensation on Walls and Windows in Your Home and Find out the Causes of Condensation and How to solve Damp and Mould with DIY Doctors Condensation Solutions

This project is sponsored by Property Repair Systems

Summary: This project aims to help define condensation, diagnose whether it is rising damp or condensation, explain the causes of window condensation and wet walls, diagnose symptoms, help you control condensation with a guide to treatment and cures for your problems, and stop condensation using paints, wallpapers, coatings and products.

Don't want to do this job yourself? Let us help you find a tradesman local to you

A condensation definition so that you know what you are dealing with! We are often asked for condensation solutions and obviously if you have a condensation problem in your home you want to know how to avoid it, and how to treat the damage that is caused by condensation on walls, windows, curtains and even clothes.

Condensation causes black mould or mildew around window frames and recesses

Condensation problems on windows and recesses create damp conditions causing flaking paint and black mould on window frames and walls

What is Condensation and How Does it Form?

However, before we explain all that, we first want to define condensation. So what is condensation and what causes condensation? Basically it is when water, which is already in the air, settles on a colder surface and makes it damp or wet. Over time this moisture can lead to damage by peeling paint off windows, and walls. This window in Bath, Somerset was in a house we were called to a few years ago, it was fitted with metal windows which are notorious for condensation, and you can see what happened to the paintwork!

Flaking paint on window caused by condensation

Condensation on windows causes flaking paint around window frames and windowsills. Flaking paint is a common symptom of condensation

The damp conditions also encourage the growth of mould spores which thrive in these moist places. This often leads to black sooty mould on windows, but you might also notice mould on walls, and even on curtains, where condensation has been allowed to build up over time.

All air contains water vapour, but the amount of water air can hold is determined by the temperature of that air. This ratio is called the ‘relative humidity’. Hot air is able to carry much more moisture than cold air, so as the temperature of air rises it is able to hold a greater volume of water. The water comes from everyday activities including, bathing, cooking and washing. Even our breathing causes condensation, and this is why car windows steam up when you sit with the windows closed and the fan off.

Once the air is saturated with water vapour (at any given temperature) it will deposit beads of water (condensation) onto any surface that is cold enough. The temperature of the surface at which this moisture will form is called the ‘dew point’. (For an example, take a cold glass bottle out of the fridge, and it will soon become wet. When the weather is hot the condensation can literally run off the bottle).

Another area that you often see this happening is on bathroom pipes – we look at how to avoid this in more detail further on in this project under ‘Best advice for Fixing Condensation problems’. This pipe (pictured below) is typical of many bathrooms in the UK. The cold mains-water comes into a warm bathroom where the water vapour in the steamy room condenses on the pipe, often leaving a puddle of water below it. This can be so bad that you might think you have a water leak from the pipe.

Condensation on kitchen and bathroom pipes leads to discolouration

Condensation forms on cold pipe work in kitchens and bathrooms leading to flaking and discoloured paint

You can see this effect outside too. Where the dew point is close to the ground, to a small layer of damp air, dew, or frost will be formed. Where a larger amount of air is involved, mist or fog will arise. If this happens to air that is rising in the atmosphere, and therefore expanding, then clouds will form.

If this happens in the home.... It's called condensation.


Condensation Symptoms

These symptoms are listed in the order they are most likely to occur. In other words, in the places most at risk of condensation forming and the way the condensation affects that area.

  • Window condensation - Beads of water on windows
  • Pools of Water on the windowsill
  • Beads of water on external walls
  • Damp patches on external walls
  • Damp patches on ceilings
  • Damp patches in the corners of rooms
  • Damp internal walls
  • Black mould on window frames, window recesses and windowsills
  • Black mould on walls – particularly corners and recesses with little air flow
  • Black mould in cupboards
  • Black mould on curtains and clothes
  • Wet internal walls
  • Black mould on carpets – can be a symptom of condensation but is more often a sign of penetrating damp especially if you don’t have any of the symptoms above

Diagnosing Condensation

Is it rising damp or condensation? Let us look at the rising damp myth, which is that all damp at low level must be rising damp. Let us be clear here, it is rarely rising damp, it is usually condensation although it can also be penetrating damp – where water is getting in through damaged or shoddily built areas such as through a roof leak, cracked render, failed wall ties, damaged brickwork or badly fitted window frames. Rising damp may occur in certain circumstances but it is very rare in modern housing.

Rising Damp or Condensation – Busting the Rising Damp Myth

Question - "I have wet walls at low level and dry walls above three foot, so it must be rising damp"

Answer - Heat rises so the lower part of the wall is where the surface is cooler and more likely to suffer from condensation; also water always tries to travel down to the lowest point, so if the condensation is running off the walls it stands to reason the lower part of the wall is most affected

Question - "I have black mould on my external walls and not on the internal walls in the same room, so it must becoming in from outside"

Answer - The interior surface of external walls tends to be the coolest surface in the room, so they naturally attract condensation.

Question - "I have black mould growing at the bottom of my walls it must be rising damp coming in from underneath the floor"

Answer - See the answers above. If you live in a house without a damp proof course, or if you think your damp proof cause is damaged or compromised, then it could be rising damp and you can find out more in our project on rising damp. If you live in a modern house or a flat above the ground floor it is most unlikely to be rising damp and you should keep reading to find out how to be sure you have condensation not rising damp or a leak.

Question - "My walls are wet, not damp or mouldy, so it can’t possibly be just condensation"

Answer - Yes it can! There is no such thing as ‘just condensation’. Condensation can be a cause of major structural damage as well as damaging paintwork, soft furnishings and clothes, if allowed to carry on unchecked. We give you some photographic evidence of this structural damage in our air brick Project.

Those are some of the most common remarks we get on our forum, but if you have a particular question you want answered then visit the damp and damp proofing DIY Forum and post your query there.

Most damp walls in modern housing are caused by condensation. You can usually tell by the pattern of the damp and damage, but do not be fooled by the amount of water involved. Some people think they must have a leak because their walls are so wet, but condensation can release a huge amount of water, so that is not a good gauge of whether the problem is condensation or water ingress.

As discussed above black mould growth is the most common effect of condensation, but mould can appear in many other colours on carpets, clothing and wood.

This picture (below) shows the effects of condensation in the corner of a room in a property in Hampshire. Note the way that the black mould climbs and concentrates on the coldest part of the wall – an outside corner. If this was rising damp it would most likely form at the same level on the whole external wall.

Black mould on external walls caused by condensation

Condensation will often settle on external walls causing mould on walls in this classic pattern, which shows where the walls are coldest

One other causes of damp walls you could consider is where a wall tie in a cavity wall has become compromised and is allowing water to travel along the tie from the external wall to the internal wall. We explain damp being caused by wall ties more thoroughly in our Damp in Cavity Walls DIY and information project.

If you are in any doubt how your walls are getting wet then you could perform a simple test. Clean and dry the area and then tape a square of aluminium foil over a test patch, about 30cm (1 foot) square should do it, and use Gaffer tape not Sellotape. Leave in place for a day or two. When you remove the foil if it is wet on the inside you may have a leak, if it is wet on the external surface then it is likely to be condensation.

If you want something a bit more scientific then you can buy Fugenex Damp Protect indicators where you feel that you might have a problem with damp, and you are not sure whether it is condensation or a leak.

These chemical strips turn blue where condensation is present and they also show how long a high humidity reading has been present. The reading is what is called 'non-reversible' which means that it always shows the highest recording even if the humidity levels then fall again, so you can always see what the maximum reading was. You can get more information from Property Repair Systems if you have any questions and they stock the indicators on their website.


Preventing Condensation

Now we have defined condensation, you know what the problem is, and what the symptoms are, the next thing you want to know is how to prevent condensation. Put simply ventilation is the cure.

How do I Stop Condensation?

As soon as warm air, containing vapour, hits a cooler surface, it will condense. This is most obvious on windows, mirrors and wall tiles, but condensation on walls and ceilings is also happening a lot of the time, quite unnoticed.

If we insulate our homes and warm the house thoroughly, then this should not happen I hear you cry..... But it will!

The only way of preventing condensation is to provide ventilation. The warm air, containing the water vapour, will rise and circulate around the room until it finds a cooler surface, unless we let that air out and some cooler air in.

In order to ensure that there is good ventilation in your home, a change of air is recommended in all rooms of the house once a day, at the very least.

Improved ventilation can help prevent condensation forming and aid in resolving problems like wet internal walls and black mould issues. However for a total solution there are some products and equipment that you can use to solve the problem.

In these simple steps below we tell you all you need to know about preventing and treating condensation.


Condensation Problems and Solutions

There are plenty of DIY condensation solutions that you can use in your home. They do not need to be expensive, and treatment will depend on the problems you are experiencing. Preventing condensation forming in the first place is the easiest solution, but in this section we run through other treatments if damage has already occurred.

As previously mentioned condensation in our homes is most obvious on mirrors, single glazed windows and wall tiles, but it may also happen on walls and ceilings, particularly in areas of high humidity such as kitchens, utility rooms and bathrooms.

Measure Relative Humidity

The amount of water vapour in the area is the key to condensation forming. The moisture content of the air is known as Relative Humidity or RH. In order to avoid condensation we need to keep the moisture content of the air between around 55-65% Relative Humidity.

If we insulate and warm our house thoroughly, you might think that condensation will be avoided, but unless we keep the moisture content of the air below level then condensation will form on the coldest surface. You can measure the moisture in the air using a Hygrometer, otherwise known as a Home Humidity Monitor pictured below. These are available online from Property Repair Systems.

Hygrometer or home humidity monitor

Hygrometer or Home Humidity Monitor

These Home Humidity monitor models have a memory feature, so you can keep a record of damp levels and whether your relative humidity level is going up or down. This will also help you gauge which household activities are causing the most condensation.

Use a Dehumidifier to Remove Moisture from the Air
To control the Relative Humidity we need to balance heating with ventilation, or use a dehumidifier to extract moisture from the air. Use a Hygrometer to show you the actual Relative Humidity level in the room. If you are not able to measure the RH it makes getting the right balance between heating and ventilation difficult to achieve.

A dehumidifier is an electrical mechanical device which removes moisture from the air using the same system of condensation that causes all those problems! A fan sucks air into the machine and passes the moist warmed air from the room over a cooled plate, the moisture in the air then condenses on the plate and runs into a tank within the machine. The tank in the dehumidifier will need to be emptied periodically (depending on the amount of moisture in the room) but it will cut off when the tank is full so it can be safely left to run if you cannot be in while it dries the area.

Dehumidifiers are useful for quickly drying out a damp room

A dehumidifier will quickly dry out a damp room and can be wheeled into each area of the house (although care should be taken using electrical equipment in kitchens and bathrooms)

You need to close windows and vents when you are running your dehumidifier, otherwise you will be trying to dehumidify the world.

Using a dehumidifier is an efficient way of removing moisture from the room if you are suffering severe condensation problems. Most Dehumidifiers give off some heat (like a fried) however we have known them to lower the room temperature, so it is not necessarily any better than opening vents and windows to ventilate the room in terms of keeping warm in your home.

Dehumidifiers can also be quite expensive to run, so they are really only a temporary measure. You don’t have to buy one as they can easily be hired from your local hire centre, and are useful to employ when a room has become very wet due to condensation build-up over time. Once the room is thoroughly dried out you can then just use ventilation to keep condensation at bay.

I have Condensation but I don’t want to leave my Window open If you don’t want to open a window (because you are a chilly mortal), or you don’t fee safe leaving a window open while you are out at work, then you can fit trickle vents to your windows. This DIY job is not difficult and we explain how you can do it yourself in our Fitting Trickle Vents Project.

The vents come in two halves, one for the inside and one for the outside of the window. Several holes are drilled through the top of the widow head to allow air passage into and out of the room.

There are integral flaps fitted so the vents can be closed in really cold weather, and insect grills keep all the creepy crawlies on the outside! We fitted them to our old house in Frome, Somerset, which had timber single-glazed windows which were prone to condensation (see below).

Trickle vents come in two halves and fit either side of the window allowing ventilation without opening a window to solve window condensation

Trickle vents come in two halves and fit either side of the window allowing ventilation without opening a window to solve window condensation

Tips To Stop Condensation

Effective condensation control is all about lifestyle, once you get the situation under control you will start to automatically put into place the measures that will stop condensation forming.

Here is our quick guide to condensation prevention:

  • Do not push furniture right up against the walls, especially external walls which are often the coldest in a room. Keeping furniture a little further away from the walls means the air has a free flow around the room.
  • Do not fill cupboards to bursting point, again, allow the air to flow.
  • Make sure that your loft insulation is not blocking the ventilation provided by the gap between the facia boards and the house wall. In modern houses there are often purpose-made ventilation grilles and vents in the loft, and you should keep these clear of blockages.
  • Install cavity wall insulation to help eliminate cold spots on the internal walls. You must first ensure this complies with building regulations for your home.
  • Have central heating or underfloor heating fitted if possible.
  • Have heating thermostatically controlled wherever possible.
  • Ventilate tumble driers externally, or have a self-condensing model
  • Install extractor fans in the kitchen and bathroom. The best models are available with a humidistat control, which means they are automatically triggered to operate when the humidity in the room rises.
  • Install trickle vents in your windows.
  • If you already have trickle vents make sure you open them for the majority of the day to allow a change of air and improve ventilation in the home.

Best Advice For Fixing Condensation Problems

Effects of Condensation

Condensation is an underestimated cause of damage to our homes. When people find they have a damp room with black mould, they usually assume it is a leak or rising damp that is causing the problem. However condensation can cause significant damage to paintwork and wallpaper, and flaking plasterwork. So once you have the relative humidity under control you will need to repair the damage once you have sorted out your underlying condensation problems.

Treating Black Mould

The main symptom of condensation is black mould on walls and ceilings. This can show up as small black spots or in bad cases can make the whole area black and 'sooty' or you could get mould which is white and fluffy which looks a bit like the walls have tonsillitis! This is unsightly and damaging to your interior decoration, but it can be harmful to your health too. People who are sensitive to irritation, such as asthmatics, can be adversely affected by the airborne spores, so removing the mould is a must!

Black mould and mildew damage to walls caused by condensation

Black mould or mildew will thrive in the moist conditions created by condensation and as well as damaging your paint and wallpaper it can be harmful to your health (pictures courtesy of Property Repair systems)

Property Repair Systems recommend that if your walls and paintwork have been damaged by black mould you should first thoroughly clean and treat the area, and then redecorate with anti-condensation paint. We look into how to do this in more detail in our guide ‘How to Treat Black Mould’ below.

You need to use a solution of anti mould cleaner to wash the affected area to kill the spores and help to prevent the mould regrowing.

This should be done before you attempt to redecorate, as simply washing off the mould with detergent and then redecorating will not kill the spores and the mould is likely to regrow, even through anti condensation paint.

This will be most effective where you have dealt with the underlying problem of condensation.

How to Clean and Treat Black Mould

  • Thoroughly clean the area first, preferably with sugar soap (mixed with water to the manufacturer’s specifications) or detergent with water. Wear Rubber gloves and a breathing mask.
  • Rinse to ensure that the area is grease-free.
  • If this hasn’t removed all the flaking material then rub the area down with fine sandpaper so that you have a smooth sound surface.
  • If you have sanded the area, rub down with a dampened lint-free cloth to remove all the dust.
  • Treat the area with an anti-mould ‘killer and primer’ which is available online from Property Repair Systems.
  • ACS Mould killer concentrate and cleaning bottles with spray head and scrubbing head

    ACS Mould Killer concentrate and cleaning bottles with spray head and with scrubbing head

  • If the area is stained, treat next with a stain and damp seal paint which is also available from Property Repair Systems.
  • ACS stain and damp seal paint

    ACS Stain and Damp Seal paint

  • Once that dries you are ready to paint. To help to avoid black mould reforming on your newly redecorated walls and woodwork, use anti-condensation paint. This is special paint which is hygroscopic, allowing the paint to ‘breathe’ and makes it ideal for kitchens, bathrooms and shower rooms. Alternatively you can incorporate anti-mould additive to your choice of regular paint for a similar effect.
  • If you are replacing damaged wallpaper it is best to strip off the old wallpaper and treat the walls as above, and then to simply repaint the walls rather than replacing with wallpaper, because mould loves to grow on damp wallpaper lining. If you feel you must hang wallpaper r again then use an anti-condensation wallpaper and anti-fungal (anti-mould) wallpaper paste.

Treating Efflorescence

Condensed moisture can also react with the salts in the skim plaster coat, masonry or stone in your walls. This can cause efflorescence or ‘salting’, which shows up as a fluffy or crystalline coating on the surface of the wall.

If this has happened to your walls you should sand off the salts and then paint on salt neutraliser to neutralise the salts before redecoration, so that they do not simply reoccur and damage all your hard work. There are many types of salt neutraliser available but we tend to use Property Repair Systems Salt Neutraliser.

Salt neutraliser to treat efflorescence on internal walls

Salt neutraliser to treat efflorescence on internal walls

If you think the salting might be caused by penetrating damp then you need to find out for sure what is causing the damp. You can visit our Diagnosing Damp Project or you can consult a structural engineer.
If the problem is diagnosed as penetrating damp then you will need to look at damp proofing before you try to treat symptoms such as efflorescence and mould.

DIY Doctor has a number of Projects about damp and damp-proofing including one about
DIY Damp proofing by injecting your walls and how to fit and repair damaged damp proof course, as well as our project about Dealing with Rising Damp.


Decorating after Condensation Treatment

Once you have got rid of your condensation problem; the walls are no longer running with water; and you have dealt with all that horrible black mould then you are ready to pick out paint colours and start redecorating.

Anti-Mould Emulsion You can purchase low odour anti-mould coating, from Property Repair Systems. This paint combines modern paint technology to make it breathable, plus highly advanced and proven biocides. The paint is guaranteed to protect the surface against unsightly and unhygienic black mould even when there is persistent condensation. The anti-mould biocide is combined throughout the paint film and it is formulated for toughness, elasticity, water resistance and durability.

Anti-Condensation Coating For areas which are not subject to abrasion and washing such as ceilings, the underside of roofing sheets, steel building frames, pipework and ducting, and the inside of cupboards, you can use Lecros Anti-Condensation Coating. This Anti-condensation Coating inhibits condensation by absorbing moisture and improving insulation. It also contains a biocide to protect the paint in the can against mould growth.

Anti-Condensation Wallpaper These are specially formulated wallpapers that inhibit mould growth and allow walls to breathe. They can be used on walls and ceilings and they are usually tough and hardwearing. They are as easy to hang as normal wallpaper so they are a DIY solution to condensation.

There are also foil wallpapers which can be hung on the wall first before papering over with normal paper, but these are not really a condensation solution. They are more suited to areas with no damp proofing or penetrating low level damp such as basements.

Condensation has caused wallpaper to peel off this wall and mildew to grow causing discolouration

Condensation has caused the wallpaper to peel off this wall, and mildew is growing on the paper causing discolouration

Practical Tips to Stop Condensation

Hopefully in this project we have provided you with a load of DIY condensation solutions which will help you deal with condensation, but because it is so long we thought we would give you a quick round-up.

10 Step Guide to Stop Condensation

  1. Ventilate your house – change the air once a day. Open a window, or fit trickle vents. Where you have trickle vents fitted make sure you leave them open.
  2. Fit a fan, and use it – make sure your bathroom fan is working while you are bathing and showering, use your kitchen extractor fan when you are cooking.
  3. Don’t dry washing indoors - if you have a tumble dryer make sure it is properly vented or buy a self condensing model.
  4. Use a Dehumidifier to quickly dry out an area that is very wet.
  5. Use a low cost digital Hygrometer to help you keep your humidity between 55-65% this is the best environment for comfortable living. Don’t forget you can use memory setting to monitor levels of relative humidity in your building throughout the day.
  6. Cold spots are building defects and may indicate failing wall ties, so use an infra red Thermometer to find any cold spots, as well as badly insulated and damp areas.
  7. Test walls and floors with a Damp Meter to make sure that you don’t have water ingress or leaks.
  8. If you have wet walls and want to be sure you have condensation use Condensation Test Strips in affected areas. They will change colour if condensation occurs, ruling out some other types of defect.
  9. Wash down all surfaces, which have been subject to mould growth, with Anti-Mould Cleaner prior to redecoration.
  10. Incorporate Anti Mould Concentrated Additive to all paints that you use (unless you are using Anti-Mould Paints).

In our video section on damp or mould you will find a short film on damp and mould control which gives a brief overview on the problems of this particularly nasty mould.

Don't fancy doing this project yourself? We work with Checkatrade to ensure that we recommend only reliable and trustworthy tradesmen.

All project content written and produced by

Project Feedback