Damp, mould and condensation in the home

Condensation occurs naturally in homes when moisture comes into contact with cold surfaces such as walls or windows.

Condensation in your home can be caused by:

  • cooking
  • cleaning
  • bathing
  • washing Up
  • drying clothes indoors
  • poor ventilation
  • inadequate heating.

Where there is a lot of condensation it can cause problems with damp and mould.

What is condensation?

Infographic showing effect of warm air on cold glass to create condensation

Condensation is water which has “condensed” from warm, moist air, on contact with a cold surface. Air holds water in the form of water vapour (moisture).

There is always some moisture in the air even if you can’t see it. Ordinary air seems to be dry. It does not cause wetness on our body or clothes and cannot be detected directly by the senses. Air has a capacity for holding water vapour (otherwise known as humidity), but it is limited by temperature. The hotter the air the more water vapour it can hold. When the air holds the maximum water possible at its temperature it is said to be ‘saturated’.

Surface condensation occurs when moist air comes into contact with a suitably cold surface - any surface including walls, floors, sub-floor areas, roof spaces, etc.

As the moist air gets close to the cold surface, the air starts to get cooled and so the relative humidity increases; the greater the air is cooled, the higher the relative humidity.

Against the cold surface the temperature of the air now drops below the dew point temperature and liquid water drops out as condensation.

Condensation example:

Take a glass which is absolutely dry on the outside. Put some ice cubes in it. Within a short time you will observe that tiny droplets of water form on the outside surface. This occurs because the air layer touching the outer surface of the glass becomes very cold and condenses the water vapour in it to droplets.

Condensation facts:

Our body cools itself by opening pores on the skin and releasing water and salts; sweating.

Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This is because the rate at which water (the sweat on the skin) evaporates depends on how much water is already in the air. On dry days, sweat evaporates quickly, which means it also carries away heat faster. On humid days, when the air is already saturated with water, sweat evaporates more slowly and therefore we find it harder to cool down.

Relative humidity

Relative humidity is the amount of moisture in the air compared to what the air can "hold" at that temperature. When the air can't "hold" all the moisture, then it is saturated and condenses as ‘dew’.

It is expressed as a percentage of how much moisture the air could possibly hold. Relative humidity is a combination of the actual moisture content of the air and the temperature. The amount of vapour that can be contained in the air increases with temperature. The higher the percentage of relative humidity, the more humid (moist) the air. Saturation is equal to 100 percent relative humidity.

Infographic showing water vapour and relative humidity at different temperatures

The usual sequence of events is as follows:

  • cold air enters the building
  • the air is warmed for the comfort of the occupants
  • the warm air takes up moisture
  • the warm, moist air comes into contact with cold surfaces, walls, windows, etc. and is cooled below its dew point
  • condensation occurs as the excess moisture is released.

Why do you have condensation in your home?

Until the 1960s condensation wasn't a major issue within buildings, but with changes in building design and in living styles condensation has become a major problem.

Condensation is the most common form of dampness in buildings. It is a problem in modern properties due to the introduction of double glazing and draught exclusion which cut down the natural ventilation of the property.

Older properties are less likely to be severely affected by surface condensation as they are not as well insulated a new properties with the latest glazing and insulation.

3 factors contribute to the condensation of water on building surfaces:

  1. high humidity of indoor air
  2. low temperature of the walls and surfaces
  3. poor ventilation.

3 factors of condensation

humidity of indoor air

Condensation appears when the indoor air in a room cannot hold the level of moisture contained (saturated).

Low temperature

Warm air can hold more moisture than cold air. When the warm humid air comes into contact with cold indoor surfaces, it cools and condensation occurs.

Poor ventilation

Humidity of indoor air can be reduced by ventilation. If there isn't enough ventilation then humidity accumulates indoors and leads to increased condensation. Walls will also remain cool when a lack of free movement of indoor air prevents warm air from reaching them.

Where does water come from in the first place?

Moisture comes from normal everyday living. The amount of water produced from normal household activities can be quite a lot including breathing by the household occupants and pets.

It is reported that a large dog can give off even more water vapour than the average adult. Uncovered fish tanks another large vapour producer.

Certain other activities such as using bottled gas and paraffin heaters add significant amounts of water to the air, water being a by-product of burning these fuels.

Drying clothes over radiators will also significantly add water vapour. The table illustrates the amount of water that could be generated in an average house during a 24 hour period.

Water Vapour source in an ‘average’ house per day Approximate water generated
4 to 5 people asleep in a property 1.5 litres
2 people active in a property 1.6 litres
Cooking 2.6 litres
Washing up 1.0 litres
Washing laundry 4.0 litres
Drying laundry 4.5 litres
Bathing or washing 0.5 litres
Approximate total: 15.7 litres

Damp walls due to rising or penetrating damp, and damp floors do not add significantly to the humidity in the air because water evaporation from such 'static' surfaces is very low compared to breathing and other active water producing activities.

Diffusion

Moisture moves around the home through a process called diffusion. Diffusion occurs if some part of your home has a higher moisture level than another part, such as the movement of moisture from the bathroom to the bedroom after a hot shower has filled the bathroom with steam.

Diffusion happens even if there is no air movement at all. Just as heat travels from a hot space to a cold space, even if it has to go through a wall, water vapour will travel from a space with a high moisture concentration to a space with a lower moisture concentration, even if it has to go through a wall.

Cold air almost always contains less water than hot air, so diffusion usually carries moisture from a warm place to a cold place.

Condensation summary

The amount of moisture that the air can hold increases as the temperature rises and decreases as it falls.

Just by living we put moisture into the air of our homes. As the temperature of the home rises and falls the amount of moisture the air can hold rises and falls. If it falls a lot, or there is a lot of moisture in the home then at some point it will "condense" on to cold surfaces. These are most often the windows or ceramic toilet cistern, but may also be walls, furniture, clothes etc.

If moisture is condensing on walls, furniture, clothes you may get mould growing in your home.
Over recent years like many landlords we have been tackling fuel poverty and carbon reduction by increasing insulation, installing better doors and double glazed windows. There is no doubt that these help to keep the heat in, but they also keep the moisture in. Unlike years ago, homes now need your help to keep the moisture low.

Moisture is measured by "relative humidity". If the relative humidity is consistently above 60 percent then you may start getting problems with damp and mould.

Every one degree Celsius drop in temperature increases the relative humidity by 5 percent and vice versa. So as your home gets cooler during the night the chance of there being a problem increases.

We know that heating fuel is expensive so first of all try to reduce moisture.

How to reduce moisture in your home

You have 2 options for reducing moisture; to reduce moisture by the biggest amount, please use them both.

Put less moisture into the air

  • put lids on saucepans
  • use a microwave rather than hob where you can
  • keep the kitchen door shut when cooking
  • don't boil too much water in your kettle
  • try to dry laundry outside whenever you can
  • if you have a tumble drier, make sure it is vented to the outside
  • try to avoid drying clothes on radiators
  • the best place to dry laundry is in the bathroom, with the door closed and an extract fan on or window slightly open
  • keep the bathroom door closed when bathing or showering, and keep it closed until the steam all goes. Use the extractor fan if you have one, and / or open a window
  • don't leave water standing, e.g. in a kitchen sink or bath
  • mop up any spillages or condensed water, don't let them evaporate into the air.

Get the moisture out of your house

  • don't close or block the trickle vents in your windows
  • use your fans and other ventilation products; They don't cost much to run
  • open windows for a few seconds in the morning; The damp air will go outside
  • where possible don't let your home get too cold.

The means of getting moisture out of the house is to ventilate, and often you will be sending warm air out with the moisture. When ventilation your kitchen or bathroom, keep the doors closed, so you are minimising the amount of heat that gets away.

By keeping your home warm you will also warm the walls and surfaces of your home. A combination of warm air and warm surfaces will allow more moisture to stay in the air within your home and will reduce the extent of condensation you experience.

If you don’t have heating in every room, open doors of unheated rooms to allow some of the heat from heated rooms to move into them.

Do not use paraffin or bottled gas heaters as when lit these produce water which will add to the level of moisture in your home.

Do not push beds or sofas against radiators as this prevents efficient circulation of warm air.

Black mould

Water vapour in the atmosphere alone causes no problems - certainly not health problems; constant inhalation of very dry air can cause health problems.

Condensation and maintenance of high humidity does lead to mould growth. Most moulds need 24 - 48 hours of moisture to begin to grow.

If a suitable material in your home is wet for more than 24 hours then you run the risk of mould starting to grow., usually detected by the musty odour associated with damp.

If condensation isn’t dealt with it can go on to help black mould to start growing on surfaces in your home.

If the conditions allow mould to grow then mould spores in large numbers may cause some to experience health problems.

Signs of black mould

The signs of mould being present are often found on north-facing walls, which is the cooler side of any home, and especially in corners of rooms or behind wardrobes and cupboards where there is little movement of air.

In places where low ventilation comes together with cold surfaces, they become the risk areas for mould growth. Most moulds grow at a relative humidity of 70 percent or higher.

Some materials mould commonly grows on in the home include:

  • wood
  • carpet
  • food
  • paper
  • insulation
  • wallpaper
  • paint
  • wallpaper glue
  • plasterboard
  • fabrics
  • cotton
  • books
  • leather
  • chipboard
  • furniture
  • dust
  • ceiling tiles
  • almost any other organic material.

The most common mould associated with condensation is the 'black spot' mould, Aspergillus niger. However, other moulds may also develop - it depends on the material and conditions.

Some moulds will grow on leather if the relative humidity is maintained around 76 percent, or grow on on brick and paint if the relative humidity of 88 percent or higher.

If you find mould

Mould is a part of the natural environment. It is found outdoors and indoors in just about every environment, including your home and office. Mould serves an essential part of our ecosystem to break down dead organic matter such as dead leaves, trees, etc.

Mould is already in your home; Mould and mildew are virtually always present in buildings.

It is only when mould is found in high levels that it can become a problem. Moulds are not picky eaters and can find 'food' to grow in any home. Many building materials (from wood to plastic foam) provide nutrients for moulds. When nutrients are combined with moisture, moulds can thrive.

Mould growths cause the most concern because they produce a musty odour, and can cause decorative and fabric spoiling in some cases.

When mould has grown, it requires persistently high relative humidity to continue to spread.

Moulds develop in areas where air flow is limited and the air remains damp and stagnant, e.g. corners, floor/wall junctions, etc.

Humid conditions suitable for mould can arise naturally during humid warmer summer months, but they do not last sufficiently long enough to maintain any mould growth.

You cannot eliminate mould from your home. But you can control the relative humidity to minimise its presence.

Health effects of mould

The most common proven health effect associated with moulds is asthma. However, smoking, pets and dust mites are also known trigger factors for asthma; dust mites also being found in greater numbers in 'damp' environments.

There have to be large numbers of mould spores or very large amounts of mould growth before health effects become noticeable in normal healthy people, although some may be more susceptible than others.

Hidden mould

Mould hidden in wall cavities does not present a significant exposure threat. Industrial Hygienists have performed hundreds of building assessments for moulds and bacteria and have never encountered a single project where sampling (air sampling or bulk sampling) was successful in discovering a hidden mould problem that was overlooked by normal visual inspection.

How to prevent condensation and mould growth

As previously mentioned, 3 factors contribute to the condensation of water on building surfaces:

  • high humidity of indoor air
  • low temperature of the walls/surfaces
  • poor ventilation.

Mould grows and feeds on organic substances such as wood or cotton. Mould should not grow on surfaces like plastic, metal or glass unless there is some other organic substance which it can feed on.

Methods to prevent black mould from growing

Dust and vacuum

You should clean regularly to reduce dirt and grime which mould can live on. It has been found that 80 percent of mould grows on dust, so dust and vacuum often to remove dust.

Watch your humidity

Many species of mould can begin to grow from humidity alone if the humidity stays high for long enough. Humidity only needs to be higher than 55 percent for some moulds to begin to grow. The best way to keep humidity low in your home is through adequate ventilation. Open the windows during the day, especially when it's hot since this is when humidity is usually the lowest outside. Close your windows when it's raining outside though.

Extractor fans will help reduce high humidity

In rooms where steam and moisture builds up it's important to ventilate the room, to remove the moisture, like the kitchen and bathroom. Extractor fans help to reduce the humidity when doing things like cooking, washing dishes or taking a shower.

Don’t leave wet clothes hanging around

A cause of moisture problems in homes is wet clothes. After you've washed your clothes you should immediately dry them. Don't leave them sitting in a wet pile for a long time. Make sure not to leave any wet clothes lying around waiting to be washed too.

Dry your laundry outside

It's best to dry your laundry outside on a clothes line if you can. Hanging them inside on an airer or indoor clothes line will not dry them as quickly and the moisture from your wet laundry will evaporate into the air within the room, raising the humidity.

If you dry your laundry in a tumble dryer inside your home, then you should make sure the hot air produced by the tumble dryer is vented outside.

Make sure the room where you're drying your laundry is well ventilated; a good method is to dry laundry in your bathroom, close the door during the drying process and ventilate the room by opening the window or using the extractor fan.

Let natural light in

Mould prefers to grow in dark spaces indoors. Open curtains or blinds in rooms during the day to allow sunlight, as this in will reduce the chances of mould growing.

Take action against mould

Once mould has begun to grow in your home it's not enough to just take away the mould's moisture source. Mould that runs out of moisture can lie dormant for a long time without dying. So if you already have mould growth in your home you need to take steps to have it removed.

How to clean mould

There are several products you can use to kill and remove mould. Some of the most effective mould removal products include:

Bleach

Bleach can kill virtually every species of indoor mould that it comes into contact with, along with its spores, leaving a surface sanitised and resistant to future mould growth. The active ingredient in bleach, sodium hypochlorite, is the main ingredient in many mould removal products, there are many reasons to use alternatives to chlorine bleach when killing mould.

Only use bleach on non-porous materials

Using bleach is only effective if the mould is growing on non-porous materials such as tiles, bathtubs, glass and countertops. Bleach cannot penetrate into porous materials and so it does not come into contact with mould growing beneath the surface of materials such as wood and drywall.

Using bleach on porous materials will kill the mould on the surface but the roots within the material will remain and the mould will soon return as the chlorine in bleach cannot penetrate into porous surfaces such as drywall or wood.  The chlorine is left on the surface of porous materials and only the water component of the bleach is absorbed into the material. This will provide more moisture for the mould to feed on. As the roots of the mould are left intact, the mould soon returns, leaving you in a cycle of repeated bleaching. 

Bleach is toxic and corrosive

Another disadvantage of bleach is that it can damage the materials it's used on as it is a harsh, corrosive chemical. Chlorine bleach also gives off harsh fumes and it even produces toxic gas when mixed with ammonia. For these reasons try to avoid using bleach and if you must use it, only use it on non-porous surfaces.

White vinegar

Vinegar is a mild acid which is natural and safe and can kill 82 percent of mould species. Vinegar is non-toxic and doesn't give off dangerous fumes like bleach does.

To kill mould with vinegar, use white distilled vinegar which you can buy cheaply from the supermarket.

How to kill mould with Vinegar

  • pour some vinegar into a spray bottle without watering it down
  • spray the vinegar onto the mouldy surface and leave it to sit for an hour
  • wipe clean the area with water and allow the surface to dry
  • Any smell from the vinegar should clear within a few hours.

If you want to use vinegar to prevent mould growing on surfaces just spray vinegar on the surface and leave it. Repeat this every few days to ensure the surface will stay mould-free.

You can even mop your tiled bathroom floor or other hard non-porous floors with vinegar if you are worried about mould growing on them.

Baking soda

Baking soda is well known as a natural and safe household cleaner, you can use to kill mould in your home. Unlike other mould killers which contain harsh chemicals, baking soda is mild (pH of 8.1) and harmless to your family and any pets.

Baking soda deodorises and using it can get rid of the smell mould leaves in your home. Baking soda also absorbs moisture to help keep mould away.

Vinegar can be used along with baking soda when cleaning up a mould problem since vinegar kills different species of mould to baking soda.

How to kill mould with Baking Soda

  • add one quarter of a tablespoon of baking soda to a spray bottle of water
  • shake the bottle to dissolve the baking soda into the water
  • spray the mouldy area with the baking soda and water solution
  • then use a sponge or scrubbing brush to make sure to remove all the mould from the surface
  • once you've scrubbed away the mould rinse the surface with water to remove any residual mould on the surface
  • spray the area with the spray bottle again and let the surface dry.
  • This will kill any left-over mould and prevent the mould returning.

Alternatively you can use a cloth instead of a spray bottle to clean mould with baking soda:

  • soak a cloth in water and then add one quarter of a tablespoon of baking soda to it
  • use the cloth on the mouldy area to remove the mould with the baking soda and water solution.

Tea tree oil (Melaleuca Alternifolia)

Tea tree oil is an antifungal and antibacterial essential oil,  which the most effective natural mould killing solutions.  Although it can expensive, a small amount of tea tree oil goes a long way in killing mould and is capable of killing all types of moulds. Tea tree oil is harmless to people and pets.

You can buy tea tree oil from most natural food stores. Make sure the tea tree oil you buy is derived from the Melaleuca Alternifolia, which is the technical name for tea tree, as not all brands always are.

How to kill mould with Tea Tree Oil

  • to kill mould using tea tree oil add water to a spray bottle, keeping in mind how many cups it takes to fill the bottle
  • next add tea tree oil at the ratio of one teaspoon per cup of water that went into the spray bottle
  • spray the solution on the mouldy surface.

There is no need to rinse since leaving the tea tree oil on the surface will kill the mould and prevent it from returning.

Alternatively you can use a rag or cloth with the tea tree oil solution to clean away mould:

  • first create a solution of tea tree oil and water in the ratio of 1 teaspoon per cup of water
  • use a cloth to apply the solution to the mouldy surface and scrub the mould away.

Again, you do not need to rinse the surface afterwards.

Tea tree oil has a strong smell but it will go away after some time. You can keep and use the solution you have made for a long time afterwards as tea tree oil does not lose its potency quickly.

Grapefruit seed extract

Grapefruit seed extract is similar to tea tree oil in that it is an expensive but very effective natural mould killer. The advantage of grapefruit seed extract over tea tree oil however is that it has almost no odour. Like tea tree oil you can buy grape fruit seed extract from most health food stores.

Grapefruit seed extract kills mould naturally as the citric acid from the grapefruit attacks mould. Grapefruit seed extract also disinfects areas and deodorises as well. Like tea tree oil, a small amount of grapefruit seed extract goes a long way in killing mould.

How to kill mould with Grapefruit Seed extract

  • to kill mould with grapefruit seed extract create a solution of grapefruit seed extract and water in a spray bottle in the ratio of 10 drops of grapefruit seed extract per cup of water
  • shake the spray bottle to mix the solution thoroughly and then spray it onto the surface where mould is growing
  • you do not need to rinse the solution away afterwards although you can use a cloth to wipe away the mould and solution after some minutes if you like. The longer grapefruit seed extract is in contact with mould the more it will cut through and kill the mould colony and prevent mould from returning.
  • Repeat if needed to more thoroughly remove mould from the surface.

The grapefruit seed extract solution in the spray bottle will remain potent for a long time and can be reused again and again as grapefruit seed extract has a long shelf life.

After cleaning up mould due to condensation, stopping the dampness from coming back means understanding and dealing with each of the causes of condensation we have explored.

Mould summary

  • The main things mould needs to grow in a home are organic materials to feed on and moisture
  • In houses there are always plenty of organic materials for mould to live on such as wood, drywall and various other building materials
  • Moisture, on the other hand, can be controlled practically and so keeping the moisture in your home low is the best way to prevent mould growth
  • Most moulds need 24 - 48 hours of moisture to begin to grow. Therefore if a suitable material in your home is wet for more than 24 hours then you run the risk of mould starting to grow
  • There are a few main things which usually cause moisture problems in the home. One is water leaks. These include things such as leaking roofs or walls, leaking pipes and leaking taps or a leaking shower
  • Condensation is another frequent cause of moisture. Condensation forms on cold surfaces when water vapour in the air cools and becomes liquid. Often you'll see condensation on metal pipes, concrete walls, water tanks and windows
  • Mould spores are everywhere in the air outside. Mould spores enter homes through windows, doors, air ducts, etc. They can also be transported inside attached to skin, clothing, hair, pets, etc. They float through buildings all the time and there is no practical way to remove all mould spores indoors
  • However if the concentration of mould spores inside is significantly higher than outside then it can start to cause health issues. A higher amount of mould spores also increases the potential for mould problems to start
  • Although you cannot eliminate all mould spores inside your home or prevent all mould spores from entering, minimising the favourable conditions for mould to thrive will lessen the chance of mould beginning to grow in your home and prevent you from suffering mould related problems.

Remember:

Produce less moisture

  • if you have a washing machine or tumble dryer in your property, ensure that it is vented correctly. From just one load of washing 2 litres of water is emitted into the air
  • where possible, try to dry your laundry outdoors to prevent excess moisture from building up in your property. If you are unable to dry your laundry outdoors, then keep them in a bathroom with the door closed and windows open until the clothes are fully dry
  • when cooking food, boiling the kettle or taking a shower, ensure that your kitchen or bathroom door is kept closed to prevent the moisture in the air from going into colder rooms, which will cause condensation to form if it touches a cold surface
  • when cooking ensure that you cover your pans with a lid to reduce moisture being created from the water boiling. Use an extractor hood if you have one above the cooker or an extractor fan if you have one installed, as these are designed to help reduce moisture created when cooking. Remember, don’t turn off your extractor fan as soon as you finish cooking as the moisture can still be in the air even when you have finished, instead leave it on afterwards for 10 to15 minutes to help to clear the humid air
  • portable gas bottles and paraffin heaters produce a lot of moisture, along with a lot of toxic fumes. Not only is this form of heat causing excess condensation in your property, it is also a health and safety hazard which is stated in most tenancy agreements as not allowed in rented flats
  • many families have house pets and plants which produce a lot of moisture. Make sure you cover up your aquarium or fish tanks to prevent excess moisture. If damp patches start to appear on your walls or you start to notice more surface condensation on your windows and walls near to your house plants then look to move them outdoors.

Ventilate to remove moisture

  • When you are taking a shower or having a bath ensure that you turn on your extractor fan to remove the steam and moisture that is created when running warm water in a cold environment. This will help reduce the amount of condensation that appears on your bathroom windows and walls
  • cooking, bathing and showering all produce steam. Open the window or put on an extractor fan, and close the door to prevent the damp air circulating into other rooms
  • at other times, leave all the doors to different rooms open to allow the air to circulate
  • if you use a room on a regular basis, such as a living room and the weather is not cold outside, open a window slightly to improve the ventilation in the room. Breathing is a major cause of condensation so this will help to improve the ventilation in your property
  • to avoid condensation in bedrooms, open the windows for 15 minutes each morning
  • do not overfill your wardrobes or kitchen cupboards. A lack of ventilation and air moisture trapped in warm overfilled cupboards can become a breeding ground for mould as the air is not able to circulate freely inside. You might notice a musty smell or clothes might have a damp feeling to them which is a sure sign that the cupboard is overfilled
  • for the same reason as above, make sure that your furniture is at least 50mm away from the surrounding walls so that air can circulate around the property. Try to ensure that your wardrobes are placed against internal walls in your bedroom which will be less cold than external walls and less likely to cause damp and mould problems.

Heating a home

  • Heating a property will improve the internal temperature of surfaces in the house and reduce the likelihood of condensation
  • the ideal temperature usually ranges between 19 to 22 degrees Celsius in living rooms, kitchens and bathrooms and 16 - 20 degrees Celsius in the bedrooms
  • Ideally, the temperature in a property should not drop under 15 degrees Celsius to avoid condensation and increased humidity levels

The Government have produced a list of energy saving tips on their Help for Households website:

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Page last updated 11 September 2025

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