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Condensation and Mould

Mould in a property can be very unpleasant and, if not addressed, a risk to your health.

It is essential to your health you ensure you are on top of the condensation that causes most mould in domestic properties.


What are the causes? - Mould

Most cases of mould in domestic properties are caused by moisture. It will usually appear as black growth in crevices, corners and around the edges of fittings, such as baths, windows, cupboards etc., but can appear anywhere.

It is very rarely caused by ‘damp’ from the ground.

Occasionally it is caused by some leak, e.g. a roof, or a gutter.

By far the most common cause of mould growth is condensation from inside the property linked directly to the people living in the property.

What affects humidity?

Simply it’s a combination of temperature and moisture!

As temperature rises, the air holds more water – we then have that ‘humid’ feeling of the place being muggy or stuffy.

As the temperature drops, it can’t hold as much moisture.

Whether hot or cold, when the air has more moisture than it can hold, the moisture comes out of the air as condensation on any cooler surface, unless the moisture is removed by ventilation or an extractor.

What are the causes? - Condensation

Over the years in our efforts to have warmer homes and reduce energy wastage we have progressively restricted the movement of air between the inside of our homes and outside. This means any moisture produced in the home tends to stay there.

This moisture is converted into liquid (condenses) when this moist air touches a surface which is relatively cold eg:

  • a window, particularly in a deep recess from the room
  • a corner furthest away from a radiator or on an outside wall
  • anywhere with cooler air than the rest of the room

These tend to be the areas where the black mould appears. Mould won’t grow without this moisture.

Seriously? How much moisture can we really produce?

Well, quite a lot actually. We put the following moisture into the air:

  • One person’s breathing – 0.5-1 litre over 24 hours
  • Cooking – 0.5 litre for each hour
  • Shower – 0.25 litre for 5 minutes. 1 bath – up to 1 litre.
  • Clothes washing and drying – 2 litres for every load of washing.

This all goes into the air and stays in the house unless it is removed.

This obviously increases when there are more people, or people are there longer (e.g. at home with children rather than out), or clothes are dried indoors, or extractor fans are not used, or doors are left open when showering to let moisture into the rest of the house. Even people exercising in the house substantially increases their breathing rate and adds their sweat to the moisture in the property.

That moisture all has to go somewhere – Just imagine what 0.5 litres (about 1 pint) of water would look like spilt on the floor!

What can be done to reduce the condensation?

Produce less moisture

  • Cooking: Cover pans; do not leave kettles boiling;
  • Washing clothes: Put washing outdoors to dry if you can, or put it in the bathroom with the door closed and the window open and/or fan on. Use the highest spin setting on the washing machine. If you have a tumble-dryer make sure you vent it to the outside (unless it is the self-condensing type). DIY kits are available for this.

Remove the moisture

  • Ventilation: Keep a small window ajar or a trickle-ventilator open when possible. You can ventilate your home without making draughts.
  • Mopping: Wipe away visible condensation and wring out the cloth into a drain.

In some conditions opening windows during cold weather can attract condensation, where this happens it is necessary to be more diligent in mopping to remove the moisture.

You need much more ventilation in the kitchen and bathroom when cooking, washing up, bathing and drying clothes. If there is an extractor fan installed, then make sure this is working and used. If not, let us know.

Close the kitchen and bathroom doors when these rooms are in use, even if your kitchen or bathroom has an extractor fan.  Doing this will help stop the moisture reaching other rooms, especially bedrooms, which are often colder and more likely to suffer condensation. The best kitchen extractors are those that push the air outside through a vent. Those which simply recirculate filtered air remove the smell of the cooking, but do not remove the moisture.

Ventilate cupboards and wardrobes. Avoid putting too many things in them, as that stops the air circulating. Leave space between the back of the wardrobe and the wall, particularly in new homes. Put floor-mounted furniture on blocks to allow air underneath. Where possible, position wardrobes and furniture against internal walls (walls which have a room on both sides) rather than against outside walls – these walls are colder and the furniture prevents airflow, making a perfect hidden space to grow mould in your home.

Make sure nothing is placed against any vents or airbricks.

Heat your home a little more

In cold weather the best way to keep rooms warm enough to avoid condensation and mould is to keep low background heating on all day, even when there is no one at home. This is very important where the bedrooms are on the ground floor, so not above a warm living room. Thermostatic radiator valves are an excellent means of doing this. There is much advice showing leaving radiators on all day at a low level is more cost effective than short periods of heating at high levels.

If you can’t afford to keep the heat on all day, it is better to spread out the periods when it is on, in particular ensuring there is a period before bedtime.


Cleaning of affected area

Mop up condensation whenever you see it. Ensure you then wring cloths out into a drain. Don’t leave cloths to dry in the house, all you have done is moved the moisture about, not got rid of it!

If mould appears clean with a detergent and thoroughly dry the area, then (unless it is a fabric surface) apply a 50/50 mix of bleach and water, an anti-mould preparation, and leave to dry. Once this has dried keep the area clean and dry. Test all treatments on an inconspicuous area first to ensure you do not damage the surface.

This cleaning is important as mould spores can survive long after the mould has apparently disappeared. Mould will reappear if the damp conditions continue. This is particularly true if you have moved furniture from a house with mould, as this will bring mould spores into your home, which will grow as soon as conditions allow.

How much humidity is there?

Low cost hygrometers are available which record temperature and humidity. Many have an app which provides a graph to show changes, which is helpful in identifying how things affect temperature and humidity.

We recommend using one and seeing how the humidity changes for example:

  • When having a shower without opening a window compared with the window being open
  • Overnight in your bedroom when there is no ventilation, compared with when the windows are locked on vent (slightly open but secure).
  • When drying clothes inside
  • When reducing or increasing the heat

This will clearly show how quickly humidity can rise to dangerous levels and how easily it can be removed, with little impact on the temperature of your home.

Ideally the humidity should not be above 65% for any prolonged period, if it is, you are at increased risk of condensation and mould.

What should you remember?

  • Produce less moisture

  • Remove the moisture through ventilation & mopping

If you eliminate the moisture you will not have an issue with mould

Your local council also produces advice on managing condensation and mould e.g.




South Cambridgeshire


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