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Understanding damp and mould

Updated: Feb 16, 2023

Damp and mould are frequent issues faced by both landlords and tenants, regardless of the age of the rental property. It's crucial for both to be aware of the reasons behind these problems and the most effective methods for preventing them from worsening.


Condensation occurs in a dwelling when warm moist air produced by ordinary activities such as showering or cooking meets a cold surface such as an external wall or window. The moisture-laden air will remain internally if ventilation does not occur and will gravitate towards the nearest cold surface where it condenses. Moisture is also naturally occurring in the air and when air temperatures drop it will release this water in droplet form. This is known as the dew point. Condensation generally occurs during cold spells of weather. It will appear on cold surfaces and also in microclimatic areas where there is little movement; for example behind a cupboard. This will often lead to the formation of mould growth and mainly occurs in corners of rooms, in cupboards or on north-facing walls, as these are generally the coldest.

Could dampness be caused by something else?

Yes. Condensation may not be the only factor when mould growth occurs.

Rising damp can occur if the damp proof course or membrane within the walls or floors of your dwelling has been breached. Prevalent in ground floor flats as well as houses due to damp proof course failure, something that generally occurs over a long period of time.

Penetrating damp. Rain may also seep through cracks in brickwork or through missing tiles on external roof surfaces.

Blocked guttering may also mean water spills over and saturates external walls.

External plumbing which is cracked may allow seepage into internal parts. It can be difficult to be certain of the exact cause of any dampness and so unless you are sure it may be wise to contact an RICS-qualified surveyor or an experienced contractor who has experience working in this particular area. Condensation will not be limited to certain areas and may cause growth in different areas of a room and you may also notice furniture and clothing becomes affected.

Examples of condensation

Lack of insulation at construction

Properties constructed during certain eras did not have the more stringent construction and insulation levels now required in new development. As a consequence, they are potentially more susceptible to the development of condensation than modern buildings. Areas, where this can occur, are areas of a property that were difficult to insulate at construction such as above or below balconies or undercrofts. Insulation causes a warming effect which helps to counteract the conditions under which condensation can develop.

Cold spots/bridging

Certain construction elements such as concrete beams by their very nature are cold and insulating them to bring them up to modern standards can prove to be difficult. As a consequence, many properties have cold spots or suffer from thermal bridging. Equally, when properties are refurbished insulation can be removed which increases the possibility of cold bridging and the like becoming a problem.

Lack of ventilation

The development of condensation can be controlled in part by increasing ventilation throughout the dwelling. In tenanted properties, there is a preconception that ventilation means higher heating bills and as such there are growing examples of condensation which is exacerbated by the blocking up of trickle vents (vents within window frames) or traditional air vents which are found individually within rooms.

Remedies and Precautions

How to avoid condensation

With the correct balance of heating and ventilation, condensation should be avoided. The heating helps keep the property warm and the ventilation will enable excess moisture-laden air to escape.

TIP: Improving ventilation (opening windows, trickle vents).

Using the thermostat. Set it on for long periods on a low setting or have it switched to operate automatically on shorter periods for at least seven hours a day. Do not adjust the thermostat manually when set but trust the system to regulate itself for you. Make sure there are no cold zones in the home by turning all radiators on

TIP: Improving heating (constant temperature space heating).

Landlords and agents should tell tenants how to best manage any problems and provide the information below:

How to combat moisture production

  • Cover saucepans.

  • Dry clothing outside rather than on radiators.

  • Wipe away condensation as quickly as it’s spotted.

  • Keep window trickle vents open constantly and open windows as much as possible (especially after cooking or showering) to allow a thorough flow of air whilst maintaining a heat balance.

  • Ensure extractor fans are operational, you can test pull by holding a sheet of tissue paper against it and seeing if it sticks.

  • Turn on the cold tap of the bath first so that when the hot water hits it doesn’t produce as much steam.

  • Close doors in wet areas to stop the spread of moisture to other rooms.

  • Where possible position cupboards and drawers etc. against internal walls.

Interesting fact

How much moisture is typically produced in the home?

• Drying clothes produces ten pints of water in an unvented tumble dryer.

• Having a bath produces two pints of moisture.

• Washing clothes produce one pint of moisture.

Fighting mould

  • Remove mould growth by wiping down walls and windows with a fungicidal wash recognised by a Health and Safety Executive ‘approval number’. Follow the manufacturer’s instructions precisely. Dry-clean mildewed clothes and shampoo carpets. Take care because disturbing mould can increase the risk of respiratory problems.

  • After treatment, redecorate using good quality fungicidal paint to help prevent mould from recurring.

  • Ensure insulation to external walls is in place

  • For landlords who want to take more intrusive measures to deal with structural problems consider:

  1. Increasing insulation (EWI, IWI, cavity fill, double glazing)

  2. Improving ventilation (fan installation, opening windows, trickle vents, Passyfier Vents)

  3. Improving Heating (constant temperature space heating)

The Regulator of Social Housing (RSH) has recently concluded an investigation into the issue of damp and mould within the social housing sector. The initial findings of the report suggest that 3-4% of local authority and housing association properties in England, approximately 120,000 to 160,000 homes, have issues with damp or mould. Out of these, 1-2% have been identified as having serious enough problems to qualify as a Category 2 hazard under the Housing Health and Safety Rating System (HHSRS), with 0.2% (about 8,000 homes) classified as a Category 1 HHSRS hazard – the most severe category.

The RSH also discovered that many social landlords had little awareness of the number of properties affected by damp and mould. They have now called for an awareness campaign to help resolve the issue. According to a spokesperson for the RSH, the main causes of damp in social housing are condensation from cooking and showering, as well as rain getting into the property and rising damp caused by faulty damp proof courses.

The study did not cover privately rented accommodation, although damp and mould issues are also common in such homes. The latest English Housing Survey found that 4% of homes had damp serious enough to warrant a HHSRS assessment, while 11% had some degree of damp, which is more than the proportion observed in the social housing sector.

Although landlords and agents in the private rented sector have made progress in tackling damp problems, with the proportion of homes affected decreasing from 10% in 2001 to the current figure of 4%, there is still work to be done. The rising cost of living may threaten this progress as tenants may lower the thermostat or stop opening windows to save on bills. Therefore, private landlords and agents must remain vigilant and proactive in addressing damp and mould issues in their properties.

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