Dampness and condensation

With the colder, wetter winter months well and truly here, we hope to answer some common questions asked regarding dampness in homes and buildings such as:

  • Why is my home damp?
  • What is causing damp in my home?
  • What is dampness?
  • Why is there black mould?
  • What is condensation?

Why is my house damp?

Dampness can be caused by many factors, can be a complex subject and is often misdiagnosed. A suitably qualified, experienced surveyor employing the correct technical equipment will be able to provide an accurate diagnosis and recommend a schedule of repair works which should aim to tackle the root cause of the problem and provide an effective solution.

The most common causes of dampness are as follows:

Building defects leading to water ingress and subsequent dampness, such as dislodged slates, leaking gutters and pipes, cracks in brick walls and raised ground levels around the perimeter of a house.

Rising dampness caused by omission or failure of a damp proof course in walls allowing dampness from the ground to rise through microscopic pores in solid materials such as brickwork.
During the colder months, dampness brought about by condensation is perhaps the most common form of dampness found in our homes.

What is condensation?

Moisture held in the air in its gas state known as water vapour. When air cools for whatever reason, its ability to hold water vapour reduces and so at a certain point known as ‘dew point’ this moisture in the air turns back into a liquid and so condensation is formed which is seen as small droplets of water.

Condensation is often seen on the inside of windows, mirrors, cold glasses and in our breath on a cold day.

What causes condensation?

Condensation is often the result of high internal humidity levels, poor heating and management of central heating, low levels of insulation and inadequate ventilation.

High humidity levels and subsequent condensation cause secondary problems such as the formation of moulds and high concentrations of dust mites both of which are known to cause health problems, specifically respiratory such as Asthma in some people. Other problems include staining, damage to decorations and decay / infestation of timber.

The appearance of other moulds can be white, yellow and green depending on the type.

Internal humidity levels are raised by day to day lifestyle activities such as cooking, bathing, showering, breathing and drying clothes inside the home. It would follow that higher occupancy levels and numbers of pets also contribute significantly to the generation of water vapour held in the air.

What is black mould?

The most common form of black mould also known as ‘Toxic mould’ is ‘Aspergilus Niger’ and in high concentrations can be injurious to health.

Condensation and associated moulds form typically near windows, corners of rooms, behind and inside furniture and at the junctions of walls and floors / ceilings.

Hydrophilic moulds such as ‘Aspergilus Niger’ require surface moisture. Internal dampness brought about by, for example, rising damp is not usually sufficient to germinate and sustain them.

How do I prevent condensation and black mould?

Black mould

  • Wipe down condensation and clean off black mould regularly using an HSE approved mould cleaner.
  • When re-decorating use a fungicidal paint or add fungicidal additive to paint and use a fungicidal paste when wallpapering.
  • Buy or hire a good de-humidifier. Many on the market are not very effective so do your research. However this should only be a temporary measure and will not address the root cause of the condensation.
  • Dry clean mouldy clothes and shampoo carpets. Hovering and brushing will release into the air millions of spores and breathing problems.

The most effective way to tackle such problems is to tackle the root cause of condensation. We would always recommend the use of a suitably qualified and experienced specialist surveyor trained and accredited by The Property Care Association (PCA) with the CSRT qualification (Certified Surveyor in Remedial treatment).


  • Manage and balance heating, ventilation and insulation. Remember condensation is caused by excessive moisture in the air coming into contact with cold surfaces.
  • Examine lifestyle activities such as bathing, showering, washing and clothes drying indoors.
  • Homes with relatively large numbers of occupants and pets compared to the volume of the property may require additional measures to prevent condensation.
  • When cooking, use the minimum amount of water with pan lids on and turn the heat down once boiling point is reached. Ideally a kitchen should have mechanical air extraction preferably with a self-regulating humidity tracking function to ensure humidity levels do not reach critical levels. A large proportion of homes either do not have any air extraction or have inadequate, poorly maintained inefficient fan units. Ventilation is now deemed to be so important that there are formal standards regarding the installation and site testing of domestic ventilation systems in new homes with minimum standards of air extraction for certain types of room.
  • A suitably qualified domestic ventilation surveyor / installer (BPEC) with the correct technical equipment should be able to assess the suitability and efficiency of existing systems.
  • When bathing, fill the bath by running the cold water first. Keep the door closed and window open if there is no mechanical air extraction.
  • Do not dry clothes indoors on radiators or clothes maidens.
  • Don’t use the cooker to heat the kitchen or use a bottled gas heater indoors as they both produce lots of water vapour in the air.
  • Use passive air vents in the wall preferably at higher level on the wall.
  • Make use of trickle vents in windows or vapour vents.
  • Close kitchen and bathroom doors when in use as vapour spreads quickly around the home and will condense out on relatively cold surfaces. Bungalows and upstairs bedrooms with poor loft insulation above tend to be more vulnerable.
  • Allow space in and around furniture. If necessary, install vents in wardrobes, remove false backs. Position furniture against warmer internal walls.

Ventilate and remove humidity

Mechanical ventilation of the correct type will not necessarily lead to draughts. Some ventilation is required to remove excessive moisture produced by lifestyle activities such as breathing, bathing and cooking. Natural ventilation can be achieved by opening windows and trickle vents in windows.

Rooms where large volumes of steam are produced such as kitchen and bathroom should ideally have mechanical ventilation. The best type is humidistat controlled where the fan will adjust its speed automatically to track humidity levels ensuring the relative humidity within a particular room remains within certain limits. When using such rooms for cooking and bathing keep the doors closed to avoid humidity spreading to other rooms.

Positive input ventilation (PIV – Very effective)

This can be a very effective way of reducing humidity levels in the whole of a property and which comprises a central unit usually sited in the loft which pushes relatively fresh air into the building causing a slight pressure. The internal relatively stale humid air is gradually pushed out through natural ventilation for example gaps in doors and dilutes the more humid internal air thereby reducing the root cause of condensation.

Central heating

Manage central heating carefully especially in the colder months when your home may need to be warmed a little more. It is better to have a relatively low constant background warmth than to have a ‘yoyo’ effect of hot and cold which will deposit condensation on cold surfaces. When the air temperature falls rapidly and the air can no longer hold any moisture. Leave the heating on this low constant setting even if you go out or at night when you are asleep. Unused or little used rooms should also be warmed in this way.

Have an efficient easy to use central heating thermostat or use thermostatic radiator valves adjusted to the characteristics and usage of a particular room.

Consider having a boiler inspection by a reputable company to assess the efficiency and suitability of the system.

Insulation and draft proofing

As insulation forms on cold surfaces any thermal improvements to that surface will reduce condensation forming on it.

A suitably qualified (CSRT) PCA registered surveyor will be able to advise on this.

Check loft insulation both in terms of thickness (current standards are the equivalent of 275mm of fibreglass insulation), and fitting. Often gaps are left especially in tight spaces towards the roof eaves and around services such as pipes and cables. Storing items on insulation which compresses it reduces dramatically its efficiency.

Solid walls (without a cavity) in old houses are prone to cold bridging and subsequent condensation / mould and also lateral damp penetration. A suitably qualified PCA registered and CSRT qualified surveyor will be able to advise on an appropriate specification and schedule of works to alleviate the problem which may also add value to your home, certainly making it more comfortable and desirable.

Ceilings in upstairs rooms which are sloping at the wall / ceiling junction are likely to be the edge of the roof structure ie the roof joists as opposed to ceiling joists and these are unlikely to be insulated in older houses. A common symptom is that the sloping part has a black shadow of mould or decorations which are spoiled.

Cavity wall insulation can be effective in warming up external walls. It is essential that the surveyor from the contractor does their job properly in assessing the suitability of the installation for that particular wall structure. There is an increasing volume of private and public housing stock that is becoming a real problem in terms of secondary issues brought about by the unsuitable installation of cavity wall insulation.

Draught proofing

Drafts account for a large proportion of heat loss in many homes. Improving draft proofing will save energy and cost, and also lead to warmer internal surfaces reducing likelihood of condensation.


Block permanent ventilators
Block unused chimney breasts, through ventilation is required by fitting air vents.
Draught proof rooms where there is a fuel burning heater or real fire.
Draught proof windows in bathrooms and kitchens