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Condensation & Mould Growth


An Explanatory leaflet from Abbey Property Renovation

(For any further information contact our office on 674323)


Dampness problems associated with condensation and mould growths occur in many buildings yet they are not always fully understood, even by the professional specifier and treater.

It results from a series of relatively simple and well established physical factors, and is directly related to standards and methods of heating, ventilating and insulating buildings.

Condensation is often confused with rising damp. Dampness in buildings can arise from a number of causes and the majority tend to be physical defects such as rising damp, penetrating damp or lack of maintenance. In the case of condensation, the problem is generally self imposed. The emphasis on improving insulation and the way in which properties are now heated and ventilated has created exactly the right conditions for increasing condensation and mould growth.

Local authorities and, indeed, any landlord receive large numbers of complaints about condensation and mould growth.

The basic principles of condensation, mould growth, the need to diagnose the problems properly and some basic remedies are reviewed here.


Condensation as the name implies is water which has "condensed" from air on contact with a cold surface.

Air normally contains water vapour in varying quantities. Its capacity to do so is related to temperature; warm air holds more water than cold air. Air is saturated when it cannot contain any more water vapour at the existing temperature; under these conditions it is said to have a relative humidity (RH) of 100%. If the temperature of the air falls until saturation point occurs the air is at a critical temperature at which it cannot hold any more water - this temperature is known as the dew point. Any further fall in temperature will result in water vapour being forced to condense out as liquid water.

Condensation in a building usually occurs when warm air comes into contact with a cold surface; the air is cooled below its saturation point causing its excess water vapour to change into liquid water. The condensed water usually appears as water droplets or water film on windows and other non absorbent surfaces. This form of condensation is SURFACE CONDENSATION. It is obvious and always occurs on the surfaces which are at or below the dew point of the air immediately adjacent.

Air inside a heated building usually contains more moisture than does the external air. Water vapour in the air exerts a pressure. This means it is a higher pressure which tends to force the warm air through the structure taking the moisture with it. Most building materials except glass, metals, plastics and certain lined elements, are to some extent permeable and do not obstruct the movement of moist air through the structure. the warm moist air will eventually cool below its due point within the fabric of the building resulting in condensation. This form of condensation is INTERSTITIAL CONDENSATION.

Interstitial condensation is rather more complex than the surface phenomenon and presents a greater hazard. The resulting high moisture content can often go undetected for long periods until serious structural damage has developed such as timber decay. It can also render ineffective any insulation within the component where it occurs.

Conditions for Condensation

In specific terms there are four main and interrelated factors which affect the occurrence of condensation in dwelling houses:

1. the moisture generated within the dwelling.

2. the ventilation system provided and the use made of it

3. the space heating system provided and the use made of it

4. the thermal capacity, response and insulation of the dwelling.

It is, however, the moisture generated within the dwelling by domestic activities which is the primary cause of condensation. Warm moist air is produced in relatively large quantities resulting in up to 15 litres of water per day (two bucketful's) within a 5 person dwelling. Condensation is found more often in kitchens and bathrooms, but warm moist air can also spread to cooler parts of the house to condense on any cold surface (at or below dew point). Here the first signs to the occupier will be mould growth.

The effect of moisture generation is aggravated by the way houses are ventilated - it is theoretically possible to avoid condensation by adequate ventilation. Up to about the 1960's, there was neutral ventilation in many homes because of the lack of double glazing, poorly fitting windows and doors, and open fireplaces. Present attitudes have eliminated natural ventilation e.g. the use of double glazing, draught excluders, fitted carpets, (preventing air movement up through suspended wooden floorboards) and the removal of open fireplaces. The ideal background ventilation rate in a typical dwelling to prevent condensation and mould growth is about one air change per hour. Buildings have been effectively sealed and therefore provide better conditions for condensation to occur.

In general terms, condensation is most likely to occur in dwellings which use the least heating. Economic pressures have forced occupants to use little or no heating: about 25% of households in the country use little or no heating: about 25% of households in the country use less than 10,000 units of energy per year (1 unit = 1Kwh electricity) for cooking, heating and lighting.

In poorly insulated dwellings condensation is also likely to occur more often as there is greater heat loss. This can result in internal wall surface temperature being much lower than the air temperature, so the dew point temperature is reached more readily with the corresponding threat of condensation.

There is always a balance between moisture generation, heating, ventilation and insulation when there is a condensation problem. It is generation of moisture under modern living standards which is the primary cause of condensation. In the majority of dwellings throughout the country, condensation, and particularly surface condensation, creates conditions for mould growth.

Mould Growth

Mould growth will appear on any damp surfaces such as plaster, wallpaper and timber and is usually associated with condensation problems. It is unacceptable because of appearance (unsightly growths of various colours - greens, yellows, pinks, black, grey or white, odour (musty and damp), fears of health, and hygiene considerations (particularly in food processing industries).

Conditions for moulds to develop exist in cold areas of a dwelling if the average relative humidity of the room is 70% or more, constantly or regularly.

Moulds are simply fungi from several groupings in the fungal classification system.

There are three principal features common to the broad range of mould fungi:

        1. Simple food requirements: able to exist on non - nutrient materials such as plaster and brick, which have traces of              contaminating organic matter.

        2. Produce vast numbers of spores which allow rapid adaptation to particular environments. Mould spores are always              present in large numbers in the outside air and easily find their way inside buildings.

        3. Grow very quickly under suitable conditions.

The main requirement for the development and growth is a source of moisture although food, oxygen and a suitable temperature are important. It is available water which is critical to mould growth and development.

Most individuals may put up with quite severe dampness from condensation, but the appearance of mould growth creates concern about damage to furnishings, decoration and possible danger to health. Mould spores can have great effect on indoor air quality. In damp buildings toxigenic as well as allergic moulds may impact on the respiratory and general health of the occupants. The main potential risk to health from mould spores are considered to be allergic responses such as coughing, sneezing and eye irritation.

Diagnosing the Problem

The actual physics of condensation are well understood, but the occurrence in a dwelling is complicated by the four interrelated factors which have been previously mentioned.

The occurrence of condensation and mould growth is a dynamic event. They need to be diagnosed properly at a site visit and not confused with other sources of dampness.

Surface condensation can be determined if relative humidity, air and wall surface temperatures are measured. This can be achieved at the time by using:

i) Whirling hygrometer, air/surface thermometers.

ii) Electronic equipment such as thermohygrometers, condensers.

Even if conditions are changing at the time of survey the risk of surface condensation can still be evaluated provided relative humidities and wall surface temperatures have been determined.

Unlike the surface phenomenon, interstitial condensation cannot be measured directly and only the risk of its occurrence can be assessed.

The factors which cause condensation (e.g. moisture production, requirements for heating, ventilation, insulation) should also be investigated wherever possible to ensure that the most appropriate remedy is proposed.

The Remedies

In principle condensation and mould growth problems can be overcome by the use of adequate heating and ventilation. This ignores, however, the financial constraints imposed in both private and tenanted accommodation.

It is a matter of understanding what can be carried out by the occupier to limit moisture production and movement and what remedial measures are needed in the dwelling itself. In this respect a number of local authorities have developed overall strategies to improve the situation in their housing stocks.

The use of approved fungicidal coatings to combat mould growth also forms part of the strategies to help in meeting the requirements of section 82 (Statutory Nuisance) Chapter 43, Environmental Protection Act 1990. There are fungicidal coatings which have been used for over 35 years and have proved to give prolonged control against mould growth if specified and used correctly.

In simple terms the remedy for condensation can be relatively inexpensive such as the use of small extractors; in others, more expensive measures, such as improved insulation or a new heating system, may be necessary.

The following represent simple and inexpensive ways of reducing condensation and mould growth as well as some of the more expensive refurbishment recommendations. The choice of remedies will depend on the magnitude of the problem, so it is essential to make sure that condensation has been identified as the source of dampness.

Simple Inexpensive Recommendations

*Limit moisture production in bathrooms and kitchens.

*Limit moisture movement from these areas by keeping doors closed.

*Use background ventilation which can be achieved in two ways. Open windows slightly on opposite sides of dwelling to give cross ventilation. Individual rooms can be ventilated by shutting doors and opening windows. (Where security is paramount trickle ventilators can be used).

*Ensure each room can be ventilated and air bricks, grills and any ventilators are not blocked.

*Fit extractors, preferably hard wired and with humidistat control, in kitchens and bathrooms.

*Ensure the existing heating system is working effectively. Maintain some low heating, even if the accommodation is unoccupied during the day, particularly in cold weather.

*Ensure tumble dryers and washing machines are being exhausted directly to the outside.

*Avoid the use of paraffin or bottled gas heaters (one litre of paraffin produces over one litre of water).

*Apply proven fungicidal and anti-condensation coatings in mould infected or mould susceptible areas.

*Allow air to circulate behind furniture (wardrobes/drawers/bedheads) to avoid build up of stagnant air pockets leading to mould growths.

Refurbishment Recommendations

*Introduce more effective heating for part or all of dwelling.

*Insulate the fabric of the building - including both roof and wall insulation (glass fibre/thermal boarding).

*Insulation and heating improvements should, where possible, be considered together and will depend on the type of building.

*Use dehumidifiers in selected areas.

*Introduce mechanical 'fresh' air systems to create constant positive air movement throughout the building.

*Install double glazing (doors and windows).

*Apply water repellents to external walling to reduce loss of heat from evaporation of absorbed water.

In the majority of cases no single remedial measure will suffice. It will be necessary to provide an overall strategy dealing with insulation, heating, ventilation and decoration.

Condensation and mould growth are widespread problems in all housing sectors but particularly in tenanted accommodation. Condensation, and associated mould growth, will remain, however, one of the most persistent and more difficult dampness problems to solve in dwelling houses.

Removal of Black Spot and other moulds from walls affected by Condensation

The moulds resulting from condensation can be removed by using a weak solution of household bleach or a product such as Mr Muscle Black Spot mould remover. The latter being the better product to use as it contains a fungicide which will help prevent the formation of moulds should the problem re-occur or persist.


If you have any questions regarding condensation, would like a visit or advice

please call our office on 674323. Or click here. We will be happy to help.