Condensation, Mould and Air Ventilation

Phillips & Co are members of the Property Care Association which has established a new group to focus on the increasingly important issue of effective ventilation in residential buildings in order to provide homeowners with a point of reference and resource to help them find the expert advice and expertise they need.

The way we are living in our homes is changing

A number of factors have an effect on the air that surrounds us in our homes.

These include increasing fuel costs which have led to a drive to reduce energy use, increased efforts of homeowners to reduce air leakage through draft proofing and insulation, increasingly sedentary lifestyles, reductions in the size of the spaces we occupy and a growing incidence of overcrowding.

Unfortunately, there can be a number of unintended consequences associated with draft proofing, insulation, reductions in fuel use and high levels of occupation.  The most important and ultimately the most damaging is an increase in the moisture held in the air.  This can lead to poor air quality, condensation, dampness and mould.  These are usually the most obvious and immediate symptoms of these changes.

WATCH the video to find out more…

Key is balancing moisture production, insulation and ventilation

Understanding how to balance moisture production, insulation, energy input (heat) and ventilation as well as a good technical understanding of the science of air moisture and condensation is the key to eliminating condensation and mould.

While a surveyor can usually provide definitive advice on moisture production, heating and ventilation they have traditionally passed the responsibility for the design of improved ventilation to a jobbing electrician who supplies what he thinks is about right.

Regulation and guidance setting out minimum requirements for domestic ventilation in homes is usually ignored or misunderstood and as a result outcomes for homeowners can be mixed.


The most common form of unwanted dampness in buildings is water from the air that forms as condensation.

The air in buildings can have a high level of relative humidity due to the activity of the occupants (e.g. cooking, drying clothes, breathing etc.). When this water laden air comes into contact with cold surfaces such as windows and cold walls it can condense, causing water to be deposited. The point at which the water held in the air changes from vapour to liquid is known as the dew point.

Condensation is often associated with poor heating and ventilation in buildings. It is more apparent in winter, as the external air temperature is low and external walls and windows are cold. 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

Walls in kitchens and bathrooms (where atmospheric moisture levels are usually highest), solid external walls, un-insulated solid floors and cold bridges such as concrete lintels set in cavity walls are commonly the areas in which condensation takes place.

Intermittent heating and cooling of the property can aggravate condensation problems, since it allows warm damp air to cool, reducing its capacity to hold water. Dew points are reduced allowing condensation to occur. When the air is reheated water is taken back into the air only to be deposited again when the air temperature drops again.

Problems caused by condensation

Running water on windows and walls is perhaps the most immediate indication of a condensation problem. If ignored this can lead to a deterioration in the decorative condition of the property, stained curtains and decay in window frames. The appearance of moulds on the surface of wallpapers and paints in poorly ventilated areas. Condensation can occur under suspended floors greatly increasing the chances of fungal decay in floor timbers.

A much less common form of condensation occurs when the Dew Point is reached, not on the surface of a wall but within the structure of the building itself. This is known as interstitial condensation and can easily be mistaken for rising damp or penetrating damp.

Overcoming Condensation

Condensation is a real problem and where it persists, a specialist surveyor should be engaged to explore the cause of the problem and provide advice or propose solutions. We have listed just a few of the possible methods of controlling the problem below.

Simply heating the air is unlikely to be a satisfactory solution, not only on grounds of cost, but also of practicality. Unless cold surfaces are eliminated and there is sufficient background ventilation, condensation is almost inevitable. Any remedial action, therefore, must involve lowering of moisture levels, ensuring sufficient ventilation and the elimination of cold surfaces.

Improved heating and ventilation coupled with specific action in relation to cold spots will usually result in a significant improvement in conditions, although there may be circumstances in which alternative methods are required. A modest but constant background heat is preferable to intermittent heating since this will help to maintain a higher ambient temperature in the fabric of the building.

The installation of suitable extractor fans in the moisture producing rooms of a property such as the kitchen, bathroom and en-suites, will help remove the majority of this moisture-laden air from these areas (that are most responsible for condensation), with minimal running costs. This is a requirement within the Building Regulations for new properties being built, whilst also applying to existing buildings. 

There are many different types of extractor fans available such as those that run continuously in the background or those which incorporate a humidistat which will control the operation of the fan within certain humidity limits. It is also possible to install fans that have an integrated heat exchanger and these have the advantage of providing effective ventilation while reducing heat loss from the property. It is very important that these types of fans are professionally specified and commissioned by a suitably trained and qualified specialist. 

Where an open fire or fixed gas fire exists, a certain amount of “natural” ventilation will occur and where additional ventilation is provided it is important that this is not blocked off.

The use of specialist insulation materials fixed to the outside of the building and insulation in cavity walls will help to improve the thermal dynamics of the building and may help overcome condensation.

An alternative to heating and ventilation for the control of moisture in the air is a dehumidifier. This is a device which draws in air, cools it to remove moisture which is collected in a reservoir and reheats it to an acceptable temperature before re-circulating it – these can be costly to run however, and may not be considered a long-term solution. 

Other devices that may be considered are positive pressure condensation control units also known as Positive Input Ventilation (PIV) units. These take dryer air from roof spaces or lofts and mix this with air in the dwelling. This is done at a very low rate (below half an air change per hour) and has the effect of lowering total moisture content and removing moist air by natural leakage.

Condensation Leaflet

The PCA has produced a leaflet to guide homeowners about the causes and how to remedy condensation in the home.

Click here to download a free copy of the leaflet – 

PCA Condensation Leaflet
Adobe Acrobat document [789.2 KB]