BUILDING BRIEFING:

Condensation Explained

Part two: how to deal with it 

 

The amount of moisture any given volume of air can hold as a vapour is dependent on air temperature: the warmer the air temperature, the greater the amount of water vapour that the air can hold. Condensation occurs because the moisture in the air can no longer be held as a vapour and hence returns to its liquid form. In dwellings, this occurs when this warm moist air comes into contact with a cooler surface. Dew point is the name given describing the temperature at which moisture vapour in the air will condense. On impermeable surfaces, condensation is obvious (e.g. on single-glazed windows). However, condensation will also occur on absorbent or porous surfaces if the air temperature drops to dew point or lower, such as furniture, clothes, paint, wall paper and other surfaces. Evidence of this occurring on such surfaces is localised damp patches causing disruption and damage to the surface. Additionally, Black Spot mould growth is a sign of chronic condensation, and because of its threat to human health, requires remedial action to deal with the causes of the condensation. 

Sources of moisture will include many typical domestic activities within a house: cooking, washing, the use of domestic appliances, clothes drying, etc. Although condensation is common in areas where these activities occur (i.e. kitchens and bathrooms), moisture vapour will also travel throughout the house and condense anywhere where there is a lowering of air or surface temperature below dew point

The degree to which the structure and fabric is thermally insulated is significant. Essentially the better the insulation, the warmer the internal air and internal surfaces will be; this is because more heat is retained for longer, reducing the opportunity for surface condensation. However, where structures are not well insulated, or where there is discontinuity in insulation (known as cold bridges), condensation will occur. 

Appropriate ventilation is another key issue: ventilation evaporates excessive moisture and, like thermal insulation, is a critical factor in reducing condensation. Homes are becoming increasingly more sealed against uncontrolled ventilation. This is good for avoiding casual or uncontrolled heat loss, but unfortunately removes the evaporative effect of ventilation and thus can increase the moisture in a dwelling. 

The minimisation of moisture production, controlled ventilation, appropriate thermal insulation and economic heating systems are all issues which need to be understood and appropriately managed if excessive and harmful condensation is to be avoided. These issues are interconnected, as such are the duties for prevention: both landlords and tenants will have separate and shared responsibilities for the management of such issues. The following sections introduce and examine the issues and responsibilities of both parties.  

Reducing moisture production

Reducing moisture production is the most obvious management response from the tenant’s point of view. Many normal domestic activities will produce moisture vapour, and some of these activities can easily be managed in a way which minimises that production. 

Kitchens

Keeping lids on pans of boiling water, the installation and judicious use of extractor fans (which are not the same as cooker hoods that deal with fumes and smells, rather than moisture vapour), and closing of kitchen doors can all significantly reduce the incidence and/or spread of moisture derived from cooking. 

Bathrooms

Closing bathroom doors, and opening windows or vents during and after taking a bath or shower are both effective in reducing the production and spread of condensation. For those who enjoy hot baths, pouring cold water prior to pouring hot will also reduce moisture vapour.  

Drying clothes inside   

 The drying of clothes inside has the greatest potential for the creation of moisture vapour. Drying clothes on radiators or on airers drives warm moisture laden air into the dwelling. Avoidance of using this method is easy to suggest, but not very practical in many circumstances. Giving tenants good, accessible, and preferably, covered rotary external clothes airers is a simple and cost effective first step. An environmentally and financially expensive option is a tumble dryer; these can be very effective at drying clothes, but their venting must be well-detailed, ensuring that it removes the moist warm air externally without creating and concentrating a condensation problem inside the dwelling (as is frequently encountered on house inspections). If tenants do want to dry clothing inside, ensure that a suitable space, such as the bathroom, is identified and that this is provided with suitably opening windows/window vents/air bricks, a suitable heat source, and a closed door during the drying process.

Clean it up, throw it out

 Any condensation which is mopped up (typically from the window cills of single glazed windows in the winter) should be disposed of, otherwise it effectively forms a reservoir of moisture for future condensation. Though a pain, the fact that single glazed windows enable condensation to occur effectively means that they are acting as a local dehumidifier, potentially reducing the likelihood of condensation occurring elsewhere.

Other defects

Ensuring that there are no obvious defects within the dwelling which may cause dampness is also important.

Ventilation

Ventilation is a necessity for human health since it allows fresh oxygen laden air to circulate in our dwellings. It is essential for the removal of unhealthy fumes, such as from WC’s and the combustion of fuels for heating and cooking. It is also essential for the removal of any excess moisture by the process of evaporation. 

Though ventilation is of critical importance to evaporating moisture, it can also create condensation if not properly managed. Specifically, if ventilation is excessive (for instance, in the case of a partially left open window during the winter months when condensation is most likely) it will lower both the air and surface temperatures in the dwelling, hence encouraging condensation. 

Thus, it is important to maintain a balance between sufficient and excessive ventilation, as well as keeping an awareness of how to attain and manage this balance. 

Extractor fans

The most commonly adopted automated response to the requirement for ventilation is the extractor fan. It is frequently found connected to a light switch and controlled by a timer or a humidistat. Required by recent building regulations in bathrooms & WCs, these electrical fans are used to remove excessive moisture and fumes, and must be regularly maintained in complete working order. 

Trickle vents and open windows

Trickle vents are usually fitted to all new windows and are a common feature to many existing ones; they are controllable openings which enable a trickle of air flow into a room or dwelling. They should always be left open, but tenants tend to frequently close them in response to perceived draughts. Open trickle vents are essential for ventilation in more recent buildings where the casual draughts from chimneys and ill-fitting windows and doors, frequently encountered in older dwellings, are now absent. Opening windows in bathrooms during and after showering or running a bath is a good idea since it helps remove excessive moisture. 

However, the time to which windows should be left open must be limited so that although moisture is evaporated, surface temperatures are not excessively lowered. This is an essential piece of understanding in order for the benefits of ventilation to be appreciated

External furniture and roof voids

A lack of air circulation can also enable condensation. Try to avoid locating large items of furniture on external walls as this can reduce the evaporative effect of ventilation. 

Adequate ventilation in insulated roof voids is essential and should be verified, particularly during the winter months when condensation is more likely. 

Thermal insulation and adequate heating 

Raising the air and surface temperature within a dwelling not only provides human comfort, but also reduces the occurrence of condensation. An adequate and suitable space heating system is therefore essential, which ideally must be economic to operate. The failure to heat also enables condensation to occur. Concerns regarding the financial costs involved in heating are becoming an increasingly encountered social and health problem. Dampness from condensation will further cool the walls, resulting in even more condensation - a vicious circle.  

Few of us can afford to continuously heat to our dwellings, and modern life¬styles encourage intermittent heating and hence intermittent cooling. This is a dangerous combination for condensation: heating air enables it to hold moisture vapour and warms any walls above dew point. However, the subsequent reduction in temperature reduces the capacity of the air to hold the moisture as a vapour, and thus may well reduce the structure’s temperature below dew point. In well insulated dwellings, this intermittent heating/cooling cycle will not result in condensation as the insulation retains heat in the walls of the building. The opposite is the case in poorly insulated buildings. 

Make sure the house is as well insulated as it possibly can be. Loft insulation is by far the most cost effective solution, but other forms of thermal insulation are worth considering too. Caution with regards to ventilation is even more critical if you are considering cavity wall insulation. Concerns about the potential for damp ingress must be discussed with a consultant and the installer. Remember also that the impact of the energy efficiency of your dwelling is, and will increasingly be, an important factor in tenant choice.

It is better and more cost-effective (at least in well-insulated buildings), to gently build up warmth in the structure during September rather than, for example, to wait for a later cold snap, and then commence heating. On a daily basis, a lower level heat input (with a far longer heating period), is preferable to limited, short bursts of high levels of heat. For example, by switching the heating system on for an hour or less will make the temperature and condensation problem worse. This short-term heat will raise the moisture content of the air without sufficiently heating the structure. Once the heat goes off, the walls then quickly attain dew point and thus resultant condensation will further cool the walls, and so on. 

“On a daily basis, a lower level heat input (with a far longer heating period), is preferable to limited, short bursts of high levels of heat.”

Some older gas fired central heating systems use the bathroom radiator as a heat sink for the water cylinder, thus keeping this condensation-vulnerable space at a higher temperature than other spaces, which is a positive thing. 

However, some forms of space heating actually give off water vapour and should be avoided – portable gas bottle heaters are a classic example of this.

Cold bridges (discontinuity in thermal insulation) can produce isolated condensation. This can be tricky to diagnose and requires appropriately specialist diagnosis.  

From the preceding discussion, it should be obvious that although the landlord bears a considerable responsibility for a variety of the issues and factors which impact on the incidence and severity of condensation, a significant proportion of the management of these issues is actually the responsibility of the tenant.  Suitable installations – such as appropriate thermal insulation, adequate ventilation, and economic and responsive heating systems - will go a long way to remove or reduce impact and severity of condensation. However, enabling and encouraging appropriate behaviour and management for the tenant, that can best reduce the opportunities for condensation, is a matter of providing clear information and explanations, such as leaflets or guidelines. 

In this regard, it is in both the tenant’s and the landlord’s best interests if such clear and unambiguous information is provided by the landlord for the tenant.

For more information about condensation, see The Letting Centre’s ‘Condensation and Damp’ leaflet inserted in this issue of the Letting Update journal.