Sources of moisture

Air moisture varies according to climate or seasonal conditions, building design and construction, and the activities of the building's occupants. As the amount of moisture in the air increases so does the vapour pressure. This causes moisture to move from its source in all directions, either by diffusion or by air currents. Therefore excess moisture generated in one room may find its way to other rooms, such as from the kitchen to a spare bedroom where, for example, doors may have been removed or left open during high moisture generation periods.

Water vapour laden air moves from a centre of high concentration (typically kitchen and bathroom) to a centre of low concentration (typically cooler bedrooms).

Water vapour condenses when it comes into close contact with a surface that is at or below the 'dew point' temperature of the internal air, depositing liquid water on the surface (figure 1). Condensation occurs on the surface of building materials where it can be easily spotted, but it can also be 'interstitial'; that is, within the thickness of walls or floors. (Incidentally, some moisture meters can be used to calculate the dew point temperature if they have first measured the air temperature and relative humidity.) It is then important to measure surface temperatures to determine the risk of condensation.

The relationship between internal air temperature and moisture content of the internal air is calculated on a Psychrometric chart to plot dew point. Refer to British Standard BS 5250:2011. See One thing leads to another for more information.

Figure 1: Condensate is streaming down the inside of this windowpane: the moisture could rot the window cill

Temperature levels, ventilation rates, and the amount of moisture available all play a role in condensation (see BRE 174), but there are other factors that contribute to the process. The condition of building elements, and indeed design, for example, are important factors because the insulating quality of roofs and walls may be compromised by significant moisture levels. One factor that is probably least appreciated is the influence of penetrating dampness on the thermal resistivity of building elements. If a wall in reasonable condition is soaked by penetrating damp from a leaking pipe, the thermal insulation properties of the wall will be reduced. Lower internal surface temperatures may result, with condensation then likely in the colder months. The link between wet external masonry and internal mould growth is often very clear cut, with the external and internal conditions being virtually mirrored.

The development of moulds on wall surfaces is frequently symptomatic of a persistent condensation problem. Moulds tend to grow in the colder parts of rooms where surface temperatures fall below the dew point - in cold corners, behind furniture or in unheated rooms. They show in a classic crescent-shaped pattern, probably due to the temperature drop towards the corner. Some authors suggest that mould growth depends on the relative humidity at the surfaces of materials, with estimations varying as to the critical RH level (with 70-80% being commonly quoted). Certainly a high RH (> 70%) is often a contributory factor, particularly if the spot is regularly or constantly at this level. For mould to develop there must be spores, a food source, moisture, oxygen and a suitable temperature.

Over a dozen common moulds are associated with condensation problems - sometimes termed 'condensation moulds' - but moulds are not always moisture-source specific. The moisture could be associated with penetrating dampness, internal leaks, or contamination from external sources, i.e. rotting garden vegetation in the soil. The most common and damaging mould to health is Aspergillius Fumigatus. Condensation mould is the end result of an often complex network of contributory factors, and diagnosis must investigate those contributory factors if you are to recommend an appropriate remedy.

Equally, a lack of mould does not mean there is no ongoing condensation problem. The condensate may have been periodically removed from surfaces, or mould-damaged surfaces may have been cleaned or redecorated. Condensation could be occurring out of view within or between materials.

Considerable guidance on the whole subject of condensation in buildings can be found in British Standard 5250. Surveyors ought to have read or be aware of advice contained in the section on diagnosis and remedial work before embarking on surveys of existing buildings. BRE Digest 297 describes the scale of the condensation problem, and offers an insight into the problems and symptoms for a range of occupancy types, referring to statistics derived from the English House Condition Survey. The report also explained the complex interrelationships between ventilation rates, relative humidity and temperature for various house types, by era.

BS 5250 offers a more detailed analysis of condensation types (see 'BS 5250' below).

There may be a long chain of events leading to the condensation-led mould noted in a survey report. The chain could lead to investigations of many areas of the building - from the internal face of a wall to the gutter at the top of the building outside (see Falling damp case study).

BRE Report 174 offers practical advice on site investigations, reminding you to visit at a time when the problem will be visible, having pre-agreed the visit so that you can also ensure that the occupants are there. Consider the seasonal effects on condensation, for example.

On diagnosis, the report alerts surveyors to the complex variables that need to be taken into account:

'...Understanding the causes and remedies encompasses so many aspects, including moisture movement, mould growth, heating, ventilation, materials and building design, as well as the views and behaviour of the occupants.'

When surveying an empty property for a pre-purchase survey report remember that conditions may be far different in the house when the buyer moves in or during the 'condensation season' (late September to May). There might be symptoms of a historic condensation problem such as mould growth, but the evidence may have been decorated over or cleaned off prior to your visit. Conversely, some occupiers may attempt to exaggerate a condition to achieve their objectives.

It is difficult to assess condensation problems fully in a single visit. The property may or may not be occupied at the time of the survey and, if occupied, condensation may be occurring at times of the day outside the usual daytime 'survey window', i.e. early in the morning.

Condensation is influenced by lifestyle. Cultural differences in such activities as cooking, cleaning, washing and heating may have a significant impact on domestic moisture generation. A dwelling may be subject to serious mould development in one occupancy, whereas the property was completely mould-free in the preceding occupancy. Data loggers may be used by specialist investigators to monitor internal conditions, but should be used with caution and sensitivity, because they may be thought to be 'monitoring lifestyles' (see Gone with the wind case study).

Figure 2: Black mould in a crescent shape formation

Figure 2 shows the junction of two external solid walls. The black mould in a crescent shape formation is a typical signature of the effects of water vapour laden air condensing on the cooler internal wall surfaces at low level. This occurs when moisture becomes trapped behind furniture near the walls where little or no air circulation takes place.

Case study A band of condensation-led mould growth had developed to the lower height of the front bedroom wall of a property in Cambridge. At first penetrating dampness caused by cavity wall defects was suspected. The cavity was inspected using a borescope, which revealed a good condition inside. Subsequent conversations with the owner-occupier revealed that there had been no condensation during the first 2 years after moving in, when the bedroom was used regularly as a study. The study was relocated, and the small bedroom became little-used, with no heating and the door usually shut during the colder months. The window had also been changed from a traditional timber casement to a PVC-U double-glazed unit with no trickle ventilation or any small openable fanlight. The new window was rarely opened in the winter. A damp-proofing specialist had recommended an expensive ventilation system for the house to alleviate the condensation problem. The author's assessment was that this was not necessary: some background heating to help keep wall surface temperatures above the dew point and improved air circulation should solve the problem. It would have been difficult to assess the problems of that room without having first interviewed the occupier. Two years on, there was still no sign of condensation and the client saved £2,000 on unnecessary specialist remediation.

BS 5250

BS 5250 lists 4 types of condensation:

• Surface condensation is probably the most common type of condensation found in dwellings. This is usually observed as water (distilled) droplets streaming down a cold windowpane or on the surface of bathroom tiling. It can also moisten more porous surfaces, typically at cold room corners, where mould colonies may then grow in a characteristic crescent-shaped pattern at the skirting or ceiling junction.
• Interstitial condensation occurs within or between the layers of the building envelope, where the dew point exists at depth within a building element, component or material (for example, where water vapour has entered the roof void and condensation has occurred to the cold underside of flat roof decking).
• Reverse condensation, a form of 'interstitial condensation' (also termed 'summer condensation'), can occur when southerly aspect walls are subjected to a period of sunlight after a wetter period, and water vapour then moves inwardly through solid masonry walls. If the internal wall is insulated condensation can form on the inside of the vapour check.
• Radiation condensation (or 'clear night condensation') is another form of 'interstitial condensation'. It occurs when the atmosphere rapidly loses heat by radiation on a clear cold night. For example, on a garage roof of thin covering material, rapid heat loss will cause condensation on the underside of the roof sheeting.

Both surface and interstitial condensation can result from 'cold bridging'. Cold bridges (or thermal breaks) are an easy route for heat loss (for example, part of a building element that has an inferior insulation quality). Condensation may also occur where cold internal surfaces result from poor insulation quality around window or door openings - typically at the head of an opening supported by a concrete lintel, cantilevered balcony slab or often where concrete floor construction promotes cold surfaces internally due to its poor insulation quality. See The golden rule of surveying for more information.