How is it possible to distinguish between rising and penetrating damp and condensation?
“The selection of an effective remedy for any dampness problem must start with a correct diagnosis of the cause.” Building Research Establishment Digest No 297, May 1985.
Dampness, defined simply, is water out of place. It is the most common defect in buildings and one of the more serious. It takes different forms and these are usually identified in terms of the source of the water, whether it is liquid or atmospheric, or its point of entry. Of all the forms of dampness, condensation has increased as a problem in recent years as a direct consequence of changes in construction to improve the insulation qualities of buildings. Compared with rising or penetrating damp, condensation might be regarded as little more than an inconvenience. However, it does have equally serious consequences ranging from the spoiling of decorations to causing wood decay. In extreme cases it can even become a health hazard.
Dampness is a relative term and it is only when it exists at such levels to cause the growth of mould and fungi that it becomes a problem. Dampness as such is usually relatively easily identified, particularly with the use of a moisture meter, but identification is only part of the problem. Accurate diagnosis is necessary as different types of dampness will have different effects and will require different forms of treatment.
Unfortunately, without careful attention to detail, it is quite easy to confuse condensation with rising or other forms of dampness.
Rising damp
The source of rising damp is the soil or subsoil. Moisture is transferred up through the structure by capillary action. The water contains a dilute solution of salts from the soil and this will leave a concentration of salts in parts of the structure. These salts are themselves capable of absorbing water from the atmosphere, so that dampness will persist even when the source of moisture has been cut off.
Dampness will rise to a height of about 1 m, but this will vary, being dependent on the pore structure of the materials used in construction, the degree of saturation of the soil, the rate of evaporation and the presence of salts.
Liquid water
The presence of liquid water is usually easy to detect and diagnose, as it is invariably related to a defect in the structure which is fairly apparent. Rainwater penetration, for example, can be the result of porosity of materials like brick, the failure of pointing, cracks in rendering or general lack of protection of any part of the external structure against weathering. It might be the result of faulty rainwater disposal caused by the blockage of downpipes of lack of sufficient fall in gutterings. The presence of liquid water can also be the result of faulty plumbing, corroded or leaking pipes for example. This form is identified as penetrating damp and is usually characterised by isolated patches of dampness. These have a tendency to increase in size after periods of heavy rain.
Atmospheric water
Condensation is not liquid water and its source is from within the atmosphere within the building rather than outside. The atmosphere always contains water and this is measured by relative humidity. This is a measure of humidity which compares the actual amount of moisture in the air with the maximum that the air can hold at that temperature. It is usually expressed as a percentage. Thus when the relative humidity is 100% the air is said to be saturated and is incapable of absorbing any more water. Warmer air will contain more moisture, so if the air is cooled, the moisture in it will eventually condense out. The critical point at which condensation occurs is known as the dew point, the temperature at which the relative humidity is 100%. Surfaces in a structure falling below this temperature will be subject to condensation.
Condensation has increased quite dramatically as a problem in modern structures, partly as a result of the way in which we live, but mainly due to the increased efficiently of the exclusion of draughts from modern buildings. With central heating, chimneys have been closed; this reduces draughts but also cuts off one of the normal escape routes for water vapour. Double-glazing, installed specifically to reduce condensation, will, ironically, tend to increase the risk of condensation elsewhere. Single-glazing effectively acts as a dehumidifier, allowing vapour to condense out and causing relatively few problems.
Condensation will also tend to occur on local cold surfaces which are the result of conductivity in certain building elements, solid concrete lintels for example. This is known as cold bridging.
In a well-insulated structure the roof space may well be the only natural escape route for water vapour. However, the increasing tendency for loft insulation reduces the temperature of the underside of the roof and this has resulted in serious condensation causing the rotting of roof timbers. In such cases it is essential that the roof space be ventilated to allow the free movement of outside air.
Condensation can also occur within the thickness of a wall or ceiling structure. This is known as interstitial condensation and results from the drop in temperature between relatively warm internal surfaces and cooler external surfaces. If the temperature within a structure falls below the dew point, condensation will occur as water vapour travels through a porous material. This sometimes happens within the cavity of a cavity wall, hence the need for cavity-wall ventilation. In other types of structure it is necessary to include a vapour barrier on the warm side of any insulating layer, preventing the vapour from travelling any further through the material to a point where condensation is likely to form. This is necessary in timber-frame constructions and condensation was one of the problems besetting this type of building, resulting in its relative unpopularity in recent years.
Heating flues have a tendency to convey rain into buildings, but they can also be a source of condensation from fuel burning. Fuels produce water which condenses out on cold flue surfaces; the condensing vapour will often become contaminated with tars and sulphates from the inside of the flue and this results in the staining of chimney breasts.
Humidity under suspended floors is often high owing to the evaporation of water in the soil. Here, it is necessary to increase ventilation to prevent the rotting of timbers.
Apart from the danger to timbers, one of the main problems resulting from condensation is mould growth. This will develop, particularly in areas of stagnant air such as built-in cupboards, corners and behind furniture. The presence of water in the atmosphere can also be the result of the use of flueless gas heaters, cooking and washing, the presence of rising and penetrating damp and even just breathing.
Solutions to the inevitable presence of water include heating, which will help by raising the temperature of the air but unfortunately not of cold surfaces. Dehumidification, which is the removal of water from the air by mechanical means, is a potential solution. However, this is usually of limited effectiveness, and tends to be both expensive and noisy.
Problems can often be alleviated by improvements in ventilation, but this will be effective only so long as the relative humidity of air outside the building is lower than that inside — clearly not always the case in Britain’s climate! In less severe cases the problem can often be resolved by a change in living habits or the use of fungicidal washes and paints which inhibit mould growth: British Standard 5250:1975 gives useful advice on these measures.
Diagnosis
“The presence of damp that has existed for some time is indicated by clearly visible signs such as damp patches on walls, peeling and blistering wall decorations, patches of efflorescence and possible rotting and splitting woodwork due to wet or dry rot. There is usually a damp and musty smell due to mould growth.” Building Research Establishment Digest 245, January 1981.
The presence of moisture itself is no guide to the source of damp, but it is always essential to establish the distribution of dampness, outlining the limits of affected areas with a moisture meter. In terms of distribution, though, condensation can often have the same appearance as rising damp because of lower levels of temperature at the bases of external walls.
Meter readings then can only be a guide. High readings will be obtained, for example, where salt concentration is high. The damp meter cannot distinguish between conductivity resulting from dampness and that which occurs solely because of the presence of salts. The only definitive answer is to take samples of affected materials to measure both moisture and salt content. It is then possible to compare the hygroscopic moisture content with the moisture content.
If the hygroscopic moisture content is higher than the ordinary moisture content, the dampness is likely to be from the atmosphere rather than the ground. If the ordinary moisture content exceeds the hygroscopic moisture content, the water is from a source other than the air.
Rising damp is often characterised by a horizontal tide-mark on decorations, but it should be recognised that this could equally be the result of rainwater penetration, defective plumbing, condensation, contamination of the surface by hygroscopic salts or even the presence of water used in construction.
Rising damp is water from the soil containing salts, chlorides and nitrates in solution. As the water evaporates from the wall, salt deposits are left behind. The presence of such salts can be established by using a salts detector.
Other characteristics are that walls affected by rising damp will be wetter in humid conditions, but will be damp more or less permanently. The dampness moves upwards but will tend to have a sharply defined upper level, usually about 1m above ground level. The skirting of the affected wall will often be wet and, while decorations will be stained or discoloured, rising damp is not normally associated with mould growth.
Moisture content decreases with height. Rising damp is seasonal, increasing in winter with rising water tables and decreasing in summer. It invariably results from the absence of a damp proof course or bridging of an existing one. Remedy is usually the insertion of a physical dpc or, more commonly (and cheaper), the injection of chemicals, under pressure, which are water repellent — silicone or aluminum stearate, for example.
Condensation, on the other hand, tends to be intermittent and will occur especially on colder and unventilated surfaces behind pictures, inside cupboards, in unheated rooms, where there are cold bridges and on the lowest parts of walls. The margin of the affected area is usually less well defined and skirtings will tend to be dry. Whereas rising damp contains salts, which inhibit mould growth, the purity of water in the atmosphere allows mould to grow. The extent of mould growth will depend upon moisture generation, ventilation, thermal insulation, heating and surface absorption. Mould spores are always present in the atmosphere, but to thrive they require water and a food supply. Such favourable conditions will almost certainly exist where the relative humidity exceeds 70%. Of course, condensation can be expected in those areas where low ventilation is combined with high water vapour content, typically bathrooms and kitchens.
Surveying for dampness should always involve an extensive test with a moisture meter so that every area of dampness can be identified and plotted. Because diagnosis will determine the cure, the cause of every area of damp must be determined with precision.
Finally, as a broad general guide, the following table summarises the major distinctions between condensation, rising and penetrating damp. It must be stressed, however, that diagnosis may be even further complicated by the presence of more than one source of dampness in any given location.
