Dampness in Buildings

E G Gobert, T A Oxley

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781135386610
• Edition: 2
• Topic: Tecnología e ingeniería
• Subtopic: Construcción e ingeniería arquitectónica

This book explains the nature of dampness in buildings, how to diagnose a dampness problem before it gets out of hand and how to deal with it. It covers the problems of rising dampness and condensation (and how to distinguish them), which are generally
not fully understood, even by many professional surveyors. The book stresses the need to use a moisture meter to obtain reliable and comprehensive information. Dampness in Buildings will be of immediate practical use to surveyors, architects, builders, housing managers and health inspectors. It will be equally valuable to house owners and potential purchasers.

1

The dampness problem

Those who have responsibility for buildings, and those who use them, are more conscious of dampness now than in the past. If they are surveyors, they have onerous responsibilities which demand that they overlook no sign of dampness, and they are expected to advise their clients on the extent, severity, and future implications of what they find. If they are building or housing managers, or enviromental managers, they will be left in no doubt by the occupants if dampness is among the defects that they suffer, and they will also be expected to prescribe the cure. But a cure is only possible on the basis of a correct diagnosis. This book, therefore, is to help the reader towards a correct diagnosis.

The great majority of surveyors use electrical meters based on the measurement of conductance for detection and evaluation of dampness; their job, insofar as it concerns building dampness, would be impossible without them. Many building, housing management, and environmental health departments also find these instruments essential. But dampness is usually not a simple problem and hence correct diagnosis is sometimes very difficult. Electrical instruments will not, themselves, diagnose the cause of dampness. They do, however, provide indications of great value, including the quantification of dampness which might otherwise not yet be detectable.

An electrical moisture meter for buildings is therefore part of a system for diagnosis, of which the other part is the knowledge and experience of the operator. It is the aim of this book to provide the knowledge and to show how a range of conductance type moisture meters, together with their accessories, and an analytical service, can provide the evidence. The remaining factor, the experience, we cannot supply. But experience will be gained more quickly if interpretations are based on understanding, which we hope this book will provide.

All dampness is water out of place, but it is convenient to classify its different manifestations by their source, and the routes by which the unwanted water enters the inhabited areas. A primary distinction is between water which enters as a liquid, and water which is condensed from the atmosphere. Until the 1960s the latter was almost unknown within buildings, but with changes in building design and in living styles, condensation has become a major problem, causing about two thirds of the complaints of dampness in houses received by housing authorities. It is this huge increase, and the difficulty of diagnosis which it often presents, which is partly responsible for the greater awareness of dampness among both professionals and laymen. Another factor is the great amount of rehabilitation of older buildings, large and small, which is now undertaken. This has often revealed dampness which was tolerated, or regarded as inevitable, by our forebears; in addition ‘modernization’ does much to increase condensation by total elimination of draughts and modern decoration makes it more obvious.

Awareness of dampness has also been stimulated by the rise of a service industry of ‘specialist’ firms devoted to curing it. This is an industry largely directed towards curing rising damp. It is generally based on the valuable and relatively cheap process of injecting water repellent materials into the lower parts of walls, thereby exploiting a principle explained later in this book for preventing the rise of water by capillarity. This is a competitive industry which uses a lot of publicity; it has spread quite widely the impression that rising damp is the main cause, or at least a very frequent cause, of dampness in buildings. In fact rising damp is relatively uncommon cause of dampness in buildings.

The specialist damp-proofing industry includes a majority of responsible firms. Many of them are members of the British Chemical Dampcourse Association and similar organizations in other countries. These firms work to the excellent codes of practice published by their Associations. However, the standard of others is not universally high. It is necessary, especially, to look critically at the survey reports produced by some who are surveyors in name only. It is one function of this book to aid such critical evaluation. It is hoped that critical customer appraisal will bring to an end the practice of attempting to cure condensation, penetrating damp, or even plumbing leaks, by irrelevant treatments for ‘rising damp’.

Decay of wood is one of the most serious consequences of dampness. It is therefore a considerable advantage of the conductance type of electrical moisture meters that they give direct measurements of water in wood. With the aid of a deep penetrating hammer electrode, the instruments especially designed for timber can show whether or not there is serious dampness within thicker pieces which is not apparent at the surface. This is important when structures are drying out after flooding or saturation during fire fighting.

If, as sometimes happens, it is impossible or uneconomic to cure a source of dampness completely, it is important to treat wood with preservative, or to use wood which has already been so treated, for replacement. Wood preservative treatments, or methods of application, are not all equally effective; some may be little better than cosmetic. Some other products are designed to be diluted on site, and if this is poorly controlled the results may be correspondingly poor. The best safeguard is to use firms who are members of the British Wood Preserving Association, the American Wood Preservers Association, or similar responsible organizations, and to use products made by such firms. The Associations’ codes of practice and lists of approved products for use in remedial or pretreatment preservation are a valuable guide.

Figure 1.1 Surveying for dampness in the house

Although this book is primarily concerned with diagnosis, we believe that it is useful to give some guidance on techniques for the cure of dampness. Therefore, each of the sections of this book on the various types of dampness problem either contains, or is followed by, an ‘answer’ outlining the principles and methods appropriate to each. These are not intended to be comprehensive, but we hope they will guard against the use of irrelevant or inappropriate measures.

Beware of the man who does not use a moisture meter because he claims it is ‘misleading’. It is more correct to say that the meter is ‘revealing’, because without it, much significant dampness would pass unnoticed. Without the graduations in dampness which a meter gives, the further information given by its accessories and the associated laboratory service, it would often be impossible to diagnose the true cause, especially in the early stages.

2

What dampness is

Dampness in buildings, if left unattended, can lead to structural deterioration; it will result in the decay of wood, it will spoil decorations and, by encouraging development of moulds and mites, can be dangerous to health.

If an absorbent material such as a piece of wood, paper, cloth or brick is placed in a very damp atmosphere (high relative humidity) it will absorb water and therefore increase in moisture content. Conversely, in a dry atmosphere (low relative humidity) it will lose water and its moisture content will fall. At any intermediate relative humidity of the air the material will either gain or lose moisture, depending on whether it was dry or wet to begin with, and finally it will settle to a certain moisture content which will remain constant so long as the humidity of the air does not change. The material is then ‘in equilibrium’ with the air.

If the humidity changes, the moisture content of the material will gradually change with it. So for every relative humidity there is a definite moisture content at which any piece of absorbent material will be in equilibrium. But this figure is different for almost every kind of building material, and even for almost every piece; certainly it varies from brick to brick and from one kind of mortar to another. But for wood, although there are differences between different varieties, and between hardwoods and softwoods, there is not nearly as great a difference as there is between other building materials. This is why moisture meters usually give a moisture content scale for wood, but do not attempt it for other materials. The accompanying graph (Figure 2.1) shows the average humidity/moisture content relationship for typical softwoods used in buildings.

Box l

How moisture is measured in a laboratory

The basic measure of moisture content is made by oven drying. A sample is weighed, dried, and weighed again. The loss in weight is assumed to be water and this is expressed as a percentage of the final oven dry weight.

Suppose a piece of brick is to be tested for its moisture content. It must, of course, have been wrapped in foil or several layers of polythene, otherwise it will have lost or gained moisture since it was removed from the structure. If large enough, it should be broken into two or three so that the determination can be carried out in duplicate or triplicate. Assume that it is broken into three pieces, dust is rejected, and the pieces are weighed.

It is specified that the sample must be dried to ‘constant weight’ at a temperature of 105°C. It is therefore dried in a thermostatically controlled oven, probably for two hours in the first instance, then for a further four hours, and then a further 16 hours:

In future the laboratory would always dry material of this type for about 18 hours having found that 6 hours is not sufficient. The laboratory would be careful to use a lower temperature for samples containing gypsum plaster because this material decomposes at high temperatures and will lose 16 or 17 per cent of its weight. This is chemically combined water (see Chapter 2) and not dampness. If a moisture as high as 16 per cent is reported by a laboratory you will know that it has been overheated. Even saturated gypsum plasters will not hold more than a few per cent by weight of water.

Figure 2.1 Wood moisture content and air relative humidity

The graph shows the approximate relationship between the relative humidity of air and the moisture content of wood. This applies to typical softwoods used in building; the curve for some of the heavier hardwoods would be different. The horizontal lines show the typical range of humidities met in various circumstances. Wood kept in these environments will gradually come into equilibrium at the moisture content levels indicated on the vertical scale. This shows why wood becomes very dry (water content 4–8 per cent) and often shrinks and cracks in centrally heated rooms, but will become much damper, though not dangerously damp (up to 16–18 per cent) in a normal room. Wood exposed to outside air continuously changes in water content over a very wide range. Although it becomes very wet in wet weather it does not decay so long as it has the opportunity to dry out in dry weather

The percentage moisture figures are derived from oven drying tests and are based on this universally accepted formula:

In the test the sample is very accurately weighed (wet weight), and it is dried in a ventilated hot air oven especially designed for the purpose at an accurately controlled temperature until it reaches a constant weight, that is, until it loses no more moisture. It is then allowed to cool in specially dried air. When cool it is again accurately weighted (dry weight). The loss in weight due to evaporation of water is expressed as a percentage of the final weight as shown above.

It follows that a heavy material has a much lower percentage moisture content than a light material which has the same amount of water in it!

Graphs could also be drawn for every other building material, but the materials are so immensely variable that such graphs would probably be different for every brick, every sample of mortar, plaster, concrete or wallboard, and all would be very different from wood. If several different materials are built into the same wall the effect of this will become obvious. For example: Wood battens are set into brickwork covered with wallboard on one side, and plaster on the other; they will exchange moisture with each other, and with the air, until all have come into equilibrium. Suppose the atmosphere is at a relative humidity averaging 50 per cent; it will be seen that the moisture content of the wood is just under 11 per cent. But the bricks may vary between, perhaps, 1½ and 2½ per cent, the plaster probably less than 1 per cent and the wallboard perhaps 9 or 10 per cent. This is the normal condition of a perfectly normal wall.

Now if this wall becomes damp, all the materials will share in the dampness until they are again in equilibrium; their moisture contents will be higher, but will continue to be widely different. If the air has not changed they will ...