Last edited 06 Nov 2020

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Institute of Historic Building Conservation Institute / association Website

Understanding dampness

Only when we understand how moisture moves within the fabric of a building can we predict how moisture will move after retrofitting works have been carried out.

As often cited in best practice publications such as BS 7913:2013, a condition survey is essential before we contemplate any work to a building, and that must include a proper understanding of dampness. It is also essential that a proper investigation of dampness is carried out that focuses on the cause and not just dealing with the symptoms.

We can measure the cause by using hygrometers, measurements of temperature, relative humidity, and the absolute humidity (total moisture content of the air). A room-by-room picture of atmospheric moisture levels can reveal previously hidden information that sheds light on the cause of problems. A particularly damp room may be above a cellar, or a suspended timber floor with poor or blocked ventilation. Often, cracked or blocked drains are revealed in this way. Symptoms of dampness such as mould can be tracked to unventilated kitchens and bathrooms.

Combined with thermal imaging, temperatures of the building fabric can be readily observed, and cold areas highlighted which give rise to condensation within the fabric. For example, within concealed spaces such as under floor voids which we cannot fully explore due to poor access, the hygrometer can be used to collect data to enable us to observe whether condensation exists or is likely if temperatures drop to dew point (the temperature at which condensation occurs) at any time.

Common terms such as ‘rising damp’, ‘breathability’, ‘penetrating damp’, and many others, all relate to moisture movement mechanisms. Hygroscopicity describes how moisture from the environment around building fabric is absorbed into it when humidity is high, as in episodes of high condensation, and is released when humidity is lowered. Commonly, moisture caused initially by other moisture movement mechanisms can bring contaminated moisture to the surface of a wall and leave hygroscopic salts behind. This frequently happens in chimney breasts, due to the high concentrations of nitrates and sulphates created by burning fossil fuels. The important issue is to be able to identify hygroscopicity and not to get it confused with other moisture movement mechanisms. This is critical as most traditional materials in older buildings are to varying degrees hygroscopic.

Another moisture movement mechanism is capillarity, often termed capillary attraction, which is related to the form and shape of pore structures. Despite common reference to capillary attraction, it is rarely a significant contributor to dampness, for a number of reasons. These include vastly different pore sizes present in materials; that they rarely interconnect; that they are often clogged up with salts; and that moisture often evaporates from a material at such a rate to minimise movement through capillary attraction.

Vapour permeability is often used to describe a host of moisture movement mechanisms. Some materials allow moisture movement and some do not: vapour permeability is the ability of building fabric to allow water vapour to diffuse through it. Water molecules are relatively light and in any given volume of air, gaseous water will rise naturally. When trapped in building fabric, diffusion acts outwards and upwards, and will find its way around areas where vapour barriers have been formed.

Traditional buildings were designed so that moisture could move. It is when this moisture movement is restricted or stopped that one finds concentrations of moisture in other areas. Many buildings have been compromised in this way through ‘improvements’ such as the replacement of timber floors with concrete, retrofitted damp-proof courses, and the installation of impermeable, solid wall insulation, to name but a few. Moisture affects the thermal performance of building fabric. In bringing these two issues together, we call it hygrothermal performance. Critically, damp building fabric is a much better conductor of heat than dry building fabric.

This increase in heat loss is cited in BS 7913: 2013 as being at least 30 per cent, and some claim it to be even greater. Therefore, managing moisture so that building fabric remains comparatively dry will save energy. Mould growth and condensation are often noted in vacant properties where heating has been turned off. Lack of heat results in many episodes of condensation as the temperature of fabric drops below dew point temperature. Constant heating at around 15°C, combined with ventilation, will reduce this risk, particularly during winter months.

In understanding dampness prior to retrofitting proposals, one has to understand how moisture movement has already been compromised, and then work out how different retrofit proposals will further effect the movement of moisture. Reinstating the original moisture movement characteristics would seem the ideal option, but often this is either impossible or cost-prohibitive.

The second option is to work out the source of moisture within the building fabric, and how the moisture is moving around. This may be easier said than done, but it is absolutely essential. It must start with understanding the design of the building, how it is constructed and the materials it is constructed with. Taking a holistic building pathological approach, assisted by appropriate tools and equipment, is essential, as is an understanding of how people are living in the building.

Analysing moisture content in timber is relatively easy using the common electric twin-pinned moisture meter. One must be aware that readings could indicate dampness in dry timber if it has been treated with chemicals. This is a distinct possibility in roof spaces. Some timber may, however, be concealed, and an experienced building pathologist may be able to determine the moisture content of adjoining building fabric and make predictions based on other factors. Micro-drilling can also be used to determine the strength of timber. This, often used where joists are bearing into walls, can indicate whether such concealed timber is rotten.

In examining the moisture content of other materials, such as masonry, an electric capacitance meter may help, but only in the hands of the very experienced building pathologist. So-called dampness readings may not be highlighting dampness at all and can be very misleading. Only in experienced hands should the use of such equipment be contemplated.

Note, however, that British Standards state that chemical methods should be used to determine dampness, and this involves the extraction of building fabric. This can be through the ‘gravimetric’ method, described in BRE Digest Rising damp in walls - diagnosis and treatment (DG 245), which involves testing the samples in a laboratory. Alternatively, the samples can be tested on site using a carbide meter, often known as the ‘speedy meter’.

Some typical dampness problems occur when external ground levels are too high. This inhibits the movement of moisture from the building fabric via the external face, and so develops a greater tendency for it to move internally in greater quantities.

Replacing timber floors with concrete incorporating a polythene damp-proof membrane removes any possibility for moisture to move through the floor. This will tend to move it towards the perimeter of the floor and into the walls. Here there will be a tendency for increased moisture within the walls. Combine this with heightened external ground levels and the situation will be even worse. The answer for many has been to install a retrofitted damp-proof course, which deals with the symptoms but not necessarily the true cause. Dealing with the cause could mean installing a limecrete floor.

When moisture penetrates the external face of a wall through any of the moisture movement mechanisms, it should have the opportunity to move out from the wall via the external face. Unfortunately, many older buildings with masonry walls have a core which is not fully mortared but merely has a rubble fill. With the advent of penetrations being formed for building services, the vibration in cutting holes through the walls can result in the settling of the rubble and the creation of hollow areas or voids.

Any moisture penetrating the face of the walls can trickle down within the core and come out at a lower level, sometimes quite a distance from where the moisture originally entered the wall. Over time the situation may get worse, as the continuing flow of moisture could increase the number and size of hollow areas when any mortar which is present is washed away.

The answer is not to just deal with the external face of the wall, as this means that moisture entering the wall may find any external access points blocked, thereby increasing the tendency for increased moisture internally, but to fill the voids with mortar or grout. However, the above view is not universally shared. There are many situations where voids can encourage the evaporation and diffusion of moisture, and act as insulation to keep the dew point down and prevent condensation. The lesson here is that all situations are different and proper analysis by a competent building pathologist is essential.

This article explains just some of the issues to look out for and is far from comprehensive. However, it should spell out the need to undertake a proper analysis of a building’s condition and, in particular, how moisture moves within building fabric. One cannot properly design retrofit improvements without knowledge of moisture movement.

This article originally appeared in IHBC's Context 149, published in May 2017. It was written by John Edwards, director of Edwards Hart Consultants and Pete Ward, a director of Heritage House Consulting.

--Institute of Historic Building Conservation

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