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18 May 2011

Tuesday 17th May 2011; Moisture

Week 6 of 12 
Module 4: Deterioration Mechanisms; Methods of Survey & Analysis

Ippolito Massari, an engineer and expert on humidity and water, spoke to us today about 'Moisture Sources and Effects' including a visit to the Basilica di San Clemente this afternoon to see some of the theory in action. We were shown classifications for the various forms of moisture ingress that that we find in buildings; capillarity, condensation, hygroscopy and infiltration. Crucially it is the movement of moisture that causes problems in buildings, a stable environment reduces the possibilities of deterioration of stone.

Ippolito highlighted the point that it is always important to think about the less obvious reasons for the causes/sources of damp, e.g. why is there more condensation in a church in the summer?- possible answer; because more people are exploiting the cool environment of a church and therefore producing a higher amount of air vapour present. Obviously in the majority of cases there are likely to be multiple sources of the damp in buildings. Equally when first assessing an area of damage it is important to identify if the source of the problem has been fixed or if the problem is ongoing.

When considering water movement around a masonry structure we need to understand the materials with which we are dealing. It is important to remember that a porous material is not necessarily permeable, and a permeable material does not necessarily have active capillaries (i.e. it cannot 'suck' water). A good example is when thinking about two very different materials; clay and gravel. Clay is composed of particles with a very small grain size. If water tries to enter from above then the clay expands like a sponge and although the water will enter to a certain level, the expansion will prevent the movement of water downwards. If water sits below a layer of clay the water is able to rise through capillary action. Gravel has almost the opposite characteristics; water entering from above will permeate very easily through the material, but the spaces between the grains are so large that there is little to no suction by capillary action. Essentially there is an inverse relationship between capillarity and permeability; these properties have commonly been utillised historically in building construction.

It is common to see rising damp appearing on walls in waves- the higher water marks are frequently interpreted as being where there is a higher input of water, however the usual explanation for this is that this is where the wall has a greater thickness. Evaporation is the major factor that reduces capillary rise and a thicker wall with the same evaporation surfaces will allow greater capillarity enabling the dampness to reach a greater height. It is important to remember that rising damp is not always the result of proximity to the water table, it can be the result of water entering the soil through rainwater dispersal, leaking underwater transport systems, etc. These sources will also produce uneven water marks but for different reasons. If you wish to solve the associated issues correct interpretation is essential.

Many historic architectural designs incorporated structures and solutions to handle the issues associated with water movement, for example the Romans commonly built vaults below their buildings to allow ventillation and evaporation of moisture before it reached the occupied spaces. The fashion for utilising these spaces causes widespread problems with damp in buildings due to the impermeable layers which are inserted to try to make these spaces dry and habitable- something for which they were never intended.

We discussed various methods for testing the water content in a building including electric resistance metres and chemical measurements, however these have issues associated with interpretation of the results; electric resistance only shows you the surface moisture in a wall and can be affected by the presence of salts, and chemical measurements are highly dependant on sampling technique and application of the method. The only true test of moisture content is by testing variation in density of a wet and dry sample of stone in a lab, but clearly this is a destructive method and not always appropriate. We tested some of the methods discussed on our site visit to the Basilica di San Clemente this afternoon.

One type of electric resistance metre being tested on a wall in the Basilica di San Clemente.

One of the underground vaults below the Basilica.

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