Greening of Masonry Walls Research Project - Why do algae grow on walls? Test walls

Why do algae grow on walls? Algal greening of walls is quite patchy, and this patchiness probably relates to the microclimatic and microenvironmental conditions on different parts of the wall. Algae need moisture in order to grow, but they also need suitable conditions of light and temperature. In order to keep things simple in our research, we try and use specially-built test walls to investigate how and why algal greening occurs.

Test walls have several advantages - for example, we know exactly what they are made of and we can also install a range of probes and other devices to monitor them (which we couldn't do on walls of real historic value). Whilst microclimate can quite easily be measured with temperature and relative humidity probes, it is much harder to measure how wet the walls themselves are. For this, we use a non destructive technique called 2D resistivity survey.

This technique allows us to 'see' inside the wall, to a depth of 20 or more cm, and visualise the moisture conditions inside. How we do it is shown in the accompanying video clip. Understanding how moisture moves within walls is crucial to understanding why algae grow on them, and we are carrying out repeat 2D resistivity surveys on our test walls as (we hope!) algae colonise and grow on them.

What is resistivity and how does it relate to moisture?

Resistivity is a material property that relates current passing through the material to the voltage recorded. Different materials have differing values of resistivity, and for any one material the resistivity observed is influenced by temperature, level of moisture, porosity and salt contents. How much moisture is in the material is one of the major controls on resistivity of porous materials and so the resistivity values (as long as other factors are equal) can be used to estimate moisture contents. Many commercial moisture meters are based on resistivity.

What are the advantages of using resistivity surveys to map moisture in walls?

Resistivity surveys are a convenient, non invasive and non destructive way of estimating moisture distributions within walls. The electrode spacing used and number of electrodes in the transect will determine the depth to which you can 'see into' the wall. Using 50 electrodes spaced 4cm apart and using the Wenner configuration, using our Geotom device we can gain a profile which stretches some 40cm into a wall. The resistivity survey technique is quite quick to use (it takes about 30 minutes per profile of 50 electrodes), and you do not need to insert any probes into the wall. We use medical electrodes to ensure good electrical contact between the resistivity equipment and the stone, meaning that we don't need to put any damaging metal pins into the surface.

What are the disadvantages of using resistivity surveys to map moisture in walls?

Like all techniques 2D resistivity surveys are not without their drawbacks. For a start, the equipment is expensive and the RES2DINV software used to calculate the apparent resistivity distribution within the wall quite complex. The fact that we are measuring resistivity variations and inferring moisture contents from them, means that this is an indirect technique - and measurements can be affected by other variables. The very near-surface layer (up to a few cm thick - depending on the electrode spacing) cannot be probed with this technique because of the geometry of the electrical fields created during measurement.

Why does moisture in walls affect surface greening?

Algae need moisture to grow, as well as light and nutrients. Walls which have surfaces which remain wet for long periods are likely to be more hospitable for algal growth than those which are totally dry or experience major swings in surface moisture levels. Understanding the movement of moisture within walls is important to understanding moisture regimes at the surface of walls - as the inner parts of walls can act as an important storage area for moisture which can then be transferred to the surface under certain conditions. Our project is using a combination of field (both real walls and test walls) and laboratory data to try and understand what moisture conditions are ideal for algal growths.

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