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20 May 2011

Wednesday 18th May 2011; Moisture

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

Our lectures continued with Ippolito Massari today discussing the diagnosis of moisture sources and methods for control.

We often discuss the issues associated with using the wrong mortar for pointing, but what about using no mortar. I have tended to think about joints missing mortar as being traps for water running down a building and allowing the water to enter the stone because it is not being 'wicked' by the mortar. In fact there's a more technical issue being faced here, which is that water trying to evaporate from the recessed mortar joint will tend to cause such an increase in water vapour within the empty joint space that it will re-condense as liquid water before it leaves the structure, and therefore retaining the water and continuously circulating it. It is the circulation or flux of water that is the danger associated with water in masonry, in a stable environment the same problems will not be experienced. Therefore if a wall that we wish to conserve is wet we have two options; 1. fully dry the wall (there is no purpose in partially drying a masonry structure) or 2. stabilise the environment.

Stabilising the environment in a dwelling is not possible like in a museum-type environment, because we aim to live in dry and warm buildings with good ventilation to prevent water from condensing on internal surfaces. As mentioned yesterday (Day 24 (17.05.2011)) our historic buildings were usually designed with an inbuilt ability to create a suitable internal living environment. Now our requirements of inside spaces have changed and most people wish to live within a draft-free environment, blocking up any possible space where a draft might enter. In doing so, however, we hinder the ventilation that is necessary for preventing the accumulation of air vapour which condenses on our walls and encourages the growth of mould. This problem is particularly prevalent in Scottish houses built with hard impermeable stones, such as granite in Aberdeen, and where they have been re-pointed using hard impermeable cement mortar. Many of these homes also have the original wooden sash and case windows, which have a natural ventilation, replaced with air-tight PVC windows with a small air vent, which home-owners commonly block-up to prevent the draft (also common in Italy apparently). The result of this is an air-tight box and any moisture created inside (e.g. steam from a kettle or water vapour from breath) will be trapped. A detached house is likely to have a more stable environment throughout the structure where there is sole occupancy, whereas in a block of flats it is likely that each set of occupants will heat or ventilate their flats differently causing large variations in moisture and vapour movement throughout the building.

The final part of this set of lectures dealt with the possible solutions to the issues associated with moisture ingress; Ippolito pointed out that although similar solutions may be appropriate for each project, there is no way of having a standard because there will always be different factors that influence the movement and accumulation of moisture in each different building. It is essential to correctly identify your moisture sources so that you can implement the appropriate solution; the solutions appropriate for rising damp due to capillarity from dispersed water will be very different from those suitable for dealing with capillarity from ground water. Any intervention work should always be carried out by an expert.

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