Moisture Measurement in Tirsted Church

Decoration on the chancel walls of Tirsted church in Denmark had been affected by salt damage over the past 100 years. To find the source of the soluble salt, the moisture content of the masonry was measured using different methods. Surface measurements carried out using a capacitance instrument gave misleading results due to the influence of salts within the plaster. The neutron probe provided more reliable data, showing high moisture levels close to the ground. By using wooden dowels it was possible to show that the wall was always in equilibrium with 100 per cent relative humidity (RH), so the moisture content was higher than the hygroscopic moisture uptake. The use of gypsum blocks indicated that the moisture was supplied mainly from the outside by rain or fog, whereas rising damp did not occur. The moisture content was also strongly influenced by heating in the winter. With moisture evaporating from the internal surfaces, salts appeared to have concentrated in the plaster over time. This assumption was supported by measurements of the room environment, showing an increase in the moisture content of the air during periods of heating.