Measurement of moisture content in solid brick walls using timber dowel

Abstract

Moisture within walls can have a serious detrimental impact on buildings and undermine their long-term durability and integrity. This paper investigates the accuracy of the timber dowel method for measuring wall moisture content in a solid brick wall. The timber dowel is inserted in the wall and removed at intervals and moisture readings obtained using a resistance-based, moisture-meter. The moisture content of timber dowels and the surrounding brick and mortar in a laboratory wall is measured and compared to the moisture content in the timber dowel, brick and mortar in different relative humidity environments (saturated salt solutions). The paper finds that a linear relationship of increasing moisture content in the dowel and laboratory wall is observed. At medium dowel moisture content (dowel moisture content >15 %), the moisture content in the dowel, brick and mortar is similar in both the laboratory wall and the RH environments. At higher moisture content there is a divergence whereby at the same dowel moisture content, the laboratory brick and mortar show higher moisture contents than those achieved in the RH environments. It appears that liquid water present in the wall has difficulty transferring to the dowel and consequently the dowel underestimates at high moisture contents. It is concluded that the dowel gives a good indication of the moisture content in the wall when the dowel readings are not higher than 15 % and indicates changes in relative wetness above this. In-situ monitoring of the moisture content of a nineteenth century brick wall following the application of lime plaster showed expected changes in moisture content. An increase in wall moisture content following the application of the plaster and the wall returned to its original moisture state by 9 months.