Service Life Prediction of Wood Claddings by in-situ Measurement of Wood Moisture Content: Status after 5 years of Outdoor Exposure

The Danish Technological Institute is in co-operation with industry partners running a project aiming at predicting the service life of different wood protecting systems. The project focuses on examining the moisture reducing effect of different protecting systems for timber claddings and the ability of these to maintain the appearance of the surfaces, when the wood is used in service class 3 (EN 335-1 1992). A façade construction is exposed to weathering at the field test area of the Danish Technological Institute (near Copenhagen). In specific locations of the construction measurements of wood moisture are done by in-situ resistance moisture meters (Lindegaard and Morsing 2006). The aim is that the test should form the basis of evaluation of the maintenance requirements and the prediction of service life of the surface treatment and the wood/construction. At the moment 60 test racks are exposed. This study examines the data from the first five years of outdoor exposure using data from a test rack with a water-borne acrylic coating and a test rack with ICP coating for case studies. The moisture content data was converted into weekly average and weekly variation values which gave a deeper insight into the performance of the tested systems. The first was used to check for moisture build-up in the construction whereas the latter gave an indication of the gradual degradation of the coating itself, i.e. the degree of cracking. A linear tendency with time was found for all tested systems and the maintenance period could be estimated on the basis of data from only 2-3 years of outdoor exposure. The risk of biological degradation was evaluated using 20 % MC and 10 °C as a limit below which the risk was negligible. The annual accumulated time above this limit was used as an indication of the risk of biological degradation. Data was corrected for variation in weather exposure between the years and a fairly linear tendency in time was found, i.e. the risk of biological degradation was found to increase in time for all tested systems. Furthermore, other combinations of moisture content and temperature were used as limits enabling a more detailed view of the risk pattern. In general, it was found that measuring the moisture content in-situ in a façade construction of the suggested design can be a valuable tool in service life prediction. Within reasonable time, it is possible to obtain enough data to predict the service life of the construction in terms of maintenance period and the risk of biological degradation of the construction.