Climatic impacts on the moisture performance of wooden decking and facades
T Bornemann, C Brischke, G Alfredsen
In outdoor usage wood is exposed to a variety of influences, which reduce its durability against biological deterioration and wetting. Besides many other factors the service life of wooden facades and cladding is dominated by occurring moisture loads on the one hand and their capability to withstand moisture on the other hand. Hence, climatic factors have a direct impact on the material climate and thus on service life. However there is still a lack of knowledge about the relationship between material climate and climatic parameters. Within this study wood moisture content and wood temperature were recorded for different wood species over a period of 34 months. By means of a dose-response-performance model the moisture and temperature dependent dose was determined for different wood species, orientations and expositions. The respective dose-time functions were calculated and expected service lives estimated. Relations of the service life to specific wood species, orientation and exposition were identified. For claddings oriented to the south higher dose values were induced compared to claddings oriented to the north. Also the horizontal exposition experienced higher moisture loads compared to the vertical. Furthermore differences between different test sites were determined and could be explained with different climatic loads impacting on the construction. Driving rain loads led to higher moisture levels in the claddings. Furthermore the wood moisture content was found to be influenced by RH levels and increased temporarily after rain events. During the summer period with higher air temperatures redrying was fostered and lower dose values were induced. No significant relationship between the sorption behavior of different wood species determined under laboratory conditions and their respective moisture performance in the field was found. However, a general trend could be deviated, which showed that the higher the wetting ability of wood the lower was the determined resistance against fungal degradation. The variation between the sorption behavior of the different wood species was not interconnected to the moisture performance or decay resistance of the respective wood specie in the field. It is recommended to take more advantage of the additional information provided by continuous MC measurements, in particular with respect for service life prediction issues. They can provide additional information about the respective material itself as well as about its performance in different exposure situations.
Keywords: service life prediction, moisture monitoring, climatic parameters, sorption, outdoor performance