The objective of this study was to determine the minimum water vapor permeability of exterior wood coatings for log homes using hygrothermal and decay modeling. The approach is based on a worst-case scenario assumption, namely a moisture trap at a given wood moisture content in a coated wall log. The minimum limit of vapor permeability of the coating system was determined by simulating the time required by the moisture trap to exit the decay risk zone, applying a dose-response model specific to brown rot fungi. The results suggest that log home walls should not be coated at the outside or, if required for decorative purposes, with a highly permeable exterior coating system with a maximum Sd-value of 0.2 m. The approach of taking the starting point in a worst-case scenario is reasonable as the service life of wooden components in Use Class 3 is typically determined not by decay attack due to an increase in average moisture content of components but attack in moisture traps. This approach could be applied not only to log homes but also to other exterior constructions made of for instance mass timber.

Download document (1.1 Mb)
free for the members of IRG. Available if purchased.

Purchase this document