The material resistance of wood to decay must be due to the combined effects of heartwood extractives and the moisture content state. Evaluating the moisture exclusion ability of wood is important for assessing its durability in outdoor use. We conducted previously double-layer outdoor exposure tests and investigated the relationship between moisture content over time and the progress of decay using an electric resistance moisture meter. In this study, we aimed to evaluate the moisture exclusion ability of wood more simply using the same double-layer test specimens. The test species were Japanese cedar, Japanese cypress, Scots pine, and Norway spruce (mainly heartwood), and moisture content was calculated from the weight change of the specimens during decay rate inspections every six months. The natural durability of the test species was evaluated using standard fungi-based Japanese laboratory decay tests. The moisture content trend derived from the specimen weight change generally corresponded to the results obtained with the electric resistance moisture meter. Furthermore, the moisture content of the upper and lower layers differed by wood species; only the upper layer exhibited low moisture content in Japanese cedar, whereas both layers exhibited low moisture content in Scots pine and Norway spruce. This indicates that Japanese cedar has good drying performance, while Scots pine and Norway spruce have low moisture permeability. From the laboratory decay test, Norway spruce exhibited low durability, similar to that of the reference specimens (Japanese cedar sapwood), yet decay progressed slowly in the double-layer test. This may be attributed to differences in the moisture content state; in the laboratory test, the moisture content of all species remained above 50%, whereas in the double-layer test, Norway spruce experienced periods with moisture content below the fibre saturation point. This study demonstrates a simple method for evaluating the moisture exclusion ability of wood without specialised equipment by calculating moisture content from the specimen weight change during the double-layer test and evaluating the upper and lower layers separately.

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