Internal moisture movement in hem-fir timbers exposed to ambient conditions following kiln drying

• Main Author: Wada, Nohara
• Language: English
• Published: University of British Columbia 2013
• Online Access: http://hdl.handle.net/2429/44050

This study aimed to investigate the changes of internal moisture content distributions of high-value hem-fir timbers after kiln drying while exposed to two different local outdoors seasonal conditions for a period of time of few weeks. Hem-fir is the most abundant species in coastal British Columbia, Canada, and high-quality thick hem-fir timbers used as construction material are one of the most important, and profitable products due to international market demand especially in Japan. Presently more-and-more of those houses are pre-fabricated and dimensional stability is paramount. Internal moisture profiles after kiln drying and their behaviour as a function of weather exposure in storage can result in dimensionally unstable products and consequently compromise quality. Conventional kiln drying of thick timbers is relatively difficult and requires long drying time to reduce final moisture content variation. Fast drying will result in steep moisture content gradients which may result in undesirable dimensional changes when products are used in normal service conditions. Thus, it is important to understand moisture behavior after drying and how that is affected by the environment. In this study, 90 x 90 mm in cross-section hem-fir timbers were dried to three different target moisture contents. Thereafter, stickered packages were stored under two diverse seasonal coastal environments thus emulating outdoor timber storage in a local sawmill. Moisture contents at 25 mm and 45 mm depths were continuously monitored for a period of three weeks. The results showed that moisture movement was observed between at 25 mm and at 45 mm depths regardless seasonal conditions while no significant net moisture content reduction took place during the cold-wet season after kiln drying. Also, regardless seasonal condition or target moisture content, moisture movement between at 25 mm and at 45 mm depths slowed down when differential moisture content between them was below 2.5%. In particular, at high target moisture content of 22%, moisture content values both at 25 mm and at 45 mm depths remained constant at moisture content difference value of 2.5% after two weeks regardless seasonal conditions, and no further drying or no further moisture content equalization were observed after that point.