Cross-scale moisture transport and moisture-gradient evolution in timber building materials during dehumidification drying

Non-uniform moisture redistribution during drying affects the quality and dimensional stability of timber, yet the connection between microscopic water-state transitions and macroscopic moisture gradients remains unclear. This study developed a cross-scale framework to investigate moisture transport in elm and Mongolian Scots pine during controlled dehumidification drying. A cross-scale characterization strategy integrating relaxation-resolved benchtop TD-NMR and depth-resolved single-sided TD-NMR was employed to correlate moisture-state transitions with spatial moisture-gradient evolution and apparent transport behavior. Free water was removed rapidly during the early stage, creating pronounced surface-to-core gradients, whereas bound-water removal was slower because of reduced molecular mobility and stronger water–cell-wall interactions. Elm exhibited a stronger stage-dependent decrease in apparent moisture diffusivity (Dapp) during the transition from free-water-dominated to bound-water-dominated drying, whereas Mongolian Scots pine maintained relatively lower and more stable Dapp values. During the free-water-dominated stage, the average Dapp was approximately 0.793 × 10−9 m2 s−1 for elm and 0.418 × 10−9 m2 s−1 for Mongolian Scots pine. Moreover, the apparent fiber saturation point (FSP) front reached the 5-mm specimen core within 8.945–11.243 h in elm, compared with 16.985–19.000 h in Mongolian Scots pine, demonstrating faster internal moisture redistribution in elm. These results provide quantitative parameters for evaluating drying uniformity and optimizing moisture-controlled timber processing.