AbstractThis paper proposes a unified sorption–desorption isotherm for the full range relative humidity (0%–100%) based on categorizing the behavior of different pore size groups for engineered wood products. The sorption and desorption isotherms are established by accumulating the wetting and drying behavior of different pore groups, which qualitatively represent the physical structure of wood material. The porosity of wood is categorized into three distinctive pore size classes, shown to be adequate for capturing the moisture capacity of different wood species with reasonable accuracy, and the saturation behavior is studied separately for each pore type. Based on physical concepts, desorption isotherms are probabilistically derived from sorption behavior. The proposed sorption-desorption isotherm is calibrated with experimental data conducted on seven engineered wood products, including Pacific Teak, Tasmanian Oak, Blackbutt, Radiata Pine, Slash Pine, laminated veneer lumber (LVL) of Radiata pine, and cross-laminated timber (CLT) of Spruce. The relative humidity range for the active participation of each pore type in the moisture content is discussed, and further conditions for the simplification of the proposed isotherm are demonstrated.