Wood is a hygroscopic material. Material properties are affected by the hygroscopicity of wood; for example, the strength value decreases with increasing moisture content of wood. Wood in the living tree generally has a moisture content (MC) of 30% or greater. The cell wall is fully saturated. After the tree is felled, the green wood is exposed to atmospheric conditions. It loses water until it comes to an adequately low moisture content to be at equilibrium with the surrounding atmosphere. This is called equilibrium moisture content (EMC) which is approximately proportional to the relative atmospheric humidity (RH). Also, EMC varies with the kind and distribution of cell-wall constituents, different wood species, affected by temperature, between heartwood and sapwood, extractives, previous exposure history and mechanical stress. The EMC decreases with decreasing relative humidity, also, increases with the increasing relative humidity of the surrounding air at a constant temperature. The important point that the EMC at a given relative humidity is not constant. It is increased or decreased to reach equilibrium depends on the level of moisture in the timber. The paper presents experimentally and theoretically approach to the effect of relative humidity and moisture to the durability of spruce and laminated timber during to drying and wetting exposure.