Water plays an essential role in fungal decay of wood, and limiting the cell wall moisture content by chemical modification can effectively improve the durability of the material. Investigating the wood-water relations of modified material under climatic conditions relevant for fungal decay are, however, experimentally challenging. Most studies in literature therefore focus on moisture sorption under conditions outside those of importance for fungal decay. This review discusses the validity of such data for characterising the wood-water relations at very humid climatic conditions, relevant for fungal decay. Moreover, the review attempts to cover the basics of fungal decay, the important role of water, and how controlling water content by modification can improve durability.