The moisture adsorption and hygrothermal dimensional stability of decking boards were investigated under simulated extreme climatic conditions. Some commercial decking boards absorbed above 4% moisture after exposure to a hot and humid environment. These decking boards underwent more than 7% swelling in thickness and ultimately developed about 5 mm longitudinal bowing based on 61 cm long boards. Both expansion and bowing were partially recovered after drying.The current practices of using high fiber loading and applying hollow profile commercial decking boards made from NFPC have raised concerns about their moisture resistance and hygrothermal dimensional stability.The moisture absorption mechanism of NFPC was studied by applying percolation theory. Two concepts, accessible fiber ratio and diffusion-permeability coefficient, were defined; one model based on percolation theory was developed to determine critical accessible fiber ratio. The results of moisture absorption and electrical conductivity behavior investigations suggested that percolation is the underlying mechanism of moisture absorption of NFPC.