Abstract This work examines the moisture sorption, the morphology, and the thermal conductivity of fully bio-based composites, namely wood flour (WF) filled poly(lactic acid) (PLA), to better understand hydrothermal deterioration and potential for long-term application. Composites containing varying WF content (0%, 10%, and 30% filler by weight) were conditioned in either a fully submerged state or a fully desiccated state at four temperatures: (a) 7 °C, (b) 25 °C, (c) 35 °C, and (d) 47 °C. Results indicate that an increase in temperature typically corresponded to an increase in the rate of moisture uptake and percent crystallinity. Higher fiber contents also typically resulted in an increase in the rate of moisture uptake and thermal conductivity. Signs of deterioration, such as cracking at low temperatures and discoloration at high temperatures were noticed for these composites. Environmental impact assessments suggest the desired application and environmental exposures for these bio-based composites could drive constituents used. While the ease of production and the popularity of PLA facilitate the potential use of PLA WPCs, these composites would be expected to exhibit deterioration under high moisture conditions even at ambient temperatures.