AbstractBio-based polyurethane (PU) spray foam insulation was prepared with soy polyol. The effects of adding wood fiber and water on the thermal and mechanical properties of the insulations were studied. The decomposition temperature (Td) of the foams increased with fiber reinforcement due to a higher degree of crosslinking. Alternatively, different fiber length contributed to different crosslinking. In addition, the neat foams, which didn’t contain wood fibers, had exceptional thermal stability with the increase of the amount of H2O by forming more stable polyurea adducts. In addition, PU spray foam blown with a larger content of H2O had higher compressive strength by forming a stiffer phase. Still, PU spray foam reinforced with fiber also had superior compressive strength due to the fiber framing into the foam struts. The effect of the fiber length on the compressive strength was evaluated. The degree of complex networks was influenced by the fiber length. However, the tensile strength was weakened with the addition of wood fiber.