Polyurethane (PUR) rigid foam was assembled to the core layer of sandwich panel due to insulating and specific construction requirements. Wood-based panels with a foam core can be manufactured either in discontinuous process in which foam core and wood-based surface layers are separately prefabricated before assembling, or in continuous process that requires additional separation of the faces for foam injection. The panel production may be executed in a continuous one-step process without separation technique in order to reduce the manufacturing stages. An application of mixed PUR reactants before pressing, during the formation of the mat, causes the expansion to take place after face layer compaction. This technique, however, requires a precisely adjusted PUR-formulation with less liquidity for the regular distribution of mixture. To improve the PUR application, wood flour was added into component mixtures to raise the viscosity of components. The reaction time and properties of heat-cured PUR foam affected by wood addition were studied in this research. Wood flour was added into PUR reactants, diphenylmethane diisocyanate (pMDI) and sucrose-based polyether, based on different mass ratios. The components were mixed then cured at 100°C referred to the using temperature in the hot press. Due to the hygroscopicity of wood, its moisture is another influencing parameter for foaming and therefore considered as blowing agent instead of distilled water. The measured foam properties of samples blown by wood moisture did not show significant difference comparing to those blown by distilled water. Furthermore, the foaming ability on high wood ratios was investigated by maximizing the wood content. Considering the optimal formulation for PUR foam in this study stage could support its application into the hot press. After all, these results may then lead to a suitable foam core material for wood-based sandwich panels produced in a continuous process.