Physical blowing agents such as n-pentane and methyl formate and, for comparison, chemical blowing agents such as water were used to prepare structural polyurethane rigid foams of different densities by reaction injection molding. Experimental runs were carried out with formulations based on oligomeric isocyanate and a mixture of polyether polyols. The constitutive equations for the vaporization rate of the two blowing agents and the polymerization kinetics data are reported. Experimental results were compared with the prediction of a simplified theoretical model, and they showed a satisfactory agreement in terms of temperatures and density profile. All the specimens were characterized by physical-mechanical properties such as hardness, impact strength, flexural strength and elastic modulus and the results were reported in function of the densities. The best mechanical performance were obtained with the physical blowing agents, due to a better density distribution profile and a thicker skin layer.