The paper presents the numerical model of the rigid polyurethane foam microstructure. The model is based on the classical approach described by Gibson and Ashby (Proc. of the Royal Society of London, 1982). The main goal of the work is to create a model that will allow to properly describe the behavior of the foam. Particularly interesting are the differences in tension and compression of the foam. The intriguing issue is to determine the (effective) Poisson's ratio, which is defined for continuous materials. The structure of the foam is discontinuous, but its relatively simple model can be presented in the form of interconnected rigid beams and/or shells. It is a real challenge to find the properties of this structure in such a way that it reflects the behavior of the actual foam. During analysis of the microstructure there are observed various phenomena, among which one of the most interesting is the loss of stability of the elements that are compressed. Creation of a reliable model of the foam microstructure is an opportunity to explain many of the fascinating phenomena observed in the macroscopic scale.