An ergonomic seat or bed must be capable of supporting optimally and evenly the user’s body for sustained periods of time. The objective of the performed nvestigations was to ascertain the impact of natural aging on the stiffness of polyurethane foams in dwelling apartment conditions and, additionally, to determine regression equations describing this dependence. Seven types of furniture foams, differing with respect to their apparent density and stiffness, were selected for the experiment. Experimental foams were exposed to aging for the period of 730 days (two years) in climatic conditions typical for dwelling facilities. Foam stiffness was determined in a uniaxial compression test determining strain characteristics in the function of deformations. The performed experiments made it possible to establish percentage changes of stiffness of some foams as well as the time required for those changes to assume a significant character.