The compressive behavior of a rigid polyurethane foam reflects its geometric structure and the physical properties of the matrix polymer. Foam properties may be modified by the inclusion of silica particles in the polymer matrix. These silica particles alter the mechanical properties of the base polymer as well as the geometrical structure of the foam. The yield stress and the modulus of the foam decrease with the initial introduction of the silica filler after which the mechanical properties of the foam increase with increasing filler content. A model is presented expressing the foam yield stress and the modulus as a function of the polymer modulus and the cell parameters. These parameters arel, a cell edge length, andd, a characteristic strut thickness. This results in the expressions,Ef=C″Ep(d/l)4and &sgr;y=‐Ep(d/l)4. This model is applicable to either filled or unfilled foams and accommodates changes in the properties of the solid phase, through the use ofEp, as well as changes induced in the cell geometry through thed/lrelationship.