The cell-strut modulus of rupture for brittle isotropic foams is not a constant, depending on the volume and Weibull modulus of solid cell struts. In this paper, the existing model describing the failure stresses for isotropic foams under multiaxial loading is modified to take into account the effect of variability in the cell-strut modulus of rupture. As a result, the failure envelopes of brittle foams with different prescribed survival probability and Weibull modulus can be generated and presented. Results suggest that the effects of cell size, Weibull modulus and prescribed survival probability on the failure envelopes of brittle foams are significant.