A numerical model of inverse segregation in a vertically cast unidirectionally solidified aluminum-copper binary alloy is presented. The model predicts the solute concentration distribution up the length of the casting. The model is validated on comparison with available analytical solutions. Initial comparisons with experiments show that the model predicts a non-physical region of positive segregation in the upper part of the casting. On accounting for microporosity formation, however, the model predictions show close agreement with experimental measurements. Application of the model also demonstrates the need to correctly account for microsegregation processes.