In determining ventilation rates, it is often necessary to combine naturally-driven ventilation, such as infiltration, with mechanical systems. Modern calculation methods are sufficiently powerful that this can be done from first principles with time varying flows, but for some purposes simplified methods of combining the mechanical and natural ventilation are required—we call this “superposition”. An example of superposition would be ventilation standards that may pre-calculate some quantities within the body of the standard. When there are balanced mechanical systems, the solution is simple additivity because a balanced system does not impact the internal pressure of the space. Unbalanced systems, however, change internal pressures and therefore can impact natural ventilation in such a way as to make it sub-additive. Several sub-additive superposition models are found in the literature. This paper presentments the results of millions of hours of simulations of the physically correct solution, which span a broad range of weather, leakage and structural conditions. This wide range of data allows for the comparison of three superposition models from the literature and two new ones. The results showed that superposition errors can be reduced significantly by using the appropriate model(s), from the 20% over-prediction from simple linear addition to 1%, or less.