Experimental data from the Helium-Air Ingress gas-Reactor Experiment (HAIRE) has provided insight into an infrequently studied flow scenario where buoyant driven flow and molecular diffusion combine during an exchange flow. The combined buoyant-diffusive exchange flow occurs because of a transition region where buoyancy and molecular diffusion combine to create flow rates higher than the separate flow regimes would predict. The buoyo-diffusive flow is described by a dimensionless number which is used to quantitatively explain the results. The flow regime is of considerable importance in the small and medium-sized accident scenarios for High-Temperature Gas-cooled Reactors (HTGRs), where the flow regime will increase the air ingress rate in comparison to previously understood theory. For medium sized breaks the estimated increase in air ingress rate is up to 5% compared to the previous theory, and for small sized breaks the increase is either 5% or greater depending on the break size. This considerable increase in air ingress rates could affect the evolution of small- and medium-sized air ingress accidents and the overall damage to the HTGR's graphite core. Buoyo-diffusive exchange flow could manifest in other areas where small physical scales and high mass diffusion are present.