In an air-cooled data center, airflow management is about delivering cool air to the IT equipment and returning warm air to the computer room air handler. Designers must pay attention to components that are sources of fugitive leaks to reduce bypass losses. In the case of HAC systems, ceilings form a return air plenum and help to separate the warm return air from the cool conditioned room. We will share recent experiment and CFD work that illustrates the importance of the ceiling system to the overall air containment strategy.