A method, apparatus, and system in which an air dam is incorporated into an air ventilation stream of a datacenter to control the temperature of an interior space of the datacenter. The datacenter may include a raised floor in which the ventilation stream is injected through perforations within the raised floor. An air dam may be positioned at an entry region of the under floor plenum to obstruct the flow of air from a supply air plenum. The air dam is used to create a positive pressure across the raised floor at a substantially constant pressure distribution.