An access floor assembly is provided that comprises a plurality of abutting access floor panels that are attached to a plurality of pedestals. A resilient and flexible gasket is provided between the abutting floor panels to provide an effective seal between the panels. The effective seal allows a pre-determined pressure to be maintained in a plenum located between the access floor panels and a sub-floor. Air under pressure in the plenum may be delivered in a controlled and consistent manner from the plenum to a space above the access floor assembly.