A well designed rack environment would include cold aisles (in front of the equipment with the AC vents) and hot aisles (behind the equipment with the air returns). Then the racks would have no gaps between the rack mounted equipment. If there are gaps between the equipment, then spacers are added to block air flow. The reason is to prevent hot and cold air flow to find alternative routes from the front of the rack and the back of the rack. That prevents equipment from recycling hot air back through the fans. Dutchess Count BOCES new data center at the Salt Point campus is a great example of this design.