What is hot aisle/cold aisle containment?

Hot aisle/cold aisle arranges server racks so that all rack fronts face one aisle (cold) and all rack backs face the adjacent aisle (hot), preventing cooled and exhaust air from mixing. Without containment, hot exhaust recirculates and forces cooling systems to work 20-40% harder. Containment adds physical barriers — curtains, doors, or rigid panels — to fully seal either the hot or cold aisle. Cold aisle containment is more common and encloses the supply air path; hot aisle containment captures exhaust and routes it directly to cooling return plenums. Proper containment improves PUE by 0.1-0.3 points and is a prerequisite for rack densities above 10 kW. It costs $500-2,000 per rack to retrofit but pays for itself in energy savings within 12-18 months.