Cooling
Containment puts a physical barrier between the cold air going into the equipment and the hot air coming out of it. Cold-aisle containment encloses the cold aisle; hot-aisle containment encloses the hot aisle. Blanking panels do the same job inside each rack. Without them, hot exhaust finds its way back to the inlets, and the cooling has to work harder to make up for it.
A way of keeping supply air and exhaust air apart. Racks are first arranged in alternating hot and cold aisles, fronts facing fronts; containment then encloses one of the two aisles. Schneider Electric's White Paper 135 explains that both approaches can improve the predictability and efficiency of the cooling, and that containment gives uniform IT inlet temperatures and removes the hot spots of uncontained rooms.
LBNL's master list describes two levels: doors at each end of the aisle with the top left open, rigid or flexible strip curtains, which allows some recirculation but does not interfere with the fire sprinklers; or aisles closed at each end and at the top, which separates the air very well, but fire code may then require sprinklers inside the enclosed aisle.
| Cold-aisle containment (CACS) | Hot-aisle containment (HACS) | |
|---|---|---|
| What is enclosed | The cold aisle, at its top and ends | The hot aisle, collecting the exhaust |
| The rest of the room becomes | A large hot-air return plenum | A large cold-air supply plenum |
| Room temperature outside the containment | Same as the hot aisle | Same as the cold aisle |
| Typical use | A convenient, cost-effective retrofit for many existing rooms | Preferred for new data centres |
Schneider Electric's comparison finds that choosing hot-aisle over cold-aisle containment can save 43% in annual cooling system energy cost, a 15% reduction in annualized PUE, mainly through more hours of economizer operation. The PUE effect is explained in PUE explained.
They close the gaps in a rack so exhaust cannot loop back to the front. LBNL's list says blanking panels should seal openings under and between racks, between equipment in partly filled racks, and in completely empty racks, that broken rows should be filled with empty racks blanked from top to bottom, and that managing blanking panels is especially important in hot and cold aisle rooms.
Panels come off during work and rarely go back on by themselves. LBNL recommends a program to minimise leakage by maintaining blanking panels and unbroken rows, and points out that a large fraction of the air from the cooling units may pass through leaks in a raised floor, so floor openings belong on the same list.
It depends on where the UPS takes its air. ASHRAE's power equipment paper notes that nearly all distributed, rack-mounted UPS units pull air from the cold aisle, with variable-speed fans controlled by internal thermal sensors, so they behave like IT equipment and benefit from the same containment. Blanking the rack space around a rack-mounted UPS keeps it breathing supply air rather than its own exhaust.
Larger UPS cabinets and their batteries often sit outside the contained aisles, in the room itself. With cold-aisle containment that room runs at hot-aisle temperature, which suits neither the UPS cabinets nor a battery string that ages fastest when warm. With hot-aisle containment the room sits at cold-aisle temperature. That makes the containment choice part of the UPS and battery plan; see the UPS battery life guide.
It can. LBNL notes that fully enclosing an aisle at the top may require sprinklers inside it under the fire code, while doors with an open top do not interfere with the sprinkler system. Gas-based suppression and detection must also reach the enclosed space; the UPS and battery room fire protection guide covers how detection, suppression and the UPS interact.
With the cheap steps first. LBNL's list starts with keeping rows unbroken, filling gaps with empty racks blanked from top to bottom, putting blanking panels in every open space, and sealing raised-floor leaks, since a large fraction of the cooling units' air can escape through them. Schneider Electric then points to cold-aisle containment as a convenient and cost-effective retrofit for many existing rooms: enclosing the tops and ends of the cold aisles.
Before closing an aisle, check three things. That the fire protection design allows it, which LBNL flags for fully enclosed aisles. That the cooling units can be controlled to the new airflow, because LBNL warns that unless airflow is fully contained, reducing it can starve equipment far from the supply. And where the UPS cabinets and batteries will end up thermally, since the open room takes on the temperature of whichever aisle is not contained.
When the density rises or the hot spots will not go away. Schneider Electric's architecture paper finds traditional uncontained cooling effective at an average of 1-2 kW per rack and no longer effective at 20 kW per rack or more without containment. Between the two, containment and blanking are usually the cheapest gain available. Beyond them, see in-row and liquid cooling for high-density racks. Cooling design, installation and maintenance are delivered through specialist cooling subcontractors. For the UPS side of a contained room, service for UPS systems in contained data centre rows covers the power equipment.
Schneider Electric finds hot-aisle containment can save 43% in annual cooling energy over cold-aisle containment, and prefers it for new rooms. Cold-aisle containment is often the easier retrofit.
Yes. LBNL calls managing blanking panels especially important in hot and cold aisle rooms, because every gap lets exhaust loop back to the equipment inlets.
If the aisle is closed at the top, fire code may require them, according to LBNL. End doors with an open top do not interfere with the sprinklers.
The cold aisle. ASHRAE notes that nearly all distributed UPS units draw air from the cold aisle, like IT equipment.
Sources, checked 2026-10-04:
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