• Containment refers to the various physical barriers used in addition to a hot aisle/cold aisle arrangement that further eliminate the mixing of cold ("supply") air and hot exhaust air. Containment structures lead to higher allowable temperatures in data centers. Higher temperatures save energy because fan speeds can be lowered, chilled water temperatures can be raised, and free cooling can be utilized more often.
  • Figure 4 shows a hot aisle/cold aisle configuration with mixing of hot and cold air. Figure 5 shows how the mixing is minimized through the use of flexible strip curtains, similar to plastic supermarket refrigeration covers. Rigid enclosures (as shown in Figure 6) are also a possibility. Note that containment barriers can contain either the hot aisle or the cold aisle.
Figure 4, through an air flow diagram, shows how mixing of hot and cold air occurs in a hot aisle/cold aisle configuration without containment.
Figure 4: Diagram of a hot aisle/cold aisle configuration with mixing of hot and cold air. (Photo courtesy of
Figure 5, through an air flow diagram, shows how mixing of hot and cold air is minimized by using plastic curtains, similar to those used in supermarket refrigeration areas, between the hot and cold aisles.
Figure 5: Diagram of data center curtains containing the airflow in the cold aisles. (Photo courtesy of
Figure 6 shows a rigid hot aisle containment structure that reduces energy consumption by exhausting hot air and preventing it from mixing with cold (supply) air.
Figure 6: Hot aisle containment reduces energy consumption by exhausting hot air and preventing it from mixing with cold (supply) air. (Photo courtesy of

Savings and Costs


  • In data centers with hot/cold aisle arrangements, containment systems can reduce energy expense by 5% to 10%.
  • Containment can reduce fan energy by 20% to 25% and deliver 20% energy savings from the chiller.18


  • PG&E's experience with containment retrofits indicated that typical paybacks can be less than two years.19
  • Other costs to consider include:
    • Adjustments to the HVAC system,
    • Electrical costs to reconfigure power distribution to the racks, and
    • Associated labor, overtime, and vendor costs, if applicable.
  • Two ENERGY STAR certified data centers – BNY Mellon and RagingWire – employed hot aisle containment during their efficiency upgrades that ultimately led to energy savings through higher chilled water temperatures. See their case studies and public service announcements recognizing them as Low Carbon IT Champions.

Cold Rooms Help Control Energy Costs

At NetApp's Global Dynamic Laboratory, air containment plays an important role in saving energy. The data center includes tunnel-like enclosures, or cold rooms, that prevent the hotter exhaust air from mixing with the cooler supply air.

This design enables the data center to set its supply air at a warmer temperature (70 to 80 degrees F) than traditional supply air temperature settings (50 to 60 degrees F). Setting a higher thermostat temperature, reduces the energy required to cool the supply air and the runtime of the chilled water system because the facility can use ambient air more often. It also lowers overall fan horsepower by about 3 percent because warmer air is lighter and easier to move.

Similarly, another cost-saving aspect of the design involves locating two air handlers above each cold room. This placement takes advantage of the fact that cool air falls, thus saving energy over a traditional raised floor, which consumes extra fan energy to push cool air upward.

In addition, the facility can use higher chilled water supply temperatures. Cooling water to 55 degrees F instead of 44 degrees F increases the efficiency by about 20 percent.

The energy savings are considerable, given the size of the data center. Its 36 cold rooms each hold 60 cabinets, for a total of 2,136 racks of equipment. At full load, this represents a n energy bill of roughly $36 million per year.


  • Depending on their location inside the data center, adjustments to fire detection and/or fire suppression systems may be necessary.
  • When the cold aisle is contained, the rest of the data center becomes warmer, which may not be ideal for staff and some equipment.
  • Though many CRAC units use return air temperature to indicate room temperature, this will not work in a hot aisle/cold aisle with containment configuration because the return air is concentrated and therefore much hotter. (Return air refers to warm air entering the CRAC for cooling.)
  • Many direct expansion CRAC units cannot be retrofitted with variable speed drives that allow the fans to run at different speeds to save energy.
  • A recent paper by American Power Conversion (APC) PDF Exit argued for hot aisle containment over cold aisle containment because the hottest air is sent back to the air conditioning unit (resulting in better cooling unit efficiency), the data center work environment can be maintained at a comfortable temperature, and the inlets of the servers have access to a large air mass that can cool the servers in case of a cooling failure.

Useful Links

18 Keyzone Computer Products, Ltd., LBNL
19 Conversation with Mark Bramfitt, former Principal Program Manager, High Technology Energy Efficiency Team, PG&E, July 14, 2010.