The data center industry now has years of experience with hot and cold aisle containment, so it’s a good time to capture some best practices to help make future projects go smoothly. In the past year alone, while Polargy completed dozens of new-build projects, we continued to see problems with containment design, drawings, and specifications. Based on our many years of experience, we compiled our own list of top challenges to good containment design that can be eliminated or minimized with shared information and expert insights.

Here are 10 easy questions and answers to help you design a hot or cold aisle containment system for your next data center project.

1. What’s your containment topology?

This may seem obvious, but it is often a point of confusion, more so than the choice between containing the hot or the cold aisles. We design around three primary topologies: ceiling supported, floor supported, and rack supported. Choose just one since it is rare to have a mixed design. The key driver for topology choice is the need for “rack independence.” If the IT deployment plan calls for individual pieces of equipment to be installed and exchanged during the life of the data center, then a rack-supported topology works since the racks themselves should remain stable. On the other hand, if the IT deployment plan calls for completely populated server racks to be installed and swapped out over time, then ceiling-supported or floor-supported containment is preferable, in order to avoid disrupting the containment when changing the racks.

Other drivers for topology choice include IT use model, cooling architecture, ceiling height, weight-bearing capacity of the building’s roof, and needs for flexible or phased deployment. For example, one may want a “ceiling independent” system in a brown field site where the shell’s roof has poor weight bearing capacity. Or, if the room has a particularly high ceiling, a floor-supported system may be more practical.

2. Are you containing hot or cold aisles?

We find the site type, enterprise vs. third party, and the site’s architecture usually drive this choice. We see most new enterprise data centers leaning towards hot aisle containment with ducted returns. For colo/wholo, we see a modest majority choose cold aisle containment, typically with roofs. Rarely do we see people containing both the cold and hot aisles.

In a traditional raised floor with perimeter cooling, we prefer hot aisle containment because the common above floor cold zone is easier and more forgiving to balance. In cold aisle containment with roofs each aisle has to be individually balanced with more care and control. However, in colo/wholo, where the aisle layout will likely change over time, then rack based cold aisle containment with roof panels tends to be the most common approach. Of course, slab floor and in-row cooling architectures leave room for either option. Floor mounted modular designs or strut ceiling systems can lend themselves to colo/wholo needs for accommodating layout changes.

3. What rack sizes need to be accommodated?

In both new construction and retrofit we see time and time again racks delivered to the site that are taller than the containment system that was specified and built, even after planning for some degree of rack height variation. We also see last minute changes to the containment system, usually to accommodate yet even taller racks, say 51U racks instead of the 45U racks. I suspect that this difficulty reflects the age-old gap between IT, the rack buyer, and facilities, the containment builder.

Take a floor mounted containment system as a case study.  These systems typically have a lower level and upper level and fixed beam between the two. This beam becomes the fixed elevation height under which the racks must fit. The beam height also sets the height of the upper panels and of any lower level InFill panels that are provided for missing racks on Day 1.

Now, add a last minute decision to change from a 42U rack design to one for 48U racks, a height change of about 12 inches. This change impacts three key items; the floor mounted frame system itself, the above rack panels have to be shortened 12 inches and the lower level InFill panels need to be extended 12 inches. Of course, the costs and schedule impacts of such last minute change are obvious on the surface. But, with containment usually being one of the last things to get installed, a late change like this can be especially painful to the schedule. 

4. Will your above-rack panels be fixed or removable?

Features drive cost: fixed panels cost less, removable ones cost more. The idea behind a removable upper panel is to provide access to cable trays/power buses through the containment.  If the design has vertical panels on the cold face of the cabinets, maybe a fixed panel is fine, though cable tray cut-outs in the panels will likely be necessary. When you have cable tray cut-outs, specify a method for sealing them. Options for sealing cut-outs in order of increasing cost include: air dam foam, vinyl flaps, and brush grommets. If the design has vertical upper panels justified to the rear face of the cabinets, the advantage of a sliding panel or hook on/off panel is ease of access to cable trays and power busways.

5. What’s your containment “air tightness” standard?

A few years back, a performance standard emerged that called for “less than X percent leakage under Y amount of pressure.” Really? How can you test that in the field, especially when the racks themselves are full of gaps and leaks? It’s just not practical.

Try something like this instead:

The HACS/CACS shall have a containment integrity with a combined open surface area of no more than 3.0% of the total contained zone surface area. The HACS/CACS shall have containment integrity such that no gaps are larger than 0.25 inches. Door bottom gaps can be up to 1.0 inches as the door bottom gaps serve as safety pressure relief paths when exhaust air and supply air fall out of balance.

This is easy to measure and verify in the field, and it is more than sufficient for proper management of airflow in the room.

6. What fire rating is required for your materials?

NFPA containment standards call for materials that meet ASTM e84 Flame and Smoke spread rating of Class B. Fortunately, most containment is made from twinwall polycarbonate that carries a Class A rating. However, when a design specifies “solid, clear” door inserts and containment panels, the fire rating also needs to be specified. Otherwise, you are going to get low quality Acrylic inserts that do not meet an e84 rating. Acrylic meets the UL-94 Flammability rating, but that standard is more for small scale applications like blanking panels and brush grommets. On that note, we recommend specifying that “gap filling” materials, used in small scale applications, meet the UL-94 rating since there are no real practical Class B rated materials for brush grommets and air dam foam.

7. Is the owner insured by Factory Mutual Global?

The implication with Factory Mutual Global is that the cost of the containment materials will be about double (yes, double!) that of a typical containment project. FM Global has unique containment requirements for their clients, and one such requirement is to use an FM4910 compliant material. One of our recent university projects illustrates the striking cost difference: The material cost using twinwall polycarbonate was $75,000. The cost for using the CPVC alternative that meets FM Global’s 4910 standard was $153,000 — double the cost! FM Global does not always require the higher cost material, but most of the time they do. An FM Global field engineer assesses the project to determine actual material requirements, and makes a determination on a case-by-case, site-by-site basis. So, there is no universal standard for this requirement.

8. What are you doing about missing racks during commissioning?

Many data centers today are using pressure differential between the hot and cold aisles to modulate the cooling. With this, full containment is needed to create a pressure differential boundary for testing and operating the cooling system. If the containment is a rack independent design and there are no racks, you will have a commissioning problem. To address this “Oops, big gap” problem, we have seen operators use cardboard, plywood and plastic sheeting to seal up the aisles during testing. Of course, after commissioning that temporary patch has to be removed, leaving the cooling system without a pressure differential to drive it until all the racks get installed.

Polargy recommends a design approach based on tool-less rack gap panels that we call “InFill Panels,” not to be confused with “Blanking Panels,” which are used to seal inside server racks. If there are no racks installed on Day 1, then deploy these InFill Panels throughout the aisles. As racks get deployed, individual InFill Panels get retired, and eventually one reaches a normal operating status with a minimal number of InFill Panels.

Often, the specifications call to “Coordinate with Owner” on the quantities of InFill panels on Day 1. The worst thing we see is “count the empty rack spaces and provide InFill panels for the missing racks.” Unfortunately, the drawings never have this detail correct. For bidding purposes we recommend clarifying the quantities and sizes of these panels to include in the bid. We know the actual quantities will vary, but at least this gives a bid leveling and prevents wasted time trying to figure how many InFill panels are needed.

9. What are you doing about empty racks during commissioning?

At a recent downtown Chicago project, the cooling system used Kyoto wheels that needed pressure differential to modulate the airflow. Unfortunately, the racks were delivered empty and rack blanking was entirely missed in the project scope. In the interest of time, the general contractor chose to wrap the racks in plastic sheeting to seal them for commissioning. The cost was two man days, $300 worth of plastic, and the racks still needed proper blanking after commissioning.

Supplying blanking panels can fall to either the rack manufacturer or the containment manufacturer. Full rack blanking panels cost around $120 per rack, so a rack may only cost $1,200 and blanking can be an additional 10% per rack. One option for quickly and cost- effectively blanking full racks is 42U blanking panels that install without tools.

10. How are you sealing various other small gaps?

Beyond containment doors and panels, every project has a unique variety of miscellaneous small gaps that need to be filled. Gaps above, below, and between racks are prime candidates, along with gaps around cable tray penetrations. Plastic skirts, air dam foam, brush grommets and vinyl flaps are the traditional choices for sealing these gaps. The UL-94 fire rating for this class of gap sealing skirts and foam is fine since these products are deployed in small scale.

We recommend that products for sealing gaps above racks and below suspended ceiling containment panels be clearly specified as either brush grommets or vinyl flaps. This 4- to 8-inch gap is an expensive one to fill. While vinyl flaps cost around $6 per linear foot, brush grommets cost closer to $14 per linear foot. Making a clear distinction helps level the bids and sets a clear owner expectation. This product detail should also be applied to cable tray penetrations; foam, vinyl or brush grommet. Gaps under and between racks can be sealed simply with fire safe air dam foam or magnetic skirts.

Be sure to use to the Containment Design Checklist available on our blog at to plan around these and a variety of other aisle containment design factors.