If your data center has a raised floor and dozens of unsealed cable cutouts -- typical for facilities built before brush grommets were standard practice -- you are probably losing close to half of the cooling capacity your CRAH and CRAC units produce before any of it reaches the IT load.
The industry rule of thumb is that up to 48 percent of conditioned air in a raised-floor data center can escape through unsealed cable cutouts and floor penetrations. Exact loss depends on cutout count, size, plenum pressure, and cabinet density, but the order of magnitude is real. Every CFM that escapes through an unsealed cutout is a CFM your chillers paid to produce that does no useful cooling work.
This guide is the engineering case for brush grommets -- where conditioned air actually goes in an unsealed plenum, how grommets stop bypass airflow without restricting cable management, sizing and selection, retrofit versus new build, and how to estimate reclaimed cooling capacity per grommet. One note: this guide does not claim a specific UL or plenum rating on any product. Fire and smoke ratings depend on your local code, AHJ, and insurance carrier. We manufacture to the spec you provide; the assembly-level certification stays with your design and installation team.
Where Conditioned Air Actually Goes
In a textbook raised-floor data center, CRAH or CRAC units pressurize the underfloor plenum with cold air, that air leaves through perforated tiles in front of cabinet inlets, cools the IT equipment, returns as warm air over the top of the cabinet, and cycles back. The reality on an unsealed floor: air leaves wherever it can find an opening. Cable cutouts are usually the path of least resistance because they are larger, have less back pressure, and are not deliberately throttled the way a perforated tile is.
Air that escapes through an unsealed cutout either rises into the room bypassing the IT load, short-cycles back into the hot aisle, or stratifies inside the cabinet without doing useful work. Beyond cable cutouts, conditioned air also escapes through perimeter gaps, floor tile seams, and other penetrations that accumulated over the life of the facility. The 48 percent rule of thumb is the upper end of cumulative loss across all leak paths in a facility that has not been actively managed for airflow.
The Bypass Airflow Problem in Plain English
Bypass airflow is conditioned air the cooling system produces but that never reaches the IT load. The economic problem: your chillers and CRAH fans use the same energy whether the air reaches the load or escapes through a cutout. If 30 percent bypasses the load, you are paying for 100 percent of cooling production and getting 70 percent of the benefit. That gap shows up directly in your PUE.
The thermal problem is more subtle. When escaping air mixes with warm room air, cooling units see a higher return air temperature than they should. Most units modulate output based on return temperature, so artificially mixed return air can throttle cooling output even while a hot aisle is starved of capacity. The fix is not more cooling -- it is sealing the bypass paths.
PUE = (Total Facility Energy) / (IT Equipment Energy). Cooling is typically 30 to 50 percent of total facility power. Reducing fan and chiller load by sealing bypass airflow directly reduces the numerator without changing the IT load. Going from 40 percent bypass to 10 percent in a cooling-dominated facility can move PUE 0.05 to 0.15 points depending on baseline.
The right metric to track
Underfloor static pressure is the most useful real-time indicator of bypass airflow improvement. As you seal cutouts, plenum pressure rises (because the air has fewer escape paths) and you can throttle CRAH fans down to maintain the same delivered CFM through the perforated tiles. Lower CRAH fan speeds equal lower fan energy. If you do not currently monitor underfloor static pressure, install a sensor before you start sealing -- it gives you a measurable before-and-after.
How Brush Grommets Solve It
A brush grommet is a rectangular frame that mounts in or over a cable cutout, fitted with two opposing rows of dense filament bristles that meet in the center. Cables pass through the gap between the rows. The bristles flex around each cable, sealing the opening regardless of cable count or diameter. When no cable is present, the bristle rows close completely against each other. Sealing efficiency on a properly sized grommet is 80 to 95 percent of the open cutout area.
The specific advantage over foam plugs and snap-in grommets is that brush grommets tolerate cable changes without resealing. Foam plugs and air pillows seal well initially but require rework every time a cable is added or removed. Snap-in grommets work for fixed cable counts but cannot accommodate changes. Brush grommets accept cable additions and removals continuously, which matters in any facility where moves, adds, and changes are routine.
Bristle material is almost always polypropylene or nylon filament. Flame-retardant grades are commonly specified for data center applications. Frames are typically aluminum, steel, or rigid polymer, sized to drop into a standard cutout or mount over the top depending on installation method.
Sizing Brush Grommets to Your Cutouts
Sizing comes down to three measurements: cutout opening, floor tile thickness (sets frame depth), and cable bundle profile (sets bristle density). The cutout opening is the most important and the easiest to get wrong -- the opening you see from above is not always the opening below the tile, and an older cutout often differs from any standard size by 5 to 10 mm. Measure the actual opening, not the nominal size.
Floor tile thickness is typically 25 mm to 50 mm. Drop-in grommets matched to tile thickness give a cleaner appearance and perform better on edge sealing. Surface-mount grommets are easier to retrofit on tiles you cannot easily replace. Cable bundle profile drives bristle density: 4 large power cables need less density than 40 small data cables. Standard density works for low to medium population; high density is recommended for cabinets with more than 30 cables through a single cutout.
Standard Cutout Sizes and Matching Grommets
Most raised floors use one of a few common cutout sizes. Here are the typical sizes and the matching grommet specs. Custom sizes are also producible for non-standard cutouts and retrofits to older facilities.
| Cutout Opening | Frame Size | Typical Use | Bristle Density |
|---|---|---|---|
| 150 mm x 250 mm (6 in x 10 in) | Drop-in or surface-mount, matched to tile thickness | Single server cabinet cable entry, low to medium cable count | Standard density, polypropylene or flame-retardant filament |
| 200 mm x 300 mm (8 in x 12 in) | Drop-in or surface-mount, matched to tile thickness | Standard server cabinet cable entry, common in modern builds | Standard to medium density depending on cable population |
| 250 mm x 400 mm (10 in x 16 in) | Drop-in, typically in 50 mm tile depth | Network and core cabinet entries with high cable counts | Medium to high density, flame-retardant filament common |
| 300 mm x 600 mm (12 in x 24 in) | Surface-mount or drop-in for tile replacement | Multi-cabinet shared cutouts and large network distribution points | High density, flame-retardant filament |
| Custom (to opening) | Built to measured opening with appropriate tile depth | Retrofits to older facilities and non-standard cutouts | Specified to cable population and rating requirement |
Fire and Plenum Rating Considerations
Fire and smoke ratings are a legitimate consideration for any component installed in a return-air plenum or any space where local code or insurance requires plenum-rated materials. The correct rating depends on your jurisdiction, space classification, and use of the cutout, so the spec for your facility is a question for your AHJ, insurance carrier, and design team.
When you specify a grommet for a space where fire and smoke matter: filament material (flame-retardant polypropylene and nylon grades are available), frame material (aluminum and steel perform differently than rigid polymer), and any adhesive in the assembly. SB5 manufactures grommets to the spec you provide -- flame-retardant filament, aluminum or steel framing, material data sheets on each shipment. Assembly-level certification (UL listing, plenum rating, AHJ approval) is the responsibility of your design and installation team and your test lab of record.
Confirm the rating requirement before specifying
Do not assume a particular fire or smoke rating is required for your space. Confirm the requirement with your AHJ and your insurance carrier before you write the spec. Over-specifying drives unnecessary cost. Under-specifying creates a real life-safety and liability problem.
Retrofit Installation: What Actually Happens
Retrofit in a live data center is one of the more disruptive cooling efficiency upgrades, and being honest about it upfront is the difference between a smooth project and one that pauses after the first cabinet.
Drop-in grommets: lift the floor tile, remove any existing seal, drop in the new grommet, reseat the tile. If the cutout opening matches the frame, that is 5 to 10 minutes per cabinet. Non-standard cutouts may need to be widened or the tile replaced. Surface-mount grommets install over the existing cutout without lifting the tile -- faster but with a slightly raised profile. In both cases, plan for 15 to 30 minutes per cabinet including cable management. The biggest practical issue is scheduling around live IT load. Coordinate with operations and work in low-utilization windows.
New Build Specifications
On new construction, grommets can be specified into the original floor tile order and installed during commissioning, before any IT load is present. The cost and disruption advantage over retrofit is significant. Each cutout on the floor plan should have a corresponding grommet specified by part number, frame material, bristle density, and rating requirement. This is also the time to rationalize cutout count -- many older facilities have more cutouts than they need because cabinets were moved without resealing abandoned openings. Cabinet cable entries are a separate consideration: grommet inserts that mount in the cabinet frame seal top, rear, and side entries, which matters as much as floor cutouts in contained cold-aisle deployments.
Estimating Your Reclaimed Cooling Capacity
Rough math for a brush grommet retrofit. Numbers are conservative starting points; actual results depend on baseline bypass, cooling efficiency, and the specific cutouts sealed.
Step 1: count unsealed cutouts. A typical mid-size data center has 50 to 200. Step 2: estimate loss per cutout. A 200 x 300 mm unsealed cutout under typical static pressure (10 to 25 Pa) loses on the order of 100 to 250 CFM; a 250 x 400 mm cutout can lose 200 to 400 CFM. Precise numbers require a tracer gas test or CFD model.
Step 3: multiply. 100 cutouts averaging 150 CFM each is roughly 15,000 CFM of bypass airflow. Step 4: convert. At standard conditions, roughly 1 ton of cooling per 400 CFM, so 15,000 CFM is 35 to 40 tons of cooling reclaimable to the IT load. Step 5: energy savings. At 0.10 USD per kWh, annual savings on a mid-size retrofit typically land in the 20,000 to 60,000 USD range. Larger facilities and higher energy costs scale proportionally.
| Metric | Before Sealing | After Sealing |
|---|---|---|
| Bypass airflow as % of cooling output | 30 - 48 percent (typical unsealed facility) | 5 - 15 percent (after grommet retrofit) |
| Underfloor static pressure | Lower (air leaks out faster than fans can pressurize) | Higher (fans can throttle down while delivering same useful CFM) |
| CRAH fan energy | Higher (fans running harder to compensate for bypass) | Lower (10 to 25 percent typical reduction after sealing) |
| Effective cooling capacity available to IT load | Reduced by bypass percentage | Increased to 85 to 95 percent of nameplate cooling output |
| PUE impact (cooling-dominated facility) | Baseline | 0.05 to 0.15 reduction typical |
Frequently Asked Questions
What sizes do brush grommets come in?
Standard sizes match common floor tile cutouts: 150 x 250 mm, 200 x 300 mm, 250 x 400 mm, and 300 x 600 mm. We also produce custom sizes for older facilities and non-standard installations. Send us the dimensions of your existing cutouts and we will quote a grommet built to that opening.
Can brush grommets be retrofitted into a live data center?
Yes, with planning. Drop-in grommets require lifting the floor tile but not powering down the cabinet above. Surface-mount grommets install on top of the tile and are even less disruptive. Plan for 15 to 30 minutes per cabinet, schedule low-utilization windows, and coordinate with operations.
What fire rating do I need for a brush grommet in a plenum space?
That depends on your local code, space classification, and insurance carrier. We do not specify the rating for you -- that belongs to your AHJ and design team. We manufacture to the spec you provide, source flame-retardant filament where required, and supply material data sheets. The assembly-level certification stays with the design and installation team.
How much cooling capacity will a brush grommet retrofit actually reclaim?
Realistic results for a mid-size facility with 50 to 200 unsealed cutouts: 20 to 30 percent reduction in bypass airflow, 10 to 25 percent reduction in CRAH fan energy, and 0.05 to 0.15 PUE improvement for cooling-dominated facilities. Specific results depend on baseline bypass and how thoroughly you seal all leak paths.
Do brush grommets work for cabinets with high cable density?
Yes, when sized correctly. Cabinets with more than 30 cables through a single cutout need high density brush stock and a frame sized to the bundle without excessive bristle compression. Sizing to cable population is the difference between an effective seal and a partial seal.
Can I add or remove cables after the grommet is installed?
Yes -- the main advantage over foam plugs and snap-in grommets. The bristles flex out of the way as a new cable is pushed through and close back around it. Cables can be added or removed continuously without any reseal step.
Do you stock standard brush grommet sizes, or are they all custom?
Common standard sizes can be warehoused for repeat customers after the first production order ships. First orders or custom sizes follow our custom brush lead time, typically 30 to 50 days. After approval, reorders ship from our Plant City, FL warehouse in 1 to 3 business days. Ask us to set up a stocking program at quote time if you have recurring need across multiple sites.
Quantify what you are losing through unsealed cutouts
Send us the dimensions of your typical floor cutouts and a photo if you have one. We will quote a grommet sized to your opening, recommend a bristle density for your cable population, and ship a free verification sample before any production commitment. For new builds, we can quote the full cutout count on a single PO.