You can reduce installation time and improve traditionalairflow patterns compared to metal ductwork by usingfabric duct in certain commercial applications

B Y C A R Y P I N K A L L A

For the past decade, the use of fabric air dispersionhas grown steadily in North America as architects,consulting engineers and hvac contractors learn ofthis alternative to metal ductwork. While many

contractors might be skeptical of installing fabric duct atfirst, those initial reservations usually are eased when theproduct’s benefits are understood.

Ten years ago fabric duct was used mainly in food pro-cessing and low-end industrial applications for makeupair. The improvement in fabric aesthetics, endurance anddiffusion performance has brought fabric duct specifica-tion to a new level.

Today, architects, engineers and contractors are using itin gymnasiums, offices, clean rooms, indoor pools, church-es and other applications. In general, the two criteria arean open-ceiling architecture and high attention to interiordesign.

While it was popular in Europe as far as 35 years ago,fabric duct has only recently come to the forefront of

American air distribution design. Five years ago fewNorth American architects were aware of fabric duct.The fact that a majority of North American architectshave seen it, read about it or used it has resulted in anincrease of projects by an estimated 500 percent in thepast five years.

Fabric duct advantagesToday’s fabric duct is made of a variety of high-end poly-ester blends that are designed to meet stringent buildingcodes and UL requirements. Plus, the range of fabricsused today has become much more diverse than 10 yearsago. For example, some fabrics are manufactured tobreathe, thus eliminating the risk of condensation.

Other fabrics are not breathable and can deliver air upto 90 feet with a terminal velocity of 150 feet per minute.Because of the varied uses and applications that fabric isused in today, there is more emphasis on fabric selectionand air delivery methods than in the past.

Fabric duct diameters typically range in size from 8 to72 inches, with the majority of installations staying withinthe 18- to 36-inch diameter range. The material cost of afabric duct system is comparable to single-wall spiralmetal duct in the 18- to 36-inch range. The cost differ-ences can vary slightly depending on the fabric, suspen-sion and air delivery system selected. When diameters aregreater than 36 inches, the material cost of the fabric ductsystem is almost always less than metal systems.

In applications using sizes less than 18 inches in diam-eter, fabric can be more expensive than metal. When con-densation is an issue, the material cost of a fabric ductsystem can range from 25-75 percent less than metal,based on diameter and the type of metal selected.

While the material cost savings for fabric are notalways obvious, the cost to install fabric duct is less thanmetal because it takes less time. It takes anywhere from20 to 80 percent less installation time to hang fabric thana comparably sized metal system.

The wide range of savings is based on the type of sus-pension system selected, the weight differences betweenfabric and metal, mounting heights and the ease of han-dling fabric that is light and flexible. Since labor is a bigcost component of hanging metal, the savings for a fabricventilation system can be significant.

Fabric duct often requires less maintenance than itsmetal counterpart, resulting in additional savings. In mod-els where the fabric breathes (allowing approximately 10percent of the total air flow to penetrate the fabric) surfacedust buildup is greatly reduced if not eliminated.

In humid environments, moisture can accumulate andprematurely age the appearance of metal duct. Fabric ductdoes not encourage condensation. You can clean the ductby disassembling via zippers and either laundering in awashing machine or dry cleaning, eliminating the needfor possibly more expensive duct-cleaning services.

Metal systems will accumulate dust and contaminatethe upper outside surface areas, which in a public spacewill require routine maintenance to remove. Air porousfabrics repel dust and prevent airborne particulates fromsettling on the duct surface. Another costly expense tomaintaining the outside of metal duct is that it may requirerepainting, which can cause business interruption and costas much as the initial investment in the metal duct system.

As for the inside of the duct, metal usually requiresusing a duct cleaning service. Fabric duct cleaning can beless expensive and done more frequently than metal ductbecause a building’s maintenance department can do thejob as described above.

Plus, facilities tend to keep their air distribution deliv-ery system cleaner when it’s an easy and inexpensive task.For example, a school gymnasium with several hundredfeet of fabric duct hanging 25 feet high can be disassem-bled in half a day, laundered and reassembled in anotherhalf day resulting in minimal downtime to the facility orhvac system.

Specialized systems It is much easier and more cost-effective to have a fabricduct system delivered to the jobsite already color selectedor dyed to match the intended space than to paint a metalduct system after it is installed. Most manufacturers offabric duct systems offer a variety of standard colors, andsome offer custom dying and the silk-screening of companylogos and slogans.

Because metal ductwork was never intended for use inopen spaces, the look and feel is industrial. Fabric, on theother hand, has an inherent soft and warm presence thatblends better into the architecture and interior design ofthe space.

Colors available typically include red, blue, white orblack. However, custom colors can be applied to matchinterior design schemes or corporate colors. Unlike metalduct, fabric duct arrives at the site pre-colored and needsno on-site painting or anti-moisture coatings. School orcorporate logos can be silk screened on the fabric as well.

Manufacturers also have expanded into many special-ized air dispersion varieties. For example, the D-shapeincorporates a special hanging system that lies flat againstthe ceiling with surface mounting. This is perfect foroffices and/or applications with low clearance. One manu-facturer offers the V-shape, which fits into a 90-degreespace created by the junction of the ceiling and wall.

Most manufacturers make a static-free version of fab-ric duct for installation in electronic control rooms to

Fabric duct’s light weight requires fewer workers to installcompared to metal ductwork.

avoid static buildupand discharge. Theyalso make antimicro-bial-treated fabric thatcontrols the growth andtransmission of bacte-ria, fungi and molds,and is especially suitedfor the food-processingindustry. Like metalductwork, fabric ductalso has standard90-degree and 45-degree elbow fittings along with Ts andadaptor rings that fit around metal collars for transitionsfrom metal to fabric.

Fabric duct can be considered a “green” product byhelping to improve IAQ. It can prevent the generation orintroduction of pollutants – especially biological contami-nants – into an occupied space. Using fabric duct also cansave valuable metal resources and thus add resource-effi-ciency benefits to a project.

Fabric has been found to absorb sound and transmitless mechanical equipment noise than metal ductwork.For diffusing air, fabric can be less noisy than metal,provided that the fabric duct is properly designed. Airusually is diffused more evenly and continuously alongfabric duct than a metal stem that positions registersevery five or 10 feet apart.

Some limitationsFabric duct disperses air eitherthrough different grades of factory-engineered orifices, through thefabric itself, through linear dif-fusers that run the length of theduct or by a combination of thesemethods. Because some fabric ductis breathable, it isn’t suitable foruse in enclosed ceilings, behindwalls, above dropped ceilings or asreturn air duct. Fabric duct is usedstrictly as a supply-side air distribu-tor in open-ceiling architecture.

One aesthetic disadvantage is thefact that fabric duct deflates duringoff-cycling of the hvac system.When deflated, fabric duct hung from a single rail orcable system looks like a decorative banner. With a dual-rail hanging system, a deflated duct holds its roundappearance when viewed from below.

Deflation is becoming less an issue as more modernhvac designs are specifying continuous ventilation in occu-pied spaces to maintain higher IAQ standards. When fabricduct is used with variable air volume (VAV) boxes, the fab-ric duct can be designed to provide a pleasing appearanceeven at 50-percent of the maximum air volume.

Additionally,fabric ductmight not be thecorrect specifi-cation in appli-cations wherecaustic chemi-cals can come incontact with thefabric or wherethere is restric-tive ceilingheights, a limitation that might apply to metal ducts too.

Installing the system Before installation, manufacturers can deliver the factory-engineered fabric duct to the job site or hvac contractor’sheadquarters. A large installation can involve dozens oflengths. Since all jobs are factory-engineered, the pieces

arrive on sitenumbered asto where theyshould beinstalled.

Sections canvary by lengthdepending onthe diameterof the duct.Smaller sizelengths canrange from

20 to 40 feet long, while larger diameters have shorterlengths for better handling ability.

While putting up a 30-foot section of 24-inch-roundspiral metal duct would necessitate a large installationcrew, a similar length of fabric duct requires only twohandlers because the fabric is approximately 90 percentlighter. On smaller projects, a contractor can save moneyby using ladders instead of scaffolding.

Tools to use for installation are no different than thoseneeded for hanging metal duct: power drills, anchors forall types of materials, screwdrivers and pliers. Fabric ductneeds no on-site cutting and typically is zippered togetherwith factory-sewn industrial zippers.

But don’t throw away those tin snips. Metal plenumsand metal duct (in enclosed ceilings) may still be neededto deliver the air from the air handler to the room(s)where fabric duct is placed.

Support systems are similar among all major fabricduct manufacturers and consist of either a cable systemor an H-track system. The majority of projects that utilizefabric duct as their air distribution system consist ofexposed steel truss roof structures (roof girder, purlin,rafter or beam). These structures can support the ductsystem.

Wire cable and small duct clips makehanging fabric duct similar to hanging ashower curtain.

The aluminum H-tracksystem allows fabricduct to be hung fromexposed steel-trussroof structures.

Cable strung over a ceiling joist is onemethod of supporting fabric duct.

Adapter rings fit around metal col-lars for transitions between fabricand metal duct.

Cable systems consist of aircraft-grade steel cable andturnbuckles for tightening. Typically, a cable system isanchored from wall-to-wall with conventional anchorsfound at any hardware store.

Long cable runs most likely will need support(s) in themiddle sections. This is accomplished no differently thansupporting piping or other types of ductwork. Threadedrod anchored to ceiling joists is the most common hang-ing method.

Going for the best fit Recent innovations have included the popular Gripple fit-ting that contractors use for hanging electrical conduit,lighting fixtures, piping, sheet metal duct, sprinkler sys-tems and other utility supplies that require leveling andon-site adjustments. The Gripple system is a faster methodthan C-clamps and threaded rod.

An installer takes a small length of aircraft cable andloops it over the bottom of the ceiling joist and under thefabric duct cable system. Both ends of the cable are easilystrung through the fitting, which can be tightened whenthe proper length is achieved (once it levels the sagging ofthe main fabric duct cable system run). The locking gripon the fitting easily can be reversed, thus freeing the cablefor readjustment or disassembly.

Once the cable system has been suspended, hangingthe fabric duct is no different than stringing a showercurtain onto a rod. The top of the duct comes with smallclips that the cable is strung through. For larger diametersof 38-inches or more, a two-row cable system is recom-mended for aesthetics, but is not mandatory. With twocable systems holding the duct at the 10 o’clock and 2

o’clock positions, potential saggingis eliminated and enhances theaesthetics of the duct run.

An alternative to the cable sus-pension system is the aluminumH-track system. This is notanchored into walls, but reliestotally on support from the ceilingstructure above. Once again thepreviously mentioned Gripple sys-tem supports the H-track from aceiling support. Each vertical sup-port cable has a special hub thatslides through the top of the H.

A snap tab that’s connected tothe fabric has a hub as well thatslides through the bottom of the H.Some manufacturers offer a locking

nut for the top hub that prevents it from sliding in the H-track once it has been set in position. Like the cable system,a two-run H-track will eliminate possible sagging on largerdiameter runs. The H-track also comes in a surface-mountstyle for low-clearance situations.

Help is availableUsually it takes only a few hours for a technician tobecome proficient at hanging either the cable or H-tracksystems. In addition, most major fabric duct manufactur-ers offer factory consultation for installation.

By the time a fabric duct project reaches the installationcontractor, the specifying, engineering and selection ofsize, airflow, color and shape have been determined by thearchitect or consulting engineer. The industry’s top fabricduct manufacturers have representatives that either helpwith system design or act as liaisons with factory engi-neers who provide the technical engineering assistance.

On design/build projects where a contractor is requiredto specify size and airflow characteristics, the manufactur-er’s representative is a vital part of the engineering team.

Incidentally, fabric duct is good for hvac contractorswho find themselves sub-contracting the sheet metal por-tion of a design/build project. Instead of sub-contracting,a contractor can install the ductwork with in-house work-ers and make additional profits on a project.◆

Cary Pinkalla is an industrial engineer and president ofDuctSox. Contact the company at 866-DUCTSOX ext. 406or visit www.ductsox.com for more information.

June 2003 ~ RSES Journal

Company or school logos can beprinted onto fabric duct, and ductcolor can be custom matched to abusiness’ corporate colors.