HVAC
Ductwork types field guide: sheet metal, flex, and ductboard
Pick the duct type that fits the pressure, the space, the budget, and the air quality, and put each one where it actually performs.
Direct answer
Ductwork carries conditioned air from the air handler to the rooms, and the type you pick sets the leakage, the friction, the insulation cost, and where the duct fits. Galvanized sheet metal is the durable commercial standard. Round and spiral lose the least to friction and leakage. Flex and fiberglass ductboard cost less but suffer field abuse.
Key takeaways
- There is no single best duct type; match it to pressure, location, budget, air-quality demand, and whether the run is exposed.
- Round duct adds roughly 15 to 25 percent less friction than equivalent rectangular, leaks less, and is stronger per gauge.
- Keep rectangular aspect ratio to about 4:1 or less; a 20 by 4 duct has roughly twice the friction of a 10 by 8.
- Reserve flex for the last short connection; field-typical compression can raise its pressure drop by a factor of four.
- Support horizontal flex every 4 to 5 ft, hold sag to about 1/2 in per foot, and use UL 181B-listed closures.
What ductwork is and why the type decides the system
Ductwork is the network of channels that carries conditioned air from the air handler out to the rooms and pulls it back again. The air handler makes the heating and cooling; the duct delivers it. Pick the wrong duct type and the equipment behind it never matters, because the air leaks out, rubs down to nothing on friction, or sweats and drips before it arrives.
The type sets five things at once: how much it leaks, how much friction it adds per foot, whether it carries its own insulation, what it costs to build, and where in the building it can physically go. Those trade against each other. The cheapest, fastest duct to install is usually the leakiest and the most friction-prone, and the duct that performs best takes the most room and the most labor.
This guide is about choosing the type and the material. How you build sheet metal to its pressure class is covered in the sheet metal duct fabrication guide, and how you prove it holds air is in the duct leakage testing and sealing guide. Here the question is which duct goes where, and what each one costs you downstream.
Sheet metal duct: the commercial standard
Sheet metal duct is the standard for commercial work and the duct everything else gets compared against. Galvanized steel is the workhorse: zinc-coated carbon steel that resists rust, takes a seam and a joint well, holds its shape under pressure, and leaks very little once it is sealed. It is rigid, it is durable, and it cleans with a brush, which is why it dominates anything that runs at real pressure or has to last decades.
The metal changes with the job. Aluminum shows up where weight or corrosion matters, on rooftop runs, in coastal air, and on some exhaust, because it does not rust the way steel does, though it costs more and dents easier. Stainless steel is the specialty metal for grease exhaust, fume hoods, pools, and corrosive process air, where ordinary galvanizing would not survive. Galvanized covers the vast majority of comfort duct, and the other metals come out only when the environment forces the upgrade.
The gauge, the seam, and the reinforcement that make sheet metal hold its pressure are a fabrication question, covered in the sheet metal duct fabrication guide. What matters for type selection is the headline: built and sealed right, sheet metal is the tightest, most durable duct on the list.
Is round or rectangular duct better?
Round duct is better than rectangular for almost everything except fitting into a tight space. Round carries its load in hoop tension instead of flexing a flat panel, so it is stronger for the same metal, it leaks less because it has fewer linear feet of seam, and it moves air with less friction, commonly on the order of 15 to 25 percent less than an equivalent rectangular duct. It also uses less metal per cfm delivered. On every performance axis, round wins.
Rectangular wins on fit. A rectangular duct can be made wide and shallow to slide through a chase or above a corridor ceiling where a round of the same area would never clear, and it lets you tap a branch anywhere along the trunk. The cost is more seam to leak, more friction, more reinforcement to stop the panels drumming, and more metal for the same air. Keep the aspect ratio reasonable, commonly 4:1 or less, because a 20 by 4 duct has the same area as a 10 by 8 but roughly twice the friction.
Flat-oval is the compromise built for that exact conflict. It is round duct flattened to fit a shallow space, keeping most of round's strength and seal while losing some height. The trade has been moving toward round and spiral wherever the ceiling allows and reaching for rectangular and oval where the structure is tight. When the spec leaves it open, the run with room should be round.
| Shape | Friction and leakage | Fit and fabrication |
|---|---|---|
| Round | Lowest friction and leakage, strongest per gauge | Needs ceiling height to swing; taps at fittings |
| Rectangular | Higher friction and leakage, needs reinforcement | Fits shallow chases; taps anywhere on the trunk |
| Flat-oval | Near-round friction and leakage | Flattened to fit shallow space; loses some height |
Spiral duct
Spiral duct is round duct made by locking a continuous strip of steel into a helical seam that spins down the length of the pipe. That helix is both the seam and the stiffener, which is why spiral runs dead straight and rigid in long pieces and needs little reinforcement. It goes up fast, the slip-and-coupling joints seal cleanly, and it reaches a tighter leakage class than rectangular for far less effort.
Spiral has a second life as a finish. Left exposed and painted, a clean spiral run with smooth gored elbows reads as architecture, which is why you see it bare in restaurants, retail, offices, and lofts where the ceiling is open on purpose. When the duct is going to be seen, the fit and finish of the fittings is part of the job, not just the airflow.
The fittings are the catalog: gored or pleated elbows, conical and saddle takeoffs, reducers, and tees, joined with couplings and sealed at the joint. Spiral is the easy duct to build tight and the natural choice for long round runs, exposed work, and any system that wants low leakage without the labor of flanged rectangular.
Is flex duct bad?
Flex duct is not bad. Flex duct used wrong is one of the worst things you can do to an air system, and it gets used wrong constantly. The product itself is an insulated flexible duct: a coiled steel wire helix holding open an inner plastic liner, wrapped in a blanket of insulation, sealed inside an outer vapor jacket. It is cheap, it is fast, and for the short final connection between a hard duct and a diffuser it is the right tool.
The trouble is that the same flexibility that makes it fast makes it easy to ruin. Flex has far more friction per foot than smooth metal even when it is installed perfectly, because the corrugated liner trips the air. Let it sag between supports, kink it over a joist, coil up the extra instead of cutting it, or crush it above a light fixture, and the friction multiplies. Field-typical compression can raise the pressure drop by a factor of four, and hard compression by close to ten.
So flex is not the villain; abused flex is. The fix is not banning it. The fix is installing it the way the design assumed, which the next section lays out. Reserve it for the last short run, and the rest of the system never has to pay for it.
Flex duct done right
Flex done right follows a short list of rules, and every one of them is something an inspector checks. Keep it short. Flex is for the last connection to the diffuser, not for a trunk and not for a long branch; the longer the run, the more its friction penalty compounds. Pull it tight. The inner liner has to be stretched taut, because a relaxed liner is a corrugated liner and that is where the friction lives.
Support it on close spacing so it cannot sag. Common practice supports horizontal flex at intervals of roughly 4 to 5 ft and holds the sag between supports to about 1/2 in per foot of run, with a saddle or a wide strap that will not bite into and crush the wire helix. Do not coil the slack. Cut the duct to length instead of looping the extra above the ceiling, because a coil is a series of kinks. Never bend it tighter than its rated radius, and keep it clear of anything that will pinch it.
The connection at each end is its own failure point. The liner gets pulled over the collar, clamped, then the insulation and vapor jacket pulled back over and sealed, with closures listed to UL 181B for flexible connectors. A flex tail that is short, stretched, supported, and clamped right disappears into the system. A long, droopy, kinked, coiled tail is the branch that starves while everyone blames the equipment.
What is ductboard?
Fiberglass ductboard is rigid duct cut and folded from boards of resin-bonded glass fiber, faced on the outside with a foil-and-scrim air barrier and vapor retarder. The board is the duct and the insulation at once, so it needs no separate wrap, and the fibrous surface inside makes it quiet, soaking up fan and air noise instead of transmitting it. It is cheaper than sheet metal to fabricate, and a shop can cut it with a knife and a groover, fold it to shape, and staple and tape the seams.
The concerns are all about the fiber facing the airstream. The inner surface is exposed glass fiber, and over years it can erode, shed fibers into the air, and, if it gets wet and dirty, grow mold in the dust that collects on it. It also cannot be mechanically cleaned the way smooth metal can; a rotary brush tears the surface and throws fiber into the building, so cleaning has to be done carefully by trained crews with soft tools and HEPA vacuums, or not at all.
That puts ductboard in a narrow lane. It shows up in low-pressure residential and light-commercial supply where cost and quiet matter, and it gets avoided in IAQ-sensitive spaces like hospitals and labs, in high-velocity systems that erode the surface, and anywhere it can get wet or take physical abuse. Where it fits, it is a low-cost, quiet duct. Where indoor air quality rules, it is the wrong choice.
Fabric and textile duct
Fabric duct is a tube of engineered textile that hangs from a cable or a track and distributes air through the cloth itself, through laser-cut perforations, or through nozzles along its length. Instead of dumping air from a few diffusers, it spreads it evenly down the whole run, which kills the drafts and the dead spots a hard duct leaves. It is light, it goes up fast, and it weighs a fraction of metal.
Where it earns its place is exposed, washable, low-pressure distribution. Gyms, pools, food processing, fitness centers, schools, warehouses, and labs run it because it can be taken down, machine-washed, and rehung, which matters where hygiene is inspected, and because it looks intentional hanging in the open. The even discharge also lets it run at lower velocity and pressure than a metal system throwing the same air.
It is not a pressure duct and not a concealed one. Fabric belongs on the low-pressure distribution end, in the open, where it can be reached and cleaned. For trunks, risers, and anything at real static, it is the wrong tool, and metal carries the air to the point where the fabric takes over.
How does pressure class match the duct type?
Pressure class is the operating static the duct has to hold, and it sorts the types as hard as anything else does. SMACNA defines the classes by inches of water gauge, and the fabrication guide ties the gauge, the seam, and the joint to that class. For type selection the point is simpler: the higher the pressure, the fewer types are still on the table.
Low-pressure duct, the residential and light-commercial range, is where flex tails, fiberglass ductboard, and light snap-lock sheet metal all live, because the static is gentle enough that the loose, cheap types still hold. Medium-pressure commercial supply moves to sealed rectangular and round sheet metal. High-pressure central systems push the duct to heavier gauge, Pittsburgh and flanged joints, and round or spiral, because a slip joint that is fine at 1 in. w.g. blows open at 6 in.
Match the type to the pressure before anything else. Flex on a high-static trunk, ductboard on a medium-pressure run, or a snap-lock seam pushed past its range is the same mistake in three costumes: a duct type that cannot hold the pressure it was handed. The gauge and reinforcement that back the higher classes are in the sheet metal duct fabrication guide; confirm the class against the project spec and the design static.
Internal liner or external wrap
Duct gets insulated two ways, and they are not interchangeable. Internal liner is glued and pinned to the inside of the duct, so it both insulates and absorbs noise, since it sits in the airstream and soaks up fan and air sound. External wrap is blanket insulation banded around the outside, so it insulates and controls condensation without putting anything in the air path. Liner buys you quiet; wrap keeps the airstream smooth and clean.
Liner has the same downside as ductboard: it faces the air, so it can erode at high velocity and it is hard to clean, which is why IAQ-sensitive jobs lean on wrap and on double-wall duct with a solid inner liner instead of exposed fiber. Wrap fails at the seams and the hangers, where a gap or a crush lets the metal sweat.
Where you insulate is set by where the duct runs. Duct in conditioned space often needs little or none. Duct in an unconditioned attic, crawlspace, or outdoors needs real thermal value, and cold supply duct in humid air needs a continuous vapor barrier, because a break in it condenses inside the insulation where nobody sees it until the ceiling stains. The insulation detail and thickness come from the project spec and the energy code.
Leakage differs by type
Leakage is conditioned air escaping the duct before it reaches a register, and how much escapes depends heavily on the type. Sealed sheet metal, especially round and spiral, is the tightest: fewer seams, clean coupling joints, and mastic over what is left. Rectangular leaks more for the same care because it carries long longitudinal seams and many transverse joints. Flex leaks at the connections, the collars and clamps at each end, far more than along its body, which is why a sloppy flex termination undoes the rest of the system.
Ductboard and lined duct seal at the taped and stapled seams, and that tape is the weak point as it ages. The thing all of them share is that the joint, not the material, is where the air leaves, and a cheap type usually has more joints and looser ones.
How you actually measure and seal leakage, the SMACNA leakage class, the allowable calculation, and the calibrated-fan test, is the whole subject of the duct leakage testing and sealing guide. For type selection the lesson is that round and spiral sheet metal start a leakage test most of the way to a pass, and flex and ductboard start it needing more help.
Friction, sizing, and why the type drives the design
Friction is the pressure the duct rubs out of the air as it flows, and it ranks cleanly by type: round smooth metal is the lowest, rectangular is higher because of the corners and the larger wetted surface, and flex is the highest by a wide margin because the corrugated liner trips the flow. The duct design sets a friction rate and sizes the duct to hold it, and the type you choose changes the size you need for the same air.
This is why the type is a design decision, not a field substitution. Swap a sized metal branch for flex without resizing and you have quietly added friction the design never budgeted, and the room downstream comes up short. The design carries a penalty for flex precisely because the field reaches for it.
How the duct gets sized, the friction rate, the equal-friction and static-regain methods, and the Manual D residential procedure, is its own discipline and is covered by topic in the duct design material. The tie-in here is direct: round buys you the smallest duct and the lowest fan energy for a given airflow, and every step toward cheaper, looser, more flexible duct costs you back in size, in static, or in the air that never arrives.
Fittings and transitions
Fittings are where the air turns, splits, and changes size, and the type and shape of the fitting waste or save more pressure than long straight runs do. The duct type drives the fittings available. Round and spiral use gored or pleated elbows, conical and saddle takeoffs, reducers, and tees, all joined with couplings. Rectangular uses formed elbows, taps, and transitions, and where a tight square elbow is unavoidable it needs turning vanes inside to guide the air around the corner.
Shape decides the loss. A wide-radius elbow turns the air with far less loss than a tight or mitered one, a conical takeoff pulls air into a branch better than a square hole cut in the trunk side, and a gradual transition beats an abrupt step that trips the flow. The vanes left out of a square elbow are a common, invisible loss until somebody reads the static.
How these fittings get built to the pressure class is in the sheet metal duct fabrication guide. For type selection, the point is that round and spiral come with low-loss fittings as standard, while rectangular depends on someone building the radius, the takeoff, and the vanes the design assumed instead of the cheap version.
Underground and under-slab duct
Underground and under-slab duct is a special case, and the trade has largely backed away from it. The problem is water. A duct buried below a slab or in the ground sits where groundwater, condensation, and the occasional plumbing leak all collect, and once water gets into sub-slab duct, mold and corrosion follow, and the duct is nearly impossible to clean or replace without breaking concrete.
When it is used, the materials and the protection are not optional. The mechanical code requires underground duct to be approved concrete, clay, metal, or plastic; metal has to be corrosion-protected or fully encased in concrete at least 2 in. thick, plastic has to meet the specified ASTM cell classification and external-load rating, the run has to slope to an accessible drain point, and a vapor barrier goes between the duct and the surrounding fill. Everything gets sealed and tested before it is buried, because there is no fixing it after the pour.
The two failure modes are crush and moisture. The duct has to survive the loads above it without collapsing, and it has to stay dry inside for the life of the slab. Where the design allows it, running duct overhead instead of under the slab avoids the whole problem. Confirm the materials and the burial detail against the adopted mechanical code.
Which ductwork is easiest to clean?
Smooth metal duct is the easiest to clean and the friendliest to indoor air quality, and that single fact sorts the types for any IAQ-sensitive job. A galvanized or stainless interior is a hard, smooth surface that a rotary brush and a vacuum clean without damaging it, and it does not shed anything into the air. That is why hospitals, labs, cleanrooms, and food plants run bare metal and double-wall duct with a solid inner liner.
The fibrous types are the hard cases. Fiberglass ductboard and internally lined duct expose glass fiber to the airstream, which can erode and shed, and which grows mold when dust and moisture collect on it. They cannot take a mechanical brush without tearing, so they have to be cleaned gently by trained crews with soft tools and HEPA vacuums, or replaced. Flex is in between: the smooth plastic liner cleans better than fiber, but the corrugations trap dust and an aggressive brush can tear the liner or crush the helix.
Match the type to how clean the air has to be. Where IAQ is inspected or the occupants are sensitive, smooth metal earns its cost back in a duct that cleans and does not contaminate. Where it does not, the fibrous and flexible types are acceptable as long as they stay dry and undisturbed.
Residential and commercial use different duct
Residential and light-commercial systems lean on flex and fiberglass ductboard, and commercial systems lean on sheet metal and spiral, and the split comes down to pressure, budget, and access. A house runs low static, short runs, and a tight budget, so flex tails off a sheet metal or ductboard trunk are fast and cheap, and the small leakage and friction penalties are tolerable on a small system.
Commercial work runs higher pressure, longer runs, and bigger airflow, where those same penalties turn into real lost air and real fan energy, so the duct moves to sealed rectangular and round sheet metal, spiral where the ceiling allows it, and flex only on the last short connection. The energy code also drives commercial duct to Seal Class A and a tested leakage class, which the loose residential types would never hit.
The mistake is carrying residential habits onto a commercial job: long flex runs, ductboard on a medium-pressure system, or unsealed joints that a house tolerates and a tested commercial system fails. The bigger the system, the less it forgives the cheap type.
Data center and large commercial duct
Data center and large central air systems are the high-stakes end of duct selection, and they run sealed sheet metal and spiral at the higher pressure classes, fed by large air handlers moving air the facility has budgeted to the cubic foot. There is no room for the cheap types here. A few percent of leakage on a large high-pressure run is a large absolute volume of cooling air leaving the duct every hour, and on a server hall that is a hot aisle nobody can explain from the unit.
Two things tighten on these jobs. The leakage class gets stricter and is proven by test rather than assumed, and the joints lean toward flanged, gasketed connections that seal repeatably at pressure, because a slip joint that is fine at 1 in. w.g. is the wrong call at 6 in. Round and spiral show up wherever the geometry allows because they hold pressure and seal for less effort.
The project spec on these systems is usually stricter than the SMACNA default and is the document that controls. Build to the spec, test to the spec, and the duct delivers the air the racks were designed around.
What is the best type of ductwork?
There is no single best type of ductwork; there is the right type for the location, the pressure, the budget, the air-quality demand, and whether the duct will be seen or hidden. The honest answer is a matrix, not a winner. Run the job through five questions and the type falls out.
How much pressure does it hold? Higher pressure forces sealed sheet metal, round or spiral, and rules out flex trunks and ductboard. Where does it run, and is it exposed? Tight chases want rectangular or flat-oval; open architectural ceilings want clean spiral; reachable low-pressure distribution can use fabric. How clean does the air have to be? IAQ-sensitive space wants smooth metal, not fiber. What is the budget, and how long must it last? Sheet metal costs more up front and lasts decades; flex and ductboard cost less and abuse easier.
Then put the types where they each belong. Sheet metal trunks and risers, round or spiral where the ceiling allows, flex only on the last short tail, ductboard on low-pressure residential where cost and quiet win, fabric for exposed even-distribution spaces. The best system usually mixes them, each type doing the job it is actually good at.
| If the job needs | Reach for |
|---|---|
| High pressure, low leakage, long life | Sealed sheet metal, round or spiral |
| A tight or shallow space | Rectangular or flat-oval |
| An exposed, finished ceiling | Painted spiral round |
| The last short connection to a diffuser | Short, tight flex |
| Low-cost, quiet, low-pressure residential supply | Fiberglass ductboard |
| Exposed, washable, even distribution | Fabric textile duct |
| IAQ-sensitive space | Smooth bare metal or double-wall |
What to document
A duct system is a mix of types, and the record of what went where is what the next crew, the commissioning agent, and the inspector read to understand it. Write down the type and material of each run, the pressure class it serves, the insulation method, and any place the install departed from the drawing, because the duct in the ceiling has to be defensible against the drawing on the plans.
The table below is the quick reference for the types themselves: what each is, where it belongs, and the watch-out that bites when it is used wrong. Keep it with the submittal so the type selection is on the record and not just in someone's head.
| Type | Material | Best use | Watch-out |
|---|---|---|---|
| Sheet metal, rectangular | Galvanized steel | Trunks in tight space, medium pressure | More seam and friction; needs reinforcement |
| Sheet metal, round or spiral | Galvanized steel | Long runs, exposed, low leakage | Needs ceiling height to swing |
| Flex | Wire helix, insulation, vapor jacket | Last short connection to a diffuser | Sag, kink, crush, and length kill airflow |
| Fiberglass ductboard | Resin-bonded glass fiber | Low-pressure residential supply | Fiber erosion, IAQ, hard to clean, hates water |
| Fabric / textile | Engineered cloth | Exposed, washable, even distribution | Low pressure only; not concealed |
| Aluminum / stainless | Aluminum, stainless steel | Corrosive, coastal, grease, pools | Cost; specialty fabrication |
Common mistakes
- Running long, kinked, coiled, or compressed flex duct, which can multiply its pressure drop several times over and starve the rooms it feeds.
- Using flex for a trunk or a long branch instead of the last short connection.
- Putting fiberglass ductboard or internal liner in an IAQ-sensitive space where shed fiber and hard cleaning are a problem.
- Sizing a rectangular duct with a high aspect ratio, which piles on friction and material for the same air.
- Substituting flex for sized metal without resizing, adding friction the design never budgeted.
- Leaving joints unsealed and treating the loose residential types as if they hold air on a tested commercial system.
- Putting ductboard or fabric where it gets wet, abused, or run at a pressure it cannot hold.
- Burying duct under a slab without the encasement, slope to drain, vapor barrier, and pre-pour test the code requires.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
SMACNA, the Sheet Metal and Air Conditioning Contractors' National Association, sets the construction standards for the metal and flexible duct types. The HVAC Duct Construction Standards, Metal and Flexible, cover the pressure classes, the gauge and reinforcement, the seams and joints, and the seal classes; the HVAC Air Duct Leakage Test Manual covers the leakage classes and the test. Both move between editions, so cite by topic and confirm against the edition the job is built to.
The listings sort the duct and the closures. UL 181 lists rigid and flexible air ducts and ductboard by class, where Class 1 carries the common flame-spread and smoke-developed limits, and it draws the line between an air duct and a length-limited air connector. UL 181A lists closures for rigid duct and UL 181B lists closures for flexible duct, which is what a tape or mastic has to meet to be a primary sealant.
The rest fills in by topic. ASHRAE Standard 90.1 and the IECC drive the sealing, insulation, and leakage testing through the energy code. NFPA 90A covers the installation of air-conditioning and ventilating systems and the fire and smoke requirements for duct, and NFPA 96 covers grease exhaust. The mechanical code governs underground duct and where dampers are required. Above all of these, the project specification controls, and on critical systems it is usually the strictest document. Confirm the adopted editions and local amendments rather than quoting a remembered number, and lean on the manufacturer's listing for any specific gauge, length, or pressure limit.
Units, terms, and conversions
Duct work crosses a few unit systems and a lot of shorthand, so the same idea reads differently across a drawing, a spec, and a catalog.
Pressure is in inches of water gauge, written in. w.g. or in. wc, and one inch of water gauge is about 249 pascals. Airflow is in cfm, cubic feet per minute, or cubic meters per second in metric work. Gauge is sheet thickness, where a lower number is thicker metal. Insulation value is given as an R-value, and flex and ductboard are commonly sold by R-value such as R-6 or R-8. Duct size is in inches, with round given by diameter and rectangular by width and height, and the aspect ratio is the width divided by the height.
- Galvanized steel
- Zinc-coated carbon steel, the standard sheet metal duct material
- Spiral duct
- Round duct made from a strip locked into a helical seam that stiffens it
- Flat-oval
- Round duct flattened to fit a shallow space, keeping most of round's strength
- Flex duct
- Insulated flexible duct: a wire helix and liner inside insulation and a vapor jacket
- Ductboard
- Rigid duct cut from resin-bonded fiberglass board with a foil facing
- Aspect ratio
- The width divided by the height of a rectangular duct
- UL 181
- The listing standard for rigid and flexible air ducts and ductboard
FAQ
What is the best type of ductwork?
There is no single best type. Sealed sheet metal, round or spiral, performs best on leakage, friction, and life, and is the commercial standard. Flex and fiberglass ductboard cost less for low-pressure residential work. Match the type to the pressure, the location, the budget, the air-quality demand, and whether the duct is exposed.
Is round or rectangular duct better?
Round duct beats rectangular on almost everything: it is stronger per gauge, leaks less with fewer seams, and adds roughly 15 to 25 percent less friction for the same air, using less metal per cfm. Rectangular wins only on fitting tight, shallow spaces and on tapping branches anywhere along the trunk.
Is flex duct bad?
Flex duct is not bad when it is used right: short, pulled tight, supported, and reserved for the last connection to a diffuser. It is bad when it is run long, kinked, coiled, or crushed, because field-typical compression can raise its pressure drop by a factor of four. Keep it off trunks and long branches.
What is ductboard?
Ductboard is rigid duct cut and folded from boards of resin-bonded fiberglass, faced outside with foil. The board is duct and insulation in one, and it is quiet and cheap. The concern is the inner glass fiber, which can erode, shed, grow mold if it gets wet, and resist mechanical cleaning, so it is avoided in IAQ-sensitive space.
Sheet metal or flex duct: which should I use?
Use sheet metal for trunks, risers, branches, and anything at real pressure, because it is durable, tight when sealed, and low friction. Use flex only for the short final connection between hard duct and a diffuser. Substituting flex for sized metal adds friction the design never budgeted and starves the rooms downstream.
How long can a flex duct run be?
Keep flex to the shortest run that reaches the diffuser, not a set maximum, because its friction penalty compounds with every foot and every sag. Pull it tight, support it on roughly 4 to 5 ft spacing, and never coil the slack. For any real distance, run metal or round and reserve flex for the last short tail.
Can I use flex duct or ductboard for a main trunk?
No. Flex has far too much friction per foot for a trunk, and fiberglass ductboard is a low-pressure product that erodes and is hard to clean. Trunks and risers belong in sealed sheet metal, round or spiral, which hold pressure, leak little, and last. Reserve flex for the last connection and ductboard for low-pressure branches.
Which duct is best for indoor air quality?
Smooth bare metal and double-wall duct with a solid inner liner are best for indoor air quality, because they clean with a brush and shed nothing into the air. Fiberglass ductboard and internally lined duct expose glass fiber that can erode and grow mold, so hospitals, labs, and food plants run smooth metal instead.
What is spiral duct used for?
Spiral duct is round duct with a helical lock seam that stiffens it, used for long round runs, low-leakage systems, and exposed architectural ceilings where it is left painted and visible. It goes up fast, seals cleanly at couplings, and reaches a tighter leakage class than rectangular for much less labor.
What do I do if a room is starved for air on a flex branch?
Check the flex first. A starved room on a flex branch is usually a sagged, kinked, coiled, or crushed run multiplying its friction, or a run far longer than it needs to be. Pull it tight, shorten it, support it, and clear any pinch. Read the static to confirm the branch is the restriction before touching the equipment.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.