Flexible duct is a wire coil wrapped in a plastic liner and insulation; rigid duct is sheet metal or duct board. Both move air, but the way flex duct is installed decides whether it moves enough, which makes the comparison as much about workmanship as material.
Rigid sheet metal moves air with the least friction and lasts longest, so it belongs on trunk lines and long runs. Flexible duct is cheaper and faster to install for short branch connections, but only if it's pulled tight and well supported; sagging or compressed flex duct can choke airflow to a room.
How they compare
| Rigid (sheet metal) | Rigid fiberglass duct board | Flexible | |
|---|---|---|---|
| Airflow resistance | Lowest, smooth interior | Higher than bare metal, from the fiberglass interior | Higher, from the ribbed liner; much higher if compressed or sagging |
| Installation | Slower, needs fittings and sealing at every joint | Cut and folded from panels, insulated in one step | Fast, bends around obstacles |
| Durability | Long-lived, hard to damage | Rigid, but the fiberglass is easier to damage with moisture or impact | Liner and jacket can tear or be crushed |
| Leakage | Seams must be sealed with mastic or tape | Seams sealed with approved tape and staples | Fewer joints, but connections must be clamped and sealed |
| Insulation | Wrapped separately where needed | Built into the panel | Usually comes insulated |
| Typical use | Trunks, long runs, returns | Trunks and plenums where built-in insulation helps | Short branch runs to registers |
Rigid fiberglass duct board is a third option: fiberglass panels cut and folded into rectangular ducts, with the insulation built into the panel. It is common for trunks and plenums because it is insulated in one step, though the fiberglass interior is not as smooth as bare metal and it is easier to damage with moisture or impact.
Why flex duct has to be pulled tight
The inside of a flex duct is a corrugated liner over a wire coil. Stretched straight, it still has more friction than smooth metal. Bunched up, sagging between supports or bent sharply, the corrugations deepen and the duct's resistance rises sharply, so the room at the end of the run gets far less air than designed.
Good flex installation means pulling the duct to its full length, supporting it so it can't sag, using long gentle bends instead of sharp ones, and cutting off the excess instead of coiling it. Flexible duct and connectors are listed to UL 181, and the manufacturer's installation instructions set the support spacing.
Sizing
Both are sized from the airflow each run carries and the friction rate the system can afford. Our ductwork sizing calculator sizes smooth sheet-metal duct; flex duct has more friction for the same airflow, so size it larger. The industry method for the whole system, including fittings, is ACCA's Manual D.
Leakage
Leaks matter more than material. The DOE puts duct air losses at about 30% of a cooling system's energy, and ENERGY STAR says leaky ducts can reduce heating and cooling efficiency by as much as 20%. Seal every rigid seam and every flex connection with mastic or approved tape, and clamp flex to its fittings; see duct leakage testing.
When to use each
- Rigid: main trunks, long straight runs, return ducts, and anywhere the duct could be stepped on or crushed.
- Flexible: short connections from the trunk to ceiling or floor registers, especially in attics where routing around framing is hard.
- Both: most good systems use rigid trunks with short flex branches.
A worked example: attic branch vs basement trunk
Picture a short branch run feeding one bedroom register, routed through an attic around trusses and framing. Flexible duct fits here: it bends around the obstacles, comes insulated for the unconditioned attic, and the run is short enough that its extra friction stays small. Pull it to full length, support it so it cannot sag, and keep the bends gentle.
Now picture the main trunk in a basement carrying the whole system's airflow. Sheet metal fits here: it moves the most air with the least friction, and it resists damage where it runs exposed along a joist. A straight basement line is also quick to fabricate and seal.
A fiberglass duct board plenum is a third option on a trunk, where its built-in insulation saves a separate wrap.
Frequently asked questions
Is flexible duct as good as rigid?
For short, straight, well-supported branch runs, it works well. For long runs, or installed with sags and kinks, it delivers noticeably less air.
How long can a flex duct run be?
Keep flex runs short and straight. Long runs add friction and invite sagging; the system design (Manual D) and the manufacturer's instructions set the limits.
Why is one room colder or hotter than the rest?
A crushed, kinked or disconnected flex run is a common cause, along with undersized ducts and leaks. Check the runs in the attic or crawl space first.
Does flex duct need to be insulated?
In unconditioned spaces such as attics, yes. Most flex duct comes with insulation built in; check that it meets your local code.
Is flex duct code-approved?
Yes, where it is listed and installed to the rules. Flexible duct and its connectors are listed to UL 181, and building and mechanical codes set where it can run, how it is supported, and the limits on length; your local code is the authority. Installed outside those rules, it is not approved.
Can you use flex duct for a return?
It is usually kept to short, well-supported return connections, not long return runs or main return trunks. Returns pull air under suction, so an unsupported or undersized flex return can collapse inward or restrict airflow more than a rigid return of the same size. Sheet metal or duct board is the safer choice for the main return path.
How long does flexible duct last?
It depends far more on installation and conditions than on age alone. Flex duct that is pulled tight, supported, and protected from sun, moisture and pests outlasts flex that sags, gets crushed, or bakes in an unconditioned attic. The liner and outer jacket fail first, so inspect runs in attics and crawl spaces for tears, kinks and disconnected joints.