How Do Rigid and Flex Duct Compare?
| Rigid Metal Duct | Flex Duct | |
|---|---|---|
| Durability | Galvanized steel; hard to damage | Fragile; easy to crush, kink or tear |
| Airflow | Smooth inside, so air moves easily | Ridged inside, so more drag |
| Size for the same airflow | 6-inch rigid | About 8-inch flex |
| Install speed | Slower, especially for single runs | Faster |
| Awkward spaces | Tight turns need custom-made fittings | Bends around obstacles |
| Insulation | Must be added in attics and crawlspaces | Comes insulated |
| Easy to install badly? | Less so | Yes |
Why Does Rigid Duct Move Air Better?
Think of riding a bike on smooth pavement versus over bumpy railroad ties. The inside of rigid duct is smooth. Flex duct is basically a giant Slinky with a plastic lining. Even pulled tight and installed correctly, it has ridges that slow the air down. That's why, where we'd run a 6-inch rigid duct, we'd need about an 8-inch flex duct for the same airflow.
Rust isn't a concern for rigid duct in Colorado's dry climate. It can be in humid or flood-prone places.
When Does Flex Make Sense?
- Tight or awkward spaces, where it can bend around rafters and beams.
- Attics, where the built-in insulation is a real advantage.
- Short final runs to each room. A common setup is a rigid metal trunk line with flex for the last stretch to each vent.
- Small add-on jobs, like a single new supply run, where rigid duct would cost more in labor and insulation.
What Does a Bad Flex Install Look Like?
- Crushed or kinked runs. Someone kneels on it while crawling through the attic, and now it's kinked.
- Not pulled tight. It shouldn't be guitar-string tight, but slack flex bunches up the ridges inside, which means more drag.
- Sharp bends. Like taking a hard right turn at 55 miles an hour, air slows down in tight turns. Gradual bends are better.
- Sagging. Flex needs to be held up off the attic insulation. Without enough support, it droops between hangers like Christmas lights. Those dips, called swags, slow the air. We support it every 3 feet, more often than code requires.
- Torn lining. The inner lining can be nicked and start coming apart if it isn't handled carefully.
Rigid duct can be installed badly too, but it's hard to tear by accident.
Rigid vs. Flex FAQ
Is Flex Duct Bad?
No. It's a good tool in the right places, installed carefully. Most problems come from how it's installed.
How Do I Know If My Flex Duct Is Installed Well?
Look for runs that are fairly straight and taut, supported every few feet, with no crushed sections, sharp bends or sagging dips.
Does Rigid Duct Need Insulation?
In an attic or crawlspace, yes. Insulating it adds time to the job. See why ductwork costs what it does.