What each material actually is
Flex duct is a wire-helix inner liner wrapped in a plastic film, surrounded by fiberglass insulation and a vapor-barrier outer jacket. It arrives on a reel, cuts to length, and connects to collars with clamps and mastic. R-6 and R-8 insulation levels are standard.
Sheet-metal duct is galvanized steel formed into rectangular or round sections, joined with mechanical seams and sealed with mastic. It's installed uninsulated and then wrapped with duct insulation (typically fiberglass with a foil-faced vapor jacket) after the ductwork is in place.
The practical trade-offs
Both materials pass Title 24 duct-leakage testing when installed correctly. The decision is driven by:
| Factor | Flex duct | Sheet metal |
|---|---|---|
| Install speed | Faster — flexible, cuts to length | Slower — fabrication and assembly |
| Static pressure | Higher — wire ribs disrupt flow | Lower — smooth walls, better long runs |
| Noise at register | Quieter — flexible walls absorb sound | Louder — rigid walls carry noise |
| Insulation | Built-in (R-6 or R-8) | Added after install (R-value depends on wrap) |
| Longevity | 15-25 years in attics (jacket UV + heat) | 40+ years when kept dry |
| Durability vs. pests/damage | Vulnerable to rodent chew-through | Resistant to physical damage |
| Cleanability | Difficult — can't scrub insides | Easier — rigid walls access-able |
| Cost | Lower material + labor | Higher material + labor |
Why the hybrid approach is the standard
A well-designed residential duct system typically uses sheet metal where it matters most — plenums, supply trunks, main return paths — and flex duct for the final branches to each register. The sheet-metal portions carry the airflow at low static pressure and resist damage; the flex branches are quiet at the register and flexible around framing obstacles.
Pure flex-duct systems are common in budget tract-home construction and often become the main source of airflow problems in those homes as the flex ages. Pure sheet-metal systems are rare in modern residential because the labor cost is substantial.
When to replace rather than repair
Flex duct installed in a hot attic reaches its practical end of life somewhere in the 15-25 year range — the plastic jacket embrittles from UV and heat, the insulation collapses in sections, and the wire helix can rust. Patching individual runs on a system with that profile just extends an ending; replacing the system catches up the whole network.
Sheet metal in good condition can run decades longer than that. Sheet-metal replacement is driven by water damage (condensation issues inside the duct), significant redesign (adding returns, resizing trunks), or demolition exposing the system during a remodel.
Common questions
Is sheet-metal ductwork worth paying more for?
On long main supply runs and return paths — yes, often. On short branch runs to individual registers — usually not, because the labor cost outweighs the practical difference. The common right answer is a mixed system where each material does what it does best.
How long does flex duct really last?
Properly installed R-8 flex in a well-ventilated attic can last 20+ years. Hot southwest-facing attics with poor ventilation accelerate aging. The telltale is the outer jacket — when it starts cracking or flaking, the system is in its last few years.
Can I upgrade existing flex to sheet metal section by section?
Technically yes, practically rarely. The labor to tear out existing flex and install sheet metal piecemeal is usually close to a full replacement. Most 'upgrade' conversations end up as either targeted repair (fix the problem flex sections) or full replacement (new mixed system).
Do rodents really chew through flex duct?
Yes — mice and rats can chew the outer jacket and insulation, then puncture the inner liner. In homes with attic rodent activity, this is a recurring issue; sealing the attic access points is the real fix, and the duct system gets patched or replaced depending on scope of damage.