Introduction
Pick up any three ductwork quotes and the biggest hidden variable is often the material: galvanized sheet metal, flexible duct, or fiberglass duct board. Each carries air differently — different interior friction, different insulation, different lifespan — and the sizing math shifts with the choice. A 6-in duct in smooth metal is not the same duct as 6-in flex pulled through a tight attic. This guide compares the three residential duct materials head to head so you can specify the right one for each part of the system. Size the runs first with the Ductwork Calculator, then match the material to the job below.
The Three Duct Materials
Galvanized sheet metal
The traditional standard: steel sheet (typically 26 gauge for round duct and fittings, 24 gauge for rectangular trunks per SMACNA) formed into round pipe or rectangular duct, with joints sealed by mastic. Interior surface is smooth, giving the lowest friction of the three materials and the most predictable sizing — ductulator tables are based on sheet metal. Lifespan is 25–50+ years; it’s damage-tolerant, cleanable, and holds its shape at hangers. Downsides: highest material and labor cost, needs separate insulation wrap (typically R-6 to R-8) outside conditioned space, and rectangular fabrication requires a shop or skilled tinsmith. Best for: trunks, long runs, and any section where friction budget is tight.
Flexible duct
A wire-reinforced plastic inner core, wrapped in fiberglass insulation (R-6 or R-8) and an outer vapor jacket — sold in 25-ft (7.6 m) boxes. Cheapest material, fastest to install, and snakes through tight framing that would need multiple metal elbows. The catch is friction: the corrugated inner core has higher resistance than smooth metal even when perfect, and dramatically higher resistance when compressed, sagging, or bent sharply. Published performance assumes the core fully stretched — at 15–30% compression, common in casual installs, a flex run behaves like a duct one to two sizes smaller. Lifespan 15–25 years. Best for: short branch runs (under 25 ft / 7.6 m) from a metal trunk to individual registers.
Fiberglass duct board
Rigid fiberglass panels with a foil exterior and sealed interior liner, fabricated into rectangular ducts — duct and insulation in one step, typically R-4.2 to R-8. Quieter than bare sheet metal (the fiberglass absorbs sound) and cheaper installed than wrapped metal. Trade-offs: higher friction than smooth metal, vulnerable to damage if the interior liner is torn (exposed fiberglass in the airstream), and harder to clean. Popular for trunks in some regions, less common in others — local contractor practice varies. Best for: rectangular trunks where insulation and noise control matter and the ducts stay protected.
Head-to-Head Comparison
| Factor | Sheet metal | Flexible duct | Duct board |
|---|---|---|---|
| Interior friction | Lowest (baseline) | Higher; much higher if compressed | Moderate |
| Typical lifespan | 25–50+ years | 15–25 years | 20–30 years |
| Installed cost | Highest | Lowest | Middle |
| Insulation | Added wrap (R-6/R-8) | Built in (R-6/R-8) | Built in (R-4.2–R-8) |
| Noise | Can drum; lined options exist | Quiet (insulated) | Quiet (absorptive) |
| Routing flexibility | Low — needs fittings | High — bends freely | Low — rectangular |
| Damage tolerance | High | Low (crush/kink) | Moderate |
| Best use | Trunks, long runs | Short branches | Insulated trunks |
How Material Changes the Sizing
Sizing tables assume smooth sheet metal. When you substitute:
- Flex for metal, same diameter: acceptable on short runs (under 25 ft / 7.6 m) installed perfectly taut. On longer runs, either upsize the flex one diameter or shorten the run — the corrugation penalty compounds with length.
- Compressed flex: no table saves you. A 6-in flex run at 30% compression can carry the air of a 4-in duct. The fix is installation, not arithmetic: stretch it tight, cut to length, support every 4 ft (1.2 m).
- Duct board for metal trunk: size per the manufacturer’s friction data or Manual D duct-board tables; expect slightly larger dimensions than smooth metal for the same CFM.
- Mixed systems: size each section in its actual material. The standard professional pattern — metal trunk, flex branches — works because each material is used where its strengths matter: low friction where airflow is highest, flexibility where runs are short. Mixing materials without adjusting the math is one of the classic errors in the common mistakes guide.
Insulation and Sealing by Material
- Sheet metal outside conditioned space: wrap with R-6 or R-8 duct insulation (R-8 in attics where code requires it), seams sealed, vapor jacket intact. Every joint gets mastic before the wrap goes on — sealing after wrapping is nearly impossible.
- Flex: insulation is built in, but the vapor jacket tears easily — inspect and tape (UL-181) any rips. Seal the inner core to the collar with mastic or listed tape before pulling the jacket and insulation over the connection.
- Duct board: joints sealed per manufacturer spec (staples + UL-181 tape or mastic); keep the interior liner unbroken — damaged liner exposes fiberglass to the airstream and must be repaired or the section replaced.
Sealing standards and insulation minimums are code issues too — the code compliance guide covers what inspectors check.
What to Specify
For most single-family homes, the value-optimized specification is:
- Trunks: galvanized sheet metal (or duct board where local practice favors it), stepped down as branches leave, sealed with mastic, wrapped to R-8 in unconditioned spaces.
- Branches: R-8 flex, 25 ft (7.6 m) maximum per run, fully stretched, supported every 4 ft (1.2 m), gentle bends only.
- Takeoffs: metal spin-in or snap-in collars with balancing dampers — never cut a raw hole and stuff flex over it.
- Boots and grilles: metal, sized to the run CFM, sealed to the drywall.
Full metal everywhere is the premium option — quietest, longest-lived, most predictable — at the highest price. Full flex everywhere is the budget option that demands the most careful installation to perform. Price brackets for each approach are in the cost guide.
Related Guides
- Complete Guide — sizing runs in any material.
- Cost Guide — installed price ranges by material.
- Pro Tips — installation practices that protect flex performance.
- Estimating Guide — counting material quantities.
- FAQ — quick material questions answered.
Frequently Asked Questions
Is metal ductwork better than flex?
Metal has lower friction, longer life (25–50+ years vs 15–25), and holds its shape; flex is cheaper and easier to route. Most pros use both: metal trunks, flex branches under 25 ft (7.6 m), installed taut.
What R-value does duct insulation need?
Energy codes commonly require R-8 for ducts in attics and R-6 elsewhere outside conditioned space — verify locally. Flex is sold in R-6 and R-8; the price difference is small, so R-8 is the usual choice.
Can duct board be used for the whole system?
It’s used for trunks and sometimes branches in some regions, but flex or metal branches are more common. Keep the interior liner intact and seal joints per manufacturer spec.
How long does flex duct last?
Typically 15–25 years. Replace it if the inner core is brittle, the insulation is compacted, sections are crushed, or there’s mold contamination.
Does the duct material affect the size calculation?
Yes. Sizing tables assume smooth sheet metal; flex has higher friction (much higher if compressed), so long flex runs may need upsizing. Always size in the material you’ll actually install.
Get Your Number
Material is a performance decision disguised as a price decision: metal where friction budget is tight, flex where runs are short and routing is tight, duct board where insulation and quiet matter. Size each section with the Ductwork Calculator in the material you plan to install — then install that material like the tables assume.
Disclaimer: Material descriptions are general planning information. Verify gauge, insulation, and sealing requirements with your local mechanical code and consult a licensed HVAC professional for system design decisions.
