Introduction
Duct material changes the friction math, the installed cost, the insulation strategy, and how long the system lasts. A CFM requirement that calls for 8-inch rigid metal may need 9-inch flex; a trunk that fits as round spiral may need rectangular ductboard to clear a joist bay. Choosing the material is therefore part of sizing — not a decision you make after.
Size your runs first with the Duct Size Calculator, then use this guide to translate the result into the right material, gauge, and insulation for each section. Cost implications are in the cost guide.
The Four Duct Materials
Galvanized sheet metal — the default for rigid duct
Galvanized steel is the workhorse: smooth interior (lowest friction of any option), rigid enough to hold its shape for decades, and repairable. Residential round and rectangular ducts are typically 26 to 30 gauge — heavier (lower gauge number) for larger ducts that must resist collapsing under negative pressure:
| Duct size (round diameter or longest rectangular side) | Typical minimum gauge |
|---|---|
| Up to 12 in | 28–30 ga |
| 13–18 in | 26 ga |
| 19–30 in | 24 ga |
Metal duct is uninsulated as manufactured — in unconditioned spaces it needs a separate insulation wrap (see R-values below). Seams and joints must be sealed; a perfectly sized but leaky metal system performs worse than a sealed flex system. Use it for: trunks, long straight runs, any run where friction budget is tight, and commercial-style exposed ductwork.
Flexible duct — cheap, fast, friction-hungry
Flex duct is a wire helix wrapped in fiberglass insulation with a vapor-barrier jacket, sold in 25-ft boxes. It bends around obstacles without fittings, which makes it the fastest (and cheapest installed) option. The catch is friction: flex’s corrugated interior loses more pressure than smooth metal, and compression, sagging, or kinks multiply that loss several times over — a compressed 8-in flex run can behave like 6-in rigid.
Planning rules for flex:
- Size one trade size up from the rigid-metal calculation (8-in rigid result → 9-in flex).
- Keep runs short and straight; pull taut with minimal compression.
- Support every ~4 ft with wide straps that don’t pinch the duct; limit sag.
- Use gradual bends — two 45° bends instead of one crushed 90°.
- Buy UL 181-listed flex from a reputable manufacturer; the listing covers the duct’s fire and mold-growth testing.
Use it for: short branch runouts in accessible attics/crawlspaces, final connections to boots and registers. Avoid it for: long trunk runs, any run that can’t be kept straight and supported.
Fiberglass ductboard — structure plus insulation in one
Ductboard is rigid fiberglass board (typically 1–2 in thick) fabricated into rectangular ducts on site, with the air stream facing a coated interior surface. It combines duct and insulation in a single product and damps sound well — noticeably quieter than bare metal. Trade-offs: fabrication quality varies with the installer (badly made ductboard leaks at the joints like anything else), the interior surface must stay intact to avoid fiber erosion, and it’s limited to rectangular shapes.
Use it for: rectangular trunks and plenums where insulation and noise control matter, especially in unconditioned spaces.
Aluminum and spiral round — specialty rigid
Aluminum duct is lighter and corrosion-resistant but pricier than galvanized steel — common in coastal or corrosive environments. Spiral round (spiral-seam) duct is the premium rigid round option: very low leakage, attractive exposed, but more expensive and a commercial/industrial staple rather than a residential default.
Insulation R-Values
Ducts in unconditioned spaces (attics, crawlspaces, garages) must be insulated or they bleed heating/cooling energy and sweat condensation in summer. Common options:
| Material / product | Typical R-value |
|---|---|
| Bare sheet metal | ≈ R-1 (essentially uninsulated) |
| Duct wrap (fiberglass blanket) | R-6 or R-8 |
| Flexible duct jacket | R-4.2, R-6, or R-8 |
| Fiberglass ductboard (1–2 in) | R-4.2 to R-8.7 |
Many energy codes require R-6 to R-8 on ducts in unconditioned spaces — but the exact requirement varies by jurisdiction and climate zone, so verify locally (see the code compliance guide). In conditioned spaces (inside the thermal envelope, e.g., a basement within insulated walls), uninsulated metal is generally acceptable. Always install the vapor-barrier jacket facing outward on insulation in humid climates; reversed or missing vapor barriers cause condensation inside the insulation.
Sealing: Mastic and Listed Tapes
Material choice means little without sealing — unsealed ducts commonly leak 20–30% of system airflow. The accepted closure systems:
- Brush-on mastic — the gold standard; paint it over every joint, seam, and takeoff. Messy, permanent, excellent.
- UL 181A/B-listed tapes — foil or film tapes tested for duct closure. 181B covers flexible duct closures; 181A covers rigid.
- Standard cloth “duct tape” — not a duct sealing method. It dries out and fails, sometimes within a season or two.
Seal first, then insulate over the sealed duct — insulation hides leaks instead of fixing them if the order is reversed.
Matching Material to the Job
| Application | Recommended material | Why |
|---|---|---|
| Main supply/return trunks | Rigid galvanized metal (or ductboard) | Lowest friction, holds shape, decades of life |
| Short branch runouts | Flexible duct (sized one up) | Fast, cheap, no fittings needed |
| Tight joist bays / soffits | Rectangular metal or ductboard | Fits shallow spaces; ductboard adds insulation |
| Unconditioned attic/crawlspace | Any + R-6 to R-8 insulation | Energy code typically requires it — verify locally |
| Noise-sensitive runs (bedrooms) | Ductboard or lined metal | Absorbs fan and airflow noise |
| Final register connections | Short flex or metal boot | Eases alignment to the grille |
Whatever you choose, size for the material actually being installed — the pro tips guide covers flex installation practices that keep its real-world friction near its rated friction, and the estimating guide walks through measuring each run.
Related Guides
- Duct Size Calculator FAQ — quick answers to the most asked duct sizing questions.
- Estimating Guide — how to measure CFM needs, lengths, and fittings.
- Common Mistakes — the sizing errors that cost the most.
- Duct Sizing Checklist — every step to verify before you buy ductwork.
- DIY vs Pro — when you can do this work yourself, and when to hire.
Get Your Number
Pick the material for each run, apply the right friction assumption (rigid as calculated, flex one size up), and run the numbers in the Duct Size Calculator — then buy UL-listed products, seal every joint with mastic, and insulate to your local code’s R-value. A licensed HVAC professional should review material selections for full-system designs.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual duct sizing depends on your system’s airflow requirements, equipment static pressure, duct material, and local code. Verify with a licensed HVAC professional, and consult your local building authority for code and permit decisions.
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