HVAC Ductwork Types: Sheet Metal vs Flex Duct vs Fiberglass Ductboard
Three fundamentally different materials handle the same job — moving conditioned air from the air handler to every room in the building. Sheet metal duct, flexible ductwork, and fiberglass ductboard each have distinct cost profiles, performance characteristics, and ideal applications. This guide compares all three across eight critical factors so you can specify the right ductwork for each part of your system.
In This Guide
1. Quick Comparison Table
| Factor | Sheet Metal | Flex Duct | Fiberglass Ductboard |
|---|---|---|---|
| Material cost (per linear ft, 6" round) | $4–$8 | $1.50–$3 | $3–$6 |
| Labor cost | High (fabrication + installation) | Low (fast installation) | Medium (cut and assemble) |
| Lifespan | 30–50 years | 15–25 years | 20–30 years |
| Pressure drop (10 ft, 300 CFM) | 0.06 in. w.g. | 0.09–0.18 in. w.g.* | 0.08 in. w.g. |
| R-value (uninsulated) | 0 (requires wrap) | R-4.2 to R-8 (built-in) | R-4.2 to R-8 (built-in) |
| Acoustic performance | Poor (requires liner) | Good (inner liner absorbs) | Excellent (integral foam) |
| Moisture resistance | Excellent | Moderate (liner can trap moisture) | Poor (absorbs moisture) |
| SMACNA code compliant | Yes — all pressure classes | Yes — limited length/bends | Yes — up to 2" w.g. |
* Properly installed flex duct: 0.09 in. w.g./10 ft; sagged or kinked: up to 0.18+ in. w.g.
2. Sheet Metal Ductwork
Sheet metal duct is the benchmark against which all other ductwork types are measured. It is manufactured from galvanized steel (zinc-coated G60 or G90) or aluminum, formed into round, rectangular, or oval cross-sections in standardized sizes from 4" to 60". In commercial and industrial installations, sheet metal accounts for roughly 60% of installed ductwork by volume.
Construction and Gauges
Sheet metal duct gauge follows SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) standards. For round spiral duct — the most common commercial product — the gauge specification is:
| Duct Diameter | SMACNA Gauge | Metal Thickness | Typical Application |
|---|---|---|---|
| 3"–12" | 26 gauge | 0.018" | Branch runs, end registers |
| 14"–26" | 24 gauge | 0.024" | Medium trunk lines |
| 28"–36" | 22 gauge | 0.030" | Main trunk lines |
| 38"–50" | 20 gauge | 0.036" | Large commercial mains |
Galvanized steel is the standard material for supply and return air systems. It handles temperatures from -20°F to 250°F (-29°C to 121°C) and static pressures up to 10" w.g. Aluminum duct is used where weight matters (overhead installations) and in corrosive environments such as kitchen exhaust and chemical processing.
Advantages of Sheet Metal Duct
- Lowest pressure drop: Smooth interior walls reduce friction losses to the SMACNA design standard of 0.1 in. w.g. per 100 feet for well-designed systems.
- Longest service life: 30–50 years with proper sealing and cleaning. The duct itself does not age — only sealants and insulation require periodic replacement.
- Inspection and cleaning: Sheet metal is the only ductwork type that can be effectively cleaned with NADCA (National Air Duct Cleaners Association) mechanical brush and vacuum equipment — important for healthcare and food-safe environments.
- Fire resistance: Sheet metal is non-combustible. It can be used in fire-rated assemblies and requires no additional protection in most UL fire-rated ceiling plenum applications.
- Flexibility for complex systems: Available in every fitting geometry — plenums, transitions, elbows, tees, wyes, reducers, boot connections — allowing any system configuration to be precisely engineered.
Disadvantages of Sheet Metal Duct
- High material and labor cost: Sheet metal duct fabrication requires a duct shop or job-site plasma cutter. Labor costs are 2–3× higher than flex duct installation for the same linear footage.
- Zero thermal insulation: Bare sheet metal conducts heat freely. All sheet metal duct in unconditioned spaces (attics, crawlspaces, unheated basements) requires external duct wrap (minimum R-6 per IECC 2021) — adding material and labor cost.
- Noise: Without acoustic lining, sheet metal transmits fan noise, airflow turbulence, and duct expansion/contraction sounds. Lined duct or fiberglass wrapping is required near occupied spaces.
3. Flexible Ductwork
Flexible ductwork — universally called "flex duct" — accounts for approximately 35% of U.S. residential duct installations and is the dominant branch duct material in new residential construction. It consists of a helical wire coil surrounded by an inner plastic liner (usually metalized polyester film), a fiberglass or foam insulation layer, and an outer jacket of reinforced foil or polymer.
Flex Duct Types by Insulation
| Type | R-Value | Use Case | IECC Compliance |
|---|---|---|---|
| Non-insulated flex duct | R-0 | Interior conditioned spaces only | Only if duct is inside conditioned envelope |
| R-4.2 insulated flex duct | R-4.2 | Attics in mild climates | Marginal — IECC 2021 requires R-6 minimum for attics |
| R-6 insulated flex duct | R-6 | Standard for most unconditioned attics | Compliant in most climate zones |
| R-8 insulated flex duct | R-8 | Extreme climate zones, deep energy retrofits | Exceeds IECC 2021 requirements |
Critical Installation Requirements
The single most important factor for flex duct performance is installation quality. According to ACCA Manual D and SMACNA guidelines:
- Support flex duct every 4 feet maximum — unsupported spans cause sag, which increases pressure drop by 50–100%.
- Extend the duct fully — compressed flex duct increases friction loss by 40–60% per foot of compression.
- Limit total length to 14 feet or less per ACCA Manual D recommendations for residential branch runs.
- Use 45° maximum bends per change of direction — a single sharp 90° flex duct elbow can add the equivalent of 15–25 feet of equivalent duct length.
- Seal all connections with UL 181B-FX listed mastic — duct tape alone does not meet code requirements and fails within 3 years.
When to Use Flex Duct
Flex duct is ideal for the final 6–8 feet of each branch run connecting the sheet metal distribution system to the ceiling or floor register. It provides vibration isolation between the system and the register, accommodates minor positional adjustments, and eliminates the need for custom-fabricated terminal fittings. Using flex duct for long trunk runs or multiple sequential elbows is a code violation in many jurisdictions and a guaranteed source of comfort complaints.
4. Fiberglass Ductboard
Fiberglass ductboard — also called duct board or rigid fiberglass duct — is manufactured from compressed fiberglass insulation faced with a foil vapor barrier. It is cut and assembled on the job site using a duct knife and specialized V-groove cut folding techniques, then sealed with UL 181B-FX mastic. Unlike sheet metal or flex duct, ductboard is a field-fabricated product — there is no duct shop required.
Ductboard Performance Characteristics
| Property | Value | Significance |
|---|---|---|
| Thermal R-value | R-4.2 to R-8 (1" to 1.5" board) | Integral insulation — no external wrap needed |
| Sound NRC (Noise Reduction Coefficient) | 0.70–0.85 | Absorbs 70–85% of airborne duct sound — excellent acoustic performance |
| Interior liner friction factor | 0.0003 (compared to 0.0001 for sheet metal) | Slightly higher friction than bare sheet metal but better than improperly installed flex |
| Maximum operating pressure | 2" w.g. positive, 1" w.g. negative | Unsuitable for high-pressure HVAC systems (>2" w.g.) |
| Temperature rating | 250°F (121°C) maximum | Cannot be used on high-temperature exhaust systems |
| Density (Type 475/IAB) | 3 lb/ft³ | NAIMA standard for air distribution ductboard |
Fiberglass Ductboard Applications
Fiberglass ductboard excels in applications where acoustic performance and thermal insulation are both required without the weight or labor of sheet metal plus acoustic liner. Typical applications include:
- Air handler plenums: The supply plenum immediately downstream of the AHU benefits from ductboard's sound absorption — it attenuates fan noise before it propagates through the duct system.
- Ceiling return air plenums: Where the ceiling cavity itself is used as the return air path, ductboard panels create defined return air channels with integral insulation and sound control.
- Commercial renovation: In existing buildings where sheet metal fabrication is impractical, ductboard allows custom duct sizes to be field-fabricated without special equipment.
- School and library HVAC: Sound-sensitive occupancies benefit from ductboard's high NRC rating — often specified as 30–50% of the duct system in these applications.
5. Cost Comparison
Total installed cost (materials + labor) for a typical residential system of 1,000 linear feet of ductwork (mix of trunk and branch runs):
| Ductwork Type | Material Cost | Labor Cost | Total Installed Cost | 25-Year Cost* |
|---|---|---|---|---|
| Sheet metal (supply trunk) + flex duct (branches) | $3,500–$5,500 | $4,000–$6,500 | $7,500–$12,000 | $8,000–$13,000 |
| All flex duct | $2,000–$3,500 | $2,500–$4,000 | $4,500–$7,500 | $7,500–$13,000† |
| Fiberglass ductboard (trunk and branch) | $3,000–$5,000 | $3,500–$5,500 | $6,500–$10,500 | $8,000–$12,500 |
* 25-year total cost includes replacement cycle at end of service life. † All-flex duct systems require full replacement after 15–25 years; second replacement cycle adds $5,000–$8,000.
The key insight: all-flex duct systems appear cheapest at installation but are most expensive over a 25-year horizon because flex duct requires full replacement rather than localized repairs. Sheet metal systems with flex duct branch runs are the most cost-effective long-term solution for residential and light commercial applications.
6. Airflow and Pressure Loss Comparison
HVAC system efficiency depends on duct pressure loss. Every additional 0.1" w.g. of system static pressure reduces air handler airflow by 5–10% (depending on the fan curve) and increases energy consumption by 3–8%. Duct selection directly impacts operating cost.
| Duct Type | Friction Rate (in. w.g./100 ft) | Condition |
|---|---|---|
| 26-gauge spiral steel, 12" round | 0.08 | SMACNA design target |
| Fiberglass ductboard, 12" round equivalent | 0.10 | New, clean interior liner |
| R-6 flex duct, 12" round | 0.09 | Fully extended, straight, per SMACNA |
| R-6 flex duct, 12" round | 0.15–0.22 | Sagging 1–2" midspan between supports |
| R-6 flex duct, 12" round | 0.30–0.45 | 25% compressed (common field condition) |
| R-6 flex duct, 12" round | 0.50+ | Single sharp 90° bend |
A 2021 field study by the Florida Solar Energy Center measured actual static pressure loss in 45 residential duct systems. Systems with properly installed flex duct had average friction rates of 0.14 in. w.g./100 ft — 55% higher than the SMACNA design target. Systems with improperly installed flex duct averaged 0.28 in. w.g./100 ft — nearly 3× the design standard, equivalent to adding 100+ feet of additional duct length to every branch run.
7. Which Ductwork Type for Each Application
| Application | Recommended Type | Reason |
|---|---|---|
| Main supply trunk (commercial) | Sheet metal (galvanized spiral) | High volume, low pressure drop, long service life required |
| Main supply trunk (residential) | Sheet metal or flex duct (R-6) | Sheet metal preferred; quality flex acceptable for single-story |
| Branch runs to registers (6–10 ft) | Flex duct (R-6 or R-8) | Fast installation, vibration isolation, positional flexibility |
| Air handler supply plenum | Fiberglass ductboard | Acoustic attenuation directly at the noise source |
| Return air plenum | Sheet metal or ductboard | Negative pressure requires rigid construction; ductboard adds sound absorption |
| Exposed ductwork (architectural) | Sheet metal (spiral or rectangular) | Visual appearance; surface can be painted |
| Kitchen exhaust (grease-laden) | Sheet metal (stainless or aluminized) | Fire code (IMC 506) requires listed grease duct — only sheet metal qualifies |
| Hospital, cleanroom, data center | Sheet metal with interior liner | Cleanability (NADCA), pressure class, fire rating requirements |
| Residential attic (retrofit) | Insulated flex duct (R-8) | Installation access, thermal performance, replace aging flex duct |
8. Code and Standards Compliance
All three ductwork types are code-compliant when used within their specified performance envelope. Key standards:
- SMACNA HVAC Duct Construction Standards: The primary technical reference for sheet metal duct gauge, joint construction, and pressure class requirements.
- UL 181 and UL 181B: The listing standard for "Factory-Made Air Ducts and Air Connectors." Flex duct must be UL 181B listed; ductboard and sheet metal connections use UL 181A (rigid) or UL 181B (flexible) listed closure systems.
- IECC 2021 Section C403 / R403: Requires duct insulation minimums based on climate zone. Attic ducts in most U.S. climate zones require R-6 minimum; some zones require R-8. All duct systems must be sealed to <4 CFM25 leakage per 100 sq ft conditioned floor area.
- IMC (International Mechanical Code) Section 603: Limits flex duct installation length (14 ft maximum per section in some jurisdictions), prohibits flex duct for kitchen exhaust, specifies support spacing.
- ASHRAE Standard 90.1: Duct sealing Class A for all duct systems in commercial buildings — all joints, seams, and connections sealed with code-listed materials (mastic preferred over tape).
Key rule of thumb: When in doubt, specify sheet metal. It is the only ductwork type that complies with every SMACNA pressure class, every UL fire rating, and every application in the IMC — with no length restrictions, no sag concerns, and no moisture absorption limitations.
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