Sheet Metal vs Fiberglass Ductwork: Which to Specify and When

Sheet metal and fiberglass ductboard are the two dominant materials for rectangular HVAC ductwork in commercial and residential light construction. Each has legitimate applications — and each is commonly misapplied by contractors and specifiers who don't understand the tradeoffs. This guide compares the two materials across the dimensions that matter: construction quality, air leakage, acoustic performance, durability, and total installed cost.

How Each Material Is Constructed

Sheet metal ductwork is fabricated from galvanized steel coil stock, typically 26-gauge for residential applications and 24-gauge or heavier for commercial. The metal is cut to pattern, formed into rectangular sections on a brake or roll-forming machine, and joined with slip-and-drive, TDC/TDF flange, or Pittsburgh seam connections. Seams are sealed with mastic or UL 181B listed tape. Sheet metal duct is then wrapped externally with fiberglass blanket insulation (typically R-4 or R-6) secured with tie wire and vapor retarder jacketing.

Fiberglass ductboard (trade names include Knauf's Ductboard, Owens Corning EasyFlo) is a rigid panel of glass fibers bonded with a resin binder, faced on one side with a foil-scrim-kraft (FSK) vapor retarder and on the other side with a resin-coated glass mat facing that forms the interior air surface. Panels are scored and snapped to form duct sections, then stapled and taped at joints with UL 181A listed tape. The insulation is integral — no separate wrap is needed.

The fundamental difference: sheet metal is a duct first, then insulated; ductboard is an insulation panel first, shaped into a duct form.

Air Leakage Performance

Air leakage is where sheet metal and fiberglass ductboard diverge most significantly — and where ductboard is most often criticized unfairly.

Sheet metal ductwork constructed to SMACNA standards with proper joint sealing achieves Seal Class A leakage performance: total leakage less than 2% of system CFM at design pressure. The metal itself is impermeable — all leakage occurs at joints. Properly sealed sheet metal joints maintain their seal for decades if mastic is used (foil tape adhesive can dry and peel over time).

Fiberglass ductboard has a different leakage characteristic: the FSK facing and glass mat interior are not perfectly impermeable, but more significantly, ductboard joints — sealed with UL 181A staples-and-tape per SMACNA's fibrous glass duct standards — can separate as the board flexes over time. Ductboard is rated for Seal Class C construction (for supply and return plenums) and Seal Class B for branches, meaning higher allowable leakage than sheet metal.

In practice, a well-installed ductboard system in a stable environment (minimal temperature cycling, no physical abuse) performs comparably to sheet metal. The problem is that ductboard is often installed in attics, crawl spaces, and other environments where temperature cycling causes expansion and contraction that works joints loose over 10–15 years.

Acoustic Performance

Fiberglass ductboard has a significant acoustic advantage over sheet metal in applications where duct noise is a concern. The fibrous interior surface absorbs sound energy — both equipment noise and airflow-generated turbulence — rather than reflecting it as smooth sheet metal does.

Acoustic insertion loss comparisons (approximate, per ASHRAE Handbook):

In open-plan offices, recording studios, and high-end residential construction, this acoustic advantage justifies ductboard for the first 10–15 feet of supply duct run downstream from the air handler — even in systems that use sheet metal elsewhere. After that distance, equipment noise has attenuated enough that the acoustic advantage diminishes.

Durability and Longevity

Sheet metal durability is essentially unlimited in dry, conditioned environments. Galvanized steel resists corrosion well unless exposed to condensation cycles or chemically aggressive atmospheres. The physical integrity of sheet metal — its ability to maintain shape and hold connections — does not degrade under normal HVAC operating conditions.

Fiberglass ductboard has a more limited service life in challenging environments. The glass fibers themselves are inert, but the resin binder that holds them together can degrade with sustained moisture exposure. The interior glass mat facing can delaminate when repeatedly wetted — this introduces glass fibers into the airstream and creates a significant indoor air quality problem. EPA and NIOSH have published guidance on ductboard delamination as an IAQ concern.

Expected service life:

• Sheet metal, dry conditioned environment: 40–50+ years
• Sheet metal, attic or crawl space with proper insulation: 20–30 years
• Fiberglass ductboard, stable conditioned environment: 15–25 years
• Fiberglass ductboard, attic or humid crawl space: 10–15 years before joint failure and possible liner degradation

Code and Standards Compliance

Both materials are code-compliant when properly applied. Key standards references:

• Sheet metal: SMACNA HVAC Duct Construction Standards (Rectangular Industrial), ASHRAE 90.1
• Fiberglass ductboard: SMACNA Fibrous Glass Duct Construction Standards, UL 181 (Class 1 air duct), ASTM C1071
• Both: IECC 2021 insulation requirements, ASHRAE 90.1 duct sealing requirements

Some jurisdictions specifically restrict or prohibit fiberglass ductboard in certain applications — check local mechanical codes before specifying.

When to Specify Each Material

Use sheet metal when:

• The system will be in an attic, crawl space, or other harsh environment
• The project requires SMACNA Seal Class A leakage performance
• Long service life with minimal maintenance is required
• Commercial or industrial applications with high airflow velocities (above 2,000 FPM)
• The system will be exposed or visible

Use fiberglass ductboard when:

• Acoustic attenuation is a primary design goal in a stable environment
• Fast field fabrication in a conditioned space is needed
• First cost is a primary driver on a residential project in a benign environment
• Short runs near equipment where acoustic benefit is highest