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Ductwork (Sheet Metal, Flex Duct, Duct Board)

Residential and light commercial air distribution uses three main duct types: galvanized sheet metal, flexible duct and fibrous glass duct board. Flex duct and duct board are sold as UL 181 listed products, and the listing class matters to the inspector.

Ref HV-0022SMACNA HVAC Duct Construction StandardsUL 181

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Sheet metal duct is galvanized steel (sometimes aluminum or stainless) formed into round, rectangular or oval duct. It is built to SMACNA construction standards for gauge, seams and reinforcement.
Flexible duct is a wire helix covered by an inner liner, a layer of insulation and an outer vapor barrier jacket. It is used for short runs to registers.
Fibrous glass duct board is rigid fiberglass board with a foil facing. It is cut and folded into rectangular duct on site and closed with listed tape or mastic.

How to identify

Sheet metal is plain metal with snap-lock, Pittsburgh or spiral seams. Flex duct has a printed jacket and a label showing the UL 181 listing, class, R-value and whether it is an air duct or an air connector. Duct board has a printed foil face with the maker, thickness, R-value and UL 181 listing.

Duct types compared

Feature	Sheet metal	Flexible duct	Duct board
Construction rules	SMACNA standards	UL 181 listing and maker's instructions	UL 181 listing and NAIMA fabrication standard
Insulation	Added wrap or liner	Built in	Built in
Airflow resistance	Lowest	Highest, rises fast if kinked or compressed	Moderate
Typical use	Trunks, plenums, commercial	Branch runs to outlets	Trunks and plenums in residential
Closure	Mechanical fasteners and sealant	Clamps plus listed tape or mastic	UL 181A tape or mastic

UL 181 listing classes

UL 181 tests ducts for flame spread, smoke and strength. Class 0 means no flame spread or smoke in the surface burning test. Class 1 allows a limited flame spread and smoke development. Codes accept Class 0 and Class 1 air ducts. UL 181 also separates air ducts from air connectors. Air connectors are lighter products that codes limit in length and location. Check the label and the mechanical code before using flex as a main duct.

Key points

Flex duct should be pulled tight, supported at the intervals the maker states and kept free of sharp bends. Compressed flex can cut airflow sharply.
R-value requirements for ducts in unconditioned spaces come from the energy code.
All duct joints should be sealed. See the duct sealants and tapes sheet.
For sizing, see the duct sizing basics chart.

Common mistakes

Using air connector flex as a long duct run.
Leaving flex duct sagging between supports or crushed at bends.
Closing duct board with cloth duct tape.
Choosing sheet metal gauge by guesswork instead of the SMACNA tables.

Governing standards: SMACNA HVAC Duct Construction Standards; UL 181 (factory-made air ducts and air connectors); NAIMA Fibrous Glass Duct Construction Standards; International Mechanical Code and local energy code.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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