10.2 Ductwork Fabrication, Sealing & Insulation

Ductwork Fabrication, Sealing & Insulation

Ductwork is the delivery system for everything a heat-load calculation and a piece of equipment promise on paper, and duct fabrication and sealing shows up heavily under the trade exam's Fabrication, Installation, and Startup domain. Poorly fabricated or unsealed ductwork is one of the most common reasons a system that should work on paper underperforms in the field — leaking supply ducts can waste a meaningful share of total system airflow before it ever reaches a room.

Sheet Metal vs. Flexible Duct

Rigid sheet-metal duct (typically galvanized steel, though aluminum and other materials appear in specialty applications) is fabricated and joined according to SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) HVAC duct construction standards. Sheet-metal gauge and joint reinforcement are selected based on duct dimension and the system's pressure classification (low, medium, or high pressure) — most residential and light-commercial systems on the C-20 exam are low-pressure. Common transverse (end-to-end) joints include S-slip and drive-cleat connections and TDC/TDF flanged joints (transverse duct connector/flange systems) on larger commercial trunks; common longitudinal (seam-running-the-length) joints include the snap-lock and Pittsburgh lock seams, with the Pittsburgh lock generally regarded as the more durable, higher-pressure-rated seam because the edges mechanically interlock along the full length of the duct.

Flexible duct is a factory-assembled product — a wire helix core wrapped in an inner liner, surrounded by insulation, and covered with an outer vapor-barrier jacket — that must be listed to Underwriters Laboratories (UL) Standard 181 for the application. Flex duct installation rules that regularly trip up new installers:

• Runs must be pulled reasonably straight and fully extended; a compressed or crushed flex run drastically increases pressure drop and starves airflow.
• Avoid bends sharper than roughly a 90-degree sweep with adequate bend radius; kinks collapse the inner core and choke airflow at that point.
• Support flex duct at reasonably close intervals (commonly cited as roughly every 4 feet) to prevent sagging that creates low spots where the liner pools and restricts flow.
• Most codes and manufacturers cap total flexible duct run length for a given branch; always check the current adopted mechanical code and the specific product's listing rather than assuming an old rule of thumb still applies.

Duct Support & Reinforcement

Rigid trunk and branch duct also needs adequate support and stiffening or it sags, rattles, and eventually pulls joints apart. Horizontal rigid duct is hung from the structure above with metal strap or trapeze hangers spaced at regular intervals (support spacing tightens as duct size and weight increase), and hangers should not be over-tightened enough to crimp or deform the duct wall. On wide, flat duct faces, sheet-metal contractors add cross-breaking (a shallow diagonal stiffening pattern pressed into the panel) or beading (a rolled-in stiffening rib) to reduce the oil-canning and rattling that a large unsupported flat panel produces when the blower cycles on and off. Flexible duct gets its own support rule: because it has no rigid structure of its own, it is hung in a smooth, saddled cradle rather than a narrow strap, since a narrow strap or wire tie cinched tightly around flex duct constricts the inner core exactly the same way a kink does.

Joint & Seam Sealing

Every duct joint, seam, and connection must be sealed — this is a direct code requirement, not just a best practice, because unsealed ductwork leaks conditioned air into attics, garages, and crawlspaces where it does no useful work. Acceptable sealants include mastic (a brush- or gun-applied adhesive compound), mastic-plus-embedded-fabric mesh systems, liquid sealants, and UL 181 listed tapes — but ordinary cloth-backed duct tape from a hardware-store shelf is not a code-compliant sealant; its adhesive breaks down with heat cycling and it is not tested or listed for permanent duct sealing.

The UL listing marks differ by duct type:

Mastic alone is appropriate for gaps up to about 1/4 inch; for wider gaps, code requires reinforcing the mastic with fiberglass mesh tape or an embedded-fabric system so the seal can bridge the gap without cracking. On a California installation, duct tightness is not just a visual sign-off — Title 24 Part 6, California's Building Energy Efficiency Standards, requires third-party Home Energy Rating System (HERS) verification of duct leakage on most new and altered duct systems, which is covered in depth in Chapter 9; the fabrication and sealing choices made here directly determine whether that leakage test passes.

Duct Insulation & R-Value Requirements

Duct insulation reduces conductive heat gain and loss as conditioned air travels through unconditioned spaces, and California's Title 24 Part 6 Energy Code sets minimum R-values by climate zone for ducts located in unconditioned spaces such as attics, garages, and vented crawlspaces:

• Climate Zones 3 and 5-7: minimum R-6 duct insulation.
• Climate Zones 1, 2, 4, and 8-16: minimum R-8 duct insulation.

Ducts that run entirely within conditioned living space are generally exempt from this insulation requirement because there is minimal temperature difference between the duct surface and the surrounding air. Insulation continuity matters as much as the R-value itself — connections and joints within unconditioned space must still carry at least R-6 insulation coverage, and a HERS rater will visually verify that continuity as part of the field inspection. On humid or exterior-exposed runs, the insulation's vapor-barrier jacket should face outward to prevent moist air from condensing on the cold duct surface underneath the insulation, which over time saturates the insulation and reduces its effectiveness.