Rigid Sheet Metal Duct

Rigid sheet metal duct is the traditional residential ductwork material — galvanized steel fabricated into rectangular trunk lines and round branch ducts. In Florida residential, it’s largely historical: common in homes built before 1985, increasingly rare in anything newer, and almost never used in new construction today. Most contemporary Florida homes have flex duct throughout. Still, understanding sheet metal matters if you live in an older home or are evaluating whether to keep an existing metal system.

What it is

Galvanized steel sheets formed into rectangular boxes (trunk lines, plenums) or round/oval pipes (branches). Typical gauges for residential:

• 26-gauge: lightweight, used for smaller branches
• 24-gauge: standard for most residential trunk and branch
• 22-gauge: heavier construction, used in commercial and high-end residential
• 20-gauge or heavier: industrial applications

Standard rectangular sizes are sold in stock dimensions (8x10, 10x12, 12x14, etc.); round pipe is sold in standard diameters from 4” to 14”+ for residential.

How it works

Air flows through the smooth interior of the metal duct from the air handler to the supply registers, or from the return grilles back to the air handler. The smooth interior surface has the lowest friction loss of any common duct material — significantly lower than flex duct, fiberboard, or lined sheet metal.

Sheet metal is uninsulated by default. In conditioned spaces (closets, finished basements), uninsulated metal is acceptable. In unconditioned spaces — Florida attics specifically — sheet metal must be wrapped in fiberglass duct insulation with a vapor barrier, typically R-6 to R-8.

Where it installs

In Florida residential, sheet metal is most commonly seen in three places, all of them upstream of where flex takes over:

• Supply and return plenums at the air handler (the rectangular boxes immediately above or beside the unit)
• Main supply trunk segments running through the attic between junction boxes — the trunk itself stays sheet metal, but the branches off each junction box are flex
• Junction boxes inline along the trunk — the sheet metal boxes where flex branches tap off via starting collars
• Return air trunks running from central return grilles back to the air handler

Pure sheet metal supply systems with sheet metal branches (instead of flex) are rare in modern residential construction but common in homes built before 1985. In typical Florida construction today, sheet metal handles the spine of the supply distribution (plenum, trunk, junction boxes) and flex handles the branches.

Common problems

Sheet metal fails in different ways than flex:

Failed joints and seams. Where two pieces of metal meet, they’re joined with sheet-metal screws, snap-lock seams, or both — then sealed with mastic, foil tape, or sometimes nothing. The sealing fails over time. Joints become primary leak sources.

Rust and corrosion. Galvanized steel resists corrosion well in dry conditions, but Florida attics get periodic moisture from condensation, roof leaks, or refrigerant line condensation drips. Rust eventually develops at low spots and seams.

Damaged insulation wrap. The insulation around the metal is more vulnerable than the metal itself. Squirrels, rats, and roofers tear it. Heat damage from sun exposure on uninsulated sections accelerates degradation.

Improper fabrication. Sheet metal duct quality depends entirely on installer skill. Poorly cut, poorly sealed, or improperly sized metal duct performs worse than even mediocre flex.

Inadequate sizing on returns. Sheet metal returns are often undersized relative to system airflow needs. This is the most common metal-duct problem in homes that otherwise have good ductwork.

Disconnected branches at starting collars. In hybrid systems where flex branches attach to sheet metal junction boxes, the starting collar connections can fail. The metal-to-flex seal is the most common failure point in the entire supply distribution. Less common with pure sheet metal branches (where screws hold the connection mechanically) but very common with flex-on-metal hybrids.

How long it lasts

Sheet metal duct, kept dry and properly insulated, lasts essentially forever. 50+ year service life is normal in dry climates. In Florida’s humid attic environment, expect 30–50 years before significant rust becomes a concern.

The insulation around the metal has a much shorter lifespan — 20–30 years before degradation becomes meaningful, less if it’s been damaged.

Inspection checklist

Walk the attic with a flashlight and look for:

• Visible rust at low points, seams, and connections
• Gaps at joints — daylight or air movement visible at connection points
• Failed mastic or tape at every visible seam
• Damaged or missing insulation around the metal
• Disconnected branches at starting collars on junction boxes
• Crushed sections from foot traffic or stored objects

When repair is appropriate

Most sheet metal duct problems are solvable without replacement:

• Resealing joints with mastic: $400–1,200 for a comprehensive job on a typical system
• Adding or replacing insulation wrap: $400–1,500
• Repairing rust spots with patching and recoating: $200–600 per location
• Reconnecting failed starting collars: $150–300 per location
• Adding return air capacity by cutting in new returns: $400–1,500

A well-built metal duct system from 1995 may need sealing and insulation work in 2026, but the actual sheet metal is likely fine and doesn’t need replacing.

When replacement makes sense

Full sheet metal replacement is justified when:

• Comprehensive rust damage compromises structural integrity
• Asbestos-containing insulation is present and disturbed (mostly pre-1980 homes; rare in Florida)
• Fundamental sizing mismatch with planned new equipment
• Major home renovation provides unusual access to otherwise sealed runs

Replacement is NOT justified by:

• Surface rust that hasn’t compromised the metal
• “It’s old” without specific failure
• A contractor pushing modern flex as inherently better (it isn’t)

The sheet-metal-vs-flex question (and the Florida reality)

For a new install or replacement decision in Florida residential, the practical answer is: you’re almost certainly getting flex. Pure sheet metal installations are uncommon and significantly more expensive. Most contractors don’t fabricate sheet metal for residential anymore, and the labor cost premium is hard to justify when properly designed flex performs adequately.

That said, if cost isn’t the driving factor and longevity matters:

• Trunk lines and plenums: sheet metal is genuinely better. Lower friction loss, longer service life, easier to seal effectively, more durable. Worth the premium on high-end installs.
• Branch runs to individual rooms: flex is acceptable and dramatically cheaper. The friction loss penalty on short branches is manageable if Manual D properly accounts for it.
• Returns: sheet metal is significantly better. Return-side leaks pull contaminated attic air into the system; quality of construction matters more here than on supply.

A premium residential install would use a hybrid: sheet metal trunks and plenums, flex branches. Florida new construction almost never does this — it’s all-flex for cost reasons. The result is a market where most homes have lower-grade ductwork than they could, but the equipment installed is sized accordingly and the systems work acceptably most of the time.

Questions to ask

• “Is the trunk line being replaced, or is this just branch work?”
• “What gauge of metal is being used?”
• “What’s the seam sealing approach — mastic, tape, both?”
• “What R-value is the insulation wrap?”
• “How will this be tested for leakage after installation?”

Pricing reality

• 24-gauge galvanized sheet (4’x10’ sheet): $30–60
• Fabricated rectangular trunk per linear foot installed: $25–45
• Round metal pipe with insulation per linear foot installed: $18–30
• Sheet metal supply trunk replacement on a 3-ton residential system: $3,500–7,500
• Full sheet metal supply system replacement: $7,000–14,000

The labor share of sheet metal duct is higher than flex — fabrication takes real skill, and that skill is increasingly priced at premium rates. Pure sheet metal installs are 30–50% more expensive than flex on the same scope, which is part of why builders default to flex.

---

For more on residential ductwork, see What’s In Your Attic: A Homeowner’s Guide to Residential Ductwork.