Flex Duct

Flex duct is the dominant residential ductwork material in Florida — essentially universal in new construction since the 1990s, and the primary material in retrofits and replacements. If you have an HVAC system in a Florida home built after 1990, you have flex duct. Older homes (1970s–80s) may have rigid metal trunks with flex branches, but pure flex installations are the norm. The construction makes it inexpensive and quick to install, which is why builders love it — but the same flexibility that makes installation easy also creates most of the problems that show up later.

What it is

Flex duct consists of a wire-reinforced flexible inner core wrapped in fiberglass insulation and an outer vapor barrier jacket. A flex duct assembly has three layers:

1. Inner core: a plastic or polyethylene film bonded to a helical steel wire that gives the duct its shape. Air flows through this layer.
2. Insulation: typically R-6 or R-8 fiberglass insulation wrapped around the inner core. This is what prevents the conditioned air from losing energy to the surrounding attic.
3. Outer jacket: a metallized or polyethylene film vapor barrier that protects the insulation and prevents condensation.

Available in standard diameters from 4” to 20” for residential applications. The most common sizes in homes are 6”, 7”, 8”, and 10”.

How it works

Air pushed by the blower travels through the inner core to wherever the duct terminates. The wire helix maintains shape under positive pressure, the insulation keeps conditioned air from losing energy to the surroundings, and the outer jacket prevents moisture infiltration that would degrade the insulation.

The key spec for flex duct is friction loss — how much resistance to airflow the duct creates. Flex duct has roughly 2–3 times the friction loss of equivalent-diameter rigid metal duct, primarily because the wire helix creates micro-turbulence in the airflow. This is why a Manual D calculation often calls for slightly larger flex duct than the equivalent rigid run would require.

Where it installs

Almost always in the attic in Florida residential construction. Flex attaches to the supply system at a starting collar — a short metal cylinder punched into the side or bottom of a junction box. The junction boxes sit inline along the supply trunk; the flex never connects to the trunk directly. From the starting collar, the flex runs through the attic to a boot at the ceiling, where it terminates at a register in the living space.

The full path of conditioned air in a typical Florida residential system: air handler → supply plenum → trunk → junction box → starting collar → flex branch → boot → register. A system might have 2–4 junction boxes chained along the trunk, each feeding 2–4 flex branches.

Common problems

Flex duct fails in predictable, almost universal ways. If you climb into the attic of any Florida home older than 10 years, you’ll see most of these:

Compression and crushing. Storage boxes, foot traffic, and bent support straps crush flex duct in unpredictable spots. Each crush point reduces airflow to everything downstream.

Kinks at the boot. Where flex enters the boot at the ceiling, installers often bend the duct sharply rather than supporting a gentle curve. The kink can reduce that branch’s airflow by 30–50%.

Sagging between supports. Code requires support every 4–5 feet for flex duct. Many installations have supports only every 8–10 feet. The duct sags between, creating low spots where the inner core can stretch and tear.

Disconnection from boots and starting collars. Flex is held to fittings with a metal band or plastic tie wrap plus mastic or foil tape. Over time the band loosens, the tape fails, and the duct pulls free. The most common location for this failure is at the starting collar on a junction box — a disconnected flex there dumps an entire room’s conditioned air directly into your attic. Disconnections at the boot end are slightly less common but happen the same way.

Inner liner deterioration. UV exposure (rare in attics but real near accessible windows), temperature cycling, and time eventually break down the polyethylene inner liner. Pinholes develop. The duct still looks intact from outside but leaks badly.

Insulation gaps. Where flex connects to fittings or where damage has occurred, the insulation gets compressed, torn, or removed. Even small bare sections in a 130°F attic become major energy losses.

How long it lasts

Manufacturers rate residential flex duct at 15–25 years of service life. In practice, expect 15–20 years before the inner liner starts degrading meaningfully. Sun-exposed or poorly-supported runs fail sooner.

Replacement isn’t usually triggered by age alone — it’s triggered by accumulated damage from the failure modes above.

Inspection checklist

Five-minute homeowner inspection with a flashlight:

• Look for completely disconnected branches — these are usually obvious, with a visible gap between the flex and either the junction-box starting collar or the boot at the ceiling
• Check for crushed sections — particularly under storage areas, near attic access, or where someone might have stepped
• Inspect support every 4–5 feet — sagging means missing supports
• Look at the boot connections — gentle curves are good, sharp kinks are bad
• Check the outer jacket — torn or stripped insulation means accelerated energy loss

When repair is appropriate

Most flex duct problems are localized. Common targeted repairs:

• Reconnection at a boot or starting collar: $150–300 per location
• Replacing a single damaged run: $200–600 depending on length
• Adding missing supports: $100–300 for a typical retrofit
• Resealing connections with mastic: included in broader duct sealing

These targeted fixes often deliver dramatic system performance improvements at modest cost. A homeowner who walks the attic with a tech and identifies five specific problems can often resolve them for under $1,000.

When replacement makes sense

Full flex duct replacement is justified when:

• Multiple runs are damaged beyond reasonable repair
• The inner liner is universally degrading (visible pinholes, tears, or shedding)
• The installation is fundamentally undersized for the system (Manual D shows the existing geometry can’t deliver design airflow)
• The system is being upgraded to substantially higher capacity

Replacement is NOT justified by:

• Age alone, if the duct is in good condition
• “While we’re in there” during a system replacement
• A contractor’s eyeball assessment without measurement

Questions to ask

• “Can you show me the specific damaged sections, with measurements?”
• “What’s the static pressure reading at the air handler? What does the manometer show?”
• “Has a Manual D calculation been done?”
• “Why is replacement the right answer instead of targeted repair?”

Pricing reality

• Standard residential flex duct material: $2–5 per linear foot
• Insulated flex with vapor barrier (R-8): $3–7 per linear foot
• Installed cost (material + labor): $8–15 per linear foot for typical residential
• Full supply-side flex replacement on a 3-ton residential system: $4,000–9,000 depending on home size and access
• Per-run replacement: $200–600 typical

A contractor quoting $1,500 to replace one damaged duct run is overcharging. A contractor quoting $15,000 to replace the entire supply system in a 1,500 sq ft house is overcharging. The honest middle is $3,000–7,000 for comprehensive supply-side flex replacement on a typical Florida home.

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